The Rust Unstable Book

# The Unstable Book

Welcome to the Unstable Book! This book consists of a number of chapters, each one organized by a "feature flag." That is, when using an unstable feature of Rust, you must use a flag, like this:

    #![feature(box_syntax)]

    fn main() {
        let five = box 5;
    }

The `box_syntax` feature has a chapter describing how to use it.

Because this documentation relates to unstable features, we make no guarantees that what is contained here is accurate or up to date. It's developed on a best-effort basis. Each page will have a link to its tracking issue with the latest developments; you might want to check those as well.

# Compiler flags

# `control-flow-guard`

The tracking issue for this feature is: #68793.

The rustc flag `-Z control-flow-guard` enables the Windows Control Flow Guard (CFG) platform security feature.

CFG is an exploit mitigation designed to enforce control-flow integrity for software running on supported Windows platforms (Windows 8.1 onwards). Specifically, CFG uses runtime checks to validate the target address of every indirect call/jump before allowing the call to complete.

During compilation, the compiler identifies all indirect calls/jumps and adds CFG checks. It also emits metadata containing the relative addresses of all address-taken functions. At runtime, if the binary is run on a CFG-aware operating system, the loader uses the CFG metadata to generate a bitmap of the address space and marks those addresses that contain valid targets. On each indirect call, the inserted check determines whether the target address is marked in this bitmap. If the target is not valid, the process is terminated.

In terms of interoperability:

- Code compiled with CFG enabled can be linked with libraries and object files that are not compiled with CFG. In this case, a CFG-aware linker can identify address-taken functions in the non-CFG libraries.
- Libraries compiled with CFG can linked into non-CFG programs. In this case, the CFG runtime checks in the libraries are not used (i.e. the mitigation is completely disabled).

CFG functionality is completely implemented in the LLVM backend and is supported for X86 (32-bit and 64-bit), ARM, and Aarch64 targets. The rustc flag adds the relevant LLVM module flags to enable the feature. This flag will be ignored for all non-Windows targets.

## When to use Control Flow Guard

The primary motivation for enabling CFG in Rust is to enhance security when linking against non-Rust code, especially C/C++ code. To achieve full CFG protection, all indirect calls (including any from Rust code) must have the appropriate CFG checks, as added by this flag. CFG can also improve security for Rust code that uses the `unsafe` keyword

## Overhead of Control Flow Guard

The CFG checks and metadata can potentially increase binary size and runtime overhead. The magnitude of any increase depends on the number and frequency of indirect calls. For example, enabling CFG for the Rust standard library increases binary size by approximately 0.14%. Enabling CFG in the SPEC CPU 2017 Integer Speed benchmark suite (compiled with Clang/LLVM) incurs approximate runtime overheads of between 0% and 8%, with a geometric mean of 2.9%.

## Testing Control Flow Guard

The rustc flag `-Z control-flow-guard=nochecks` instructs LLVM to emit the list of valid call targets without inserting runtime checks. This flag should only be used for testing purposes as it does not provide security enforcement.

## Control Flow Guard in libraries

It is strongly recommended to also enable CFG checks for all linked libraries, including the standard library.

To enable CFG in the standard library, use the cargo `-Z build-std` functionality to recompile the standard library with the same configuration options as the main program.

For example:

    rustup toolchain install --force nightly
    rustup component add rust-src
    SET RUSTFLAGS=-Z control-flow-guard
    cargo +nightly build -Z build-std --target x86_64-pc-windows-msvc

    rustup toolchain install --force nightly
    rustup component add rust-src
    $Env:RUSTFLAGS = "-Z control-flow-guard"
    cargo +nightly build -Z build-std --target x86_64-pc-windows-msvc

Alternatively, if you are building the standard library from source, you can set `control-flow-guard = true` in the config.toml file.

# `emit-stack-sizes`

The tracking issue for this feature is: #54192

The rustc flag `-Z emit-stack-sizes` makes LLVM emit stack size metadata.

> NOTE: This LLVM feature only supports the ELF object format as of LLVM 8.0. Using this flag with targets that use other object formats (e.g. macOS and Windows) will result in it being ignored.

Consider this crate:

    #![crate_type = "lib"]

    use std::ptr;

    pub fn foo() {
        // this function doesn't use the stack
    }

    pub fn bar() {
        let xs = [0u32; 2];

        // force LLVM to allocate `xs` on the stack
        unsafe { ptr::read_volatile(&xs.as_ptr()); }
    }

Using the `-Z emit-stack-sizes` flag produces extra linker sections in the output object file.

    $ rustc -C opt-level=3 --emit=obj foo.rs

    $ size -A foo.o
    foo.o  :
    section                                 size   addr
    .text                                      0      0
    .text._ZN3foo3foo17he211d7b4a3a0c16eE      1      0
    .text._ZN3foo3bar17h1acb594305f70c2eE     22      0
    .note.GNU-stack                            0      0
    .eh_frame                                 72      0
    Total                                     95

    $ rustc -C opt-level=3 --emit=obj -Z emit-stack-sizes foo.rs

    $ size -A foo.o
    foo.o  :
    section                                 size   addr
    .text                                      0      0
    .text._ZN3foo3foo17he211d7b4a3a0c16eE      1      0
    .stack_sizes                               9      0
    .text._ZN3foo3bar17h1acb594305f70c2eE     22      0
    .stack_sizes                               9      0
    .note.GNU-stack                            0      0
    .eh_frame                                 72      0
    Total                                    113

As of LLVM 7.0 the data will be written into a section named `.stack_sizes` and the format is "an array of pairs of function symbol values (pointer size) and stack sizes (unsigned LEB128)".

    $ objdump -d foo.o

    foo.o:     file format elf64-x86-64

    Disassembly of section .text._ZN3foo3foo17he211d7b4a3a0c16eE:

    0000000000000000 <_ZN3foo3foo17he211d7b4a3a0c16eE>:
       0:   c3                      retq

    Disassembly of section .text._ZN3foo3bar17h1acb594305f70c2eE:

    0000000000000000 <_ZN3foo3bar17h1acb594305f70c2eE>:
       0:   48 83 ec 10             sub    $0x10,%rsp
       4:   48 8d 44 24 08          lea    0x8(%rsp),%rax
       9:   48 89 04 24             mov    %rax,(%rsp)
       d:   48 8b 04 24             mov    (%rsp),%rax
      11:   48 83 c4 10             add    $0x10,%rsp
      15:   c3                      retq

    $ objdump -s -j .stack_sizes foo.o

    foo.o:     file format elf64-x86-64

    Contents of section .stack_sizes:
     0000 00000000 00000000 00                 .........
    Contents of section .stack_sizes:
     0000 00000000 00000000 10                 .........

It's important to note that linkers will discard this linker section by default. To preserve the section you can use a linker script like the one shown below.

    /* file: keep-stack-sizes.x */
    SECTIONS
    {
      /* `INFO` makes the section not allocatable so it won't be loaded into memory */
      .stack_sizes (INFO) :
      {
        KEEP(*(.stack_sizes));
      }
    }

The linker script must be passed to the linker using a rustc flag like `-C link-arg`.

    // file: src/main.rs
    use std::ptr;

    #[inline(never)]
    fn main() {
        let xs = [0u32; 2];

        // force LLVM to allocate `xs` on the stack
        unsafe { ptr::read_volatile(&xs.as_ptr()); }
    }

    $ RUSTFLAGS="-Z emit-stack-sizes" cargo build --release

    $ size -A target/release/hello | grep stack_sizes || echo section was not found
    section was not found

    $ RUSTFLAGS="-Z emit-stack-sizes" cargo rustc --release -- \
        -C link-arg=-Wl,-Tkeep-stack-sizes.x \
        -C link-arg=-N

    $ size -A target/release/hello | grep stack_sizes
    .stack_sizes                               90   176272

    $ # non-allocatable section (flags don't contain the "A" (alloc) flag)
    $ readelf -S target/release/hello
    Section Headers:
      [Nr]   Name              Type             Address           Offset
           Size              EntSize            Flags  Link  Info  Align
    (..)
      [1031] .stack_sizes      PROGBITS         000000000002b090  0002b0f0
           000000000000005a  0000000000000000   L       5     0     1

    $ objdump -s -j .stack_sizes target/release/hello

    target/release/hello:     file format elf64-x86-64

    Contents of section .stack_sizes:
     2b090 c0040000 00000000 08f00400 00000000  ................
     2b0a0 00080005 00000000 00000810 05000000  ................
     2b0b0 00000000 20050000 00000000 10400500  .... ........@..
     2b0c0 00000000 00087005 00000000 00000080  ......p.........
     2b0d0 05000000 00000000 90050000 00000000  ................
     2b0e0 00a00500 00000000 0000               ..........

> Author note: I'm not entirely sure why, in this case, `-N` is required in addition to `-Tkeep-stack-sizes.x`. For example, it's not required when producing statically linked files for the ARM Cortex-M architecture.

# `profile`

The tracking issue for this feature is: #42524.

This feature allows the generation of code coverage reports.

Set the `-Zprofile` compiler flag in order to enable gcov profiling.

For example:

    cargo new testgcov --bin
    cd testgcov
    export RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Copt-level=0 -Clink-dead-code -Coverflow-checks=off -Zpanic_abort_tests -Cpanic=abort"
    export CARGO_INCREMENTAL=0
    cargo build
    cargo run

Once you've built and run your program, files with the `gcno` (after build) and `gcda` (after execution) extensions will be created. You can parse them with llvm-cov gcov or grcov.

Please note that `RUSTFLAGS` by default applies to everything that cargo builds and runs during a build! When the `--target` flag is explicitly passed to cargo, the `RUSTFLAGS` no longer apply to build scripts and procedural macros. For more fine-grained control consider passing a `RUSTC_WRAPPER` program to cargo that only adds the profiling flags to rustc for the specific crates you want to profile.

# `report-time`

The tracking issue for this feature is: #64888

The `report-time` feature adds a possibility to report execution time of the tests generated via `libtest`.

This is unstable feature, so you have to provide `-Zunstable-options` to get this feature working.

Sample usage command:

    ./test_executable -Zunstable-options --report-time

Available options:

    --report-time [plain|colored]
                    Show execution time of each test. Available values:
                    plain = do not colorize the execution time (default);
                    colored = colorize output according to the `color`
                    parameter value;
                    Threshold values for colorized output can be
                    configured via
                    `RUST_TEST_TIME_UNIT`, `RUST_TEST_TIME_INTEGRATION`
                    and
                    `RUST_TEST_TIME_DOCTEST` environment variables.
                    Expected format of environment variable is
                    `VARIABLE=WARN_TIME,CRITICAL_TIME`.
                    Not available for --format=terse
    --ensure-time
                    Treat excess of the test execution time limit as
                    error.
                    Threshold values for this option can be configured via
                    `RUST_TEST_TIME_UNIT`, `RUST_TEST_TIME_INTEGRATION`
                    and
                    `RUST_TEST_TIME_DOCTEST` environment variables.
                    Expected format of environment variable is
                    `VARIABLE=WARN_TIME,CRITICAL_TIME`.
                    `CRITICAL_TIME` here means the limit that should not be
                    exceeded by test.

Example of the environment variable format:

    RUST_TEST_TIME_UNIT=100,200

where 100 stands for warn time, and 200 stands for critical time.

## Examples

    cargo test --tests -- -Zunstable-options --report-time
        Finished dev [unoptimized + debuginfo] target(s) in 0.02s
         Running target/debug/deps/example-27fb188025bec02c

    running 3 tests
    test tests::unit_test_quick ... ok <0.000s>
    test tests::unit_test_warn ... ok <0.055s>
    test tests::unit_test_critical ... ok <0.110s>

    test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

         Running target/debug/deps/tests-cedb06f6526d15d9

    running 3 tests
    test unit_test_quick ... ok <0.000s>
    test unit_test_warn ... ok <0.550s>
    test unit_test_critical ... ok <1.100s>

    test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

# `sanitizer`

The tracking issue for this feature is: #39699.

This feature allows for use of one of following sanitizers:

- AddressSanitizer a fast memory error detector.
- LeakSanitizer a run-time memory leak detector.
- MemorySanitizer a detector of uninitialized reads.
- ThreadSanitizer a fast data race detector.

To enable a sanitizer compile with `-Zsanitizer=address`, `-Zsanitizer=leak`, `-Zsanitizer=memory` or `-Zsanitizer=thread`.

# AddressSanitizer

AddressSanitizer is a memory error detector. It can detect the following types of bugs:

- Out of bound accesses to heap, stack and globals
- Use after free
- Use after return (runtime flag `ASAN_OPTIONS=detect_stack_use_after_return=1`)
- Use after scope
- Double-free, invalid free
- Memory leaks

AddressSanitizer is supported on the following targets:

- `x86_64-apple-darwin`
- `x86_64-unknown-linux-gnu`

AddressSanitizer works with non-instrumented code although it will impede its ability to detect some bugs. It is not expected to produce false positive reports.

## Examples

Stack buffer overflow:

    fn main() {
        let xs = [0, 1, 2, 3];
        let _y = unsafe { *xs.as_ptr().offset(4) };
    }

    $ export RUSTFLAGS=-Zsanitizer=address RUSTDOCFLAGS=-Zsanitizer=address
    $ cargo run -Zbuild-std --target x86_64-unknown-linux-gnu
    ==37882==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffe400e6250 at pc 0x5609a841fb20 bp 0x7ffe400e6210 sp 0x7ffe400e6208
    READ of size 4 at 0x7ffe400e6250 thread T0
        #0 0x5609a841fb1f in example::main::h628ffc6626ed85b2 /.../src/main.rs:3:23
        ...

    Address 0x7ffe400e6250 is located in stack of thread T0 at offset 48 in frame
        #0 0x5609a841f8af in example::main::h628ffc6626ed85b2 /.../src/main.rs:1

      This frame has 1 object(s):
        [32, 48) 'xs' (line 2) <== Memory access at offset 48 overflows this variable
    HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
          (longjmp and C++ exceptions *are* supported)
    SUMMARY: AddressSanitizer: stack-buffer-overflow /.../src/main.rs:3:23 in example::main::h628ffc6626ed85b2
    Shadow bytes around the buggy address:
      0x100048014bf0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
      0x100048014c00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
      0x100048014c10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
      0x100048014c20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
      0x100048014c30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    =>0x100048014c40: 00 00 00 00 f1 f1 f1 f1 00 00[f3]f3 00 00 00 00
      0x100048014c50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
      0x100048014c60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
      0x100048014c70: f1 f1 f1 f1 00 00 f3 f3 00 00 00 00 00 00 00 00
      0x100048014c80: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
      0x100048014c90: 00 00 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
    Shadow byte legend (one shadow byte represents 8 application bytes):
      Addressable:           00
      Partially addressable: 01 02 03 04 05 06 07
      Heap left redzone:       fa
      Freed heap region:       fd
      Stack left redzone:      f1
      Stack mid redzone:       f2
      Stack right redzone:     f3
      Stack after return:      f5
      Stack use after scope:   f8
      Global redzone:          f9
      Global init order:       f6
      Poisoned by user:        f7
      Container overflow:      fc
      Array cookie:            ac
      Intra object redzone:    bb
      ASan internal:           fe
      Left alloca redzone:     ca
      Right alloca redzone:    cb
      Shadow gap:              cc
    ==37882==ABORTING

Use of a stack object after its scope has already ended:

    static mut P: *mut usize = std::ptr::null_mut();

    fn main() {
        unsafe {
            {
                let mut x = 0;
                P = &mut x;
            }
            std::ptr::write_volatile(P, 123);
        }
    }

    $ export RUSTFLAGS=-Zsanitizer=address RUSTDOCFLAGS=-Zsanitizer=address
    $ cargo run -Zbuild-std --target x86_64-unknown-linux-gnu
    =================================================================
    ==39249==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7ffc7ed3e1a0 at pc 0x55c98b262a8e bp 0x7ffc7ed3e050 sp 0x7ffc7ed3e048
    WRITE of size 8 at 0x7ffc7ed3e1a0 thread T0
        #0 0x55c98b262a8d in core::ptr::write_volatile::he21f1df5a82f329a /.../src/rust/src/libcore/ptr/mod.rs:1048:5
        #1 0x55c98b262cd2 in example::main::h628ffc6626ed85b2 /.../src/main.rs:9:9
        ...

    Address 0x7ffc7ed3e1a0 is located in stack of thread T0 at offset 32 in frame
        #0 0x55c98b262bdf in example::main::h628ffc6626ed85b2 /.../src/main.rs:3

      This frame has 1 object(s):
        [32, 40) 'x' (line 6) <== Memory access at offset 32 is inside this variable
    HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
          (longjmp and C++ exceptions *are* supported)
    SUMMARY: AddressSanitizer: stack-use-after-scope /.../src/rust/src/libcore/ptr/mod.rs:1048:5 in core::ptr::write_volatile::he21f1df5a82f329a
    Shadow bytes around the buggy address:
      0x10000fd9fbe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
      0x10000fd9fbf0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
      0x10000fd9fc00: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
      0x10000fd9fc10: f8 f8 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
      0x10000fd9fc20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    =>0x10000fd9fc30: f1 f1 f1 f1[f8]f3 f3 f3 00 00 00 00 00 00 00 00
      0x10000fd9fc40: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
      0x10000fd9fc50: 00 00 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
      0x10000fd9fc60: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 00 f3 f3
      0x10000fd9fc70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
      0x10000fd9fc80: 00 00 00 00 f1 f1 f1 f1 00 00 f3 f3 00 00 00 00
    Shadow byte legend (one shadow byte represents 8 application bytes):
      Addressable:           00
      Partially addressable: 01 02 03 04 05 06 07
      Heap left redzone:       fa
      Freed heap region:       fd
      Stack left redzone:      f1
      Stack mid redzone:       f2
      Stack right redzone:     f3
      Stack after return:      f5
      Stack use after scope:   f8
      Global redzone:          f9
      Global init order:       f6
      Poisoned by user:        f7
      Container overflow:      fc
      Array cookie:            ac
      Intra object redzone:    bb
      ASan internal:           fe
      Left alloca redzone:     ca
      Right alloca redzone:    cb
      Shadow gap:              cc
    ==39249==ABORTING

# MemorySanitizer

MemorySanitizer is detector of uninitialized reads. It is only supported on the `x86_64-unknown-linux-gnu` target.

MemorySanitizer requires all program code to be instrumented. C/C++ dependencies need to be recompiled using Clang with `-fsanitize=memory` option. Failing to achieve that will result in false positive reports.

## Example

Detecting the use of uninitialized memory. The `-Zbuild-std` flag rebuilds and instruments the standard library, and is strictly necessary for the correct operation of the tool. The `-Zsanitizer-track-origins` enables tracking of the origins of uninitialized memory:

    use std::mem::MaybeUninit;

    fn main() {
        unsafe {
            let a = MaybeUninit::<[usize; 4]>::uninit();
            let a = a.assume_init();
            println!("{}", a[2]);
        }
    }

    $ export \
      CC=clang \
      CXX=clang++ \
      CFLAGS='-fsanitize=memory -fsanitize-memory-track-origins' \
      CXXFLAGS='-fsanitize=memory -fsanitize-memory-track-origins' \
      RUSTFLAGS='-Zsanitizer=memory -Zsanitizer-memory-track-origins' \
      RUSTDOCFLAGS='-Zsanitizer=memory -Zsanitizer-memory-track-origins'
    $ cargo clean
    $ cargo run -Zbuild-std --target x86_64-unknown-linux-gnu
    ==9416==WARNING: MemorySanitizer: use-of-uninitialized-value
        #0 0x560c04f7488a in core::fmt::num::imp::fmt_u64::haa293b0b098501ca $RUST/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/src/rust/src/libcore/fmt/num.rs:202:16
    ...
      Uninitialized value was stored to memory at
        #0 0x560c04ae898a in __msan_memcpy.part.0 $RUST/src/llvm-project/compiler-rt/lib/msan/msan_interceptors.cc:1558:3
        #1 0x560c04b2bf88 in memory::main::hd2333c1899d997f5 $CWD/src/main.rs:6:16

      Uninitialized value was created by an allocation of 'a' in the stack frame of function '_ZN6memory4main17hd2333c1899d997f5E'
        #0 0x560c04b2bc50 in memory::main::hd2333c1899d997f5 $CWD/src/main.rs:3

# ThreadSanitizer

ThreadSanitizer is a data race detection tool. It is supported on the following targets:

- `x86_64-apple-darwin`
- `x86_64-unknown-linux-gnu`

To work correctly ThreadSanitizer needs to be "aware" of all synchronization operations in a program. It generally achieves that through combination of library interception (for example synchronization performed through `pthread_mutex_lock` / `pthread_mutex_unlock`) and compile time instrumentation (e.g. atomic operations). Using it without instrumenting all the program code can lead to false positive reports.

ThreadSanitizer does not support atomic fences `std::sync::atomic::fence`, nor synchronization performed using inline assembly code.

## Example

    static mut A: usize = 0;

    fn main() {
        let t = std::thread::spawn(|| {
            unsafe { A += 1 };
        });
        unsafe { A += 1 };

        t.join().unwrap();
    }

    $ export RUSTFLAGS=-Zsanitizer=thread RUSTDOCFLAGS=-Zsanitizer=thread
    $ cargo run -Zbuild-std --target x86_64-unknown-linux-gnu
    ==================
    WARNING: ThreadSanitizer: data race (pid=10574)
      Read of size 8 at 0x5632dfe3d030 by thread T1:
        #0 example::main::_$u7b$$u7b$closure$u7d$$u7d$::h23f64b0b2f8c9484 ../src/main.rs:5:18 (example+0x86cec)
        ...

      Previous write of size 8 at 0x5632dfe3d030 by main thread:
        #0 example::main::h628ffc6626ed85b2 /.../src/main.rs:7:14 (example+0x868c8)
        ...
        #11 main <null> (example+0x86a1a)

      Location is global 'example::A::h43ac149ddf992709' of size 8 at 0x5632dfe3d030 (example+0x000000bd9030)

# Instrumentation of external dependencies and std

The sanitizers to varying degrees work correctly with partially instrumented code. On the one extreme is LeakSanitizer that doesn't use any compile time instrumentation, on the other is MemorySanitizer that requires that all program code to be instrumented (failing to achieve that will inevitably result in false positives).

It is strongly recommended to combine sanitizers with recompiled and instrumented standard library, for example using cargo `-Zbuild-std` functionality.

# Build scripts and procedural macros

Use of sanitizers together with build scripts and procedural macros is technically possible, but in almost all cases it would be best avoided. This is especially true for procedural macros which would require an instrumented version of rustc.

In more practical terms when using cargo always remember to pass `--target` flag, so that rustflags will not be applied to build scripts and procedural macros.

# Symbolizing the Reports

Sanitizers produce symbolized stacktraces when llvm-symbolizer binary is in `PATH`.

# Additional Information

- Sanitizers project page
- AddressSanitizer in Clang
- LeakSanitizer in Clang
- MemorySanitizer in Clang
- ThreadSanitizer in Clang

# `self-profile`

The `-Zself-profile` compiler flag enables rustc's internal profiler. When enabled, the compiler will output three binary files in the specified directory (or the current working directory if no directory is specified). These files can be analyzed by using the tools in the `measureme` repository.

To control the data recorded in the trace files, use the `-Zself-profile-events` flag.

For example:

First, run a compilation session and provide the `-Zself-profile` flag:

    $ rustc --crate-name foo -Zself-profile

This will generate three files in the working directory such as:

- `foo-1234.events`
- `foo-1234.string_data`
- `foo-1234.string_index`

Where `foo` is the name of the crate and `1234` is the process id of the rustc process.

To get a summary of where the compiler is spending its time:

    $ ../measureme/target/release/summarize summarize foo-1234

To generate a flamegraph of the same data:

    $ ../measureme/target/release/inferno foo-1234

To dump the event data in a Chromium-profiler compatible format:

    $ ../measureme/target/release/crox foo-1234

For more information, consult the `measureme` documentation.

# `self-profile-events`

The `-Zself-profile-events` compiler flag controls what events are recorded by the self-profiler when it is enabled via the `-Zself-profile` flag.

This flag takes a comma delimited list of event types to record.

For example:

    $ rustc -Zself-profile -Zself-profile-events=default,args

## Event types

- `query-provider`

  - Traces each query used internally by the compiler.

- `generic-activity`

  - Traces other parts of the compiler not covered by the query system.

- `query-cache-hit`

  - Adds tracing information that records when the in-memory query cache is "hit" and does not need to re-execute a query which has been cached.
  - Disabled by default because this significantly increases the trace file size.

- `query-blocked`

  - Tracks time that a query tries to run but is blocked waiting on another thread executing the same query to finish executing.
  - Query blocking only occurs when the compiler is built with parallel mode support.

- `incr-cache-load`

  - Tracks time that is spent loading and deserializing query results from the incremental compilation on-disk cache.

- `query-keys`

  - Adds a serialized representation of each query's query key to the tracing data.
  - Disabled by default because this significantly increases the trace file size.

- `function-args`

  - Adds additional tracing data to some `generic-activity` events.
  - Disabled by default for parity with `query-keys`.

- `llvm`

  - Adds tracing information about LLVM passes and codegeneration.
  - Disabled by default because this only works when `-Znew-llvm-pass-manager` is enabled.

## Event synonyms

- `none`

  - Disables all events. Equivalent to the self-profiler being disabled.

- `default`

  - The default set of events which stikes a balance between providing detailed tracing data and adding additional overhead to the compilation.

- `args`

  - Equivalent to `query-keys` and `function-args`.

- `all`

  - Enables all events.

## Examples

Enable the profiler and capture the default set of events (both invocations are equivalent):

    $ rustc -Zself-profile
    $ rustc -Zself-profile -Zself-profile-events=default

Enable the profiler and capture the default events and their arguments:

    $ rustc -Zself-profile -Zself-profile-events=default,args

# `src-hash-algorithm`

The tracking issue for this feature is: #70401.

The `-Z src-hash-algorithm` compiler flag controls which algorithm is used when hashing each source file. The hash is stored in the debug info and can be used by a debugger to verify the source code matches the executable.

Supported hash algorithms are: `md5`, and `sha1`. Note that not all hash algorithms are supported by all debug info formats.

By default, the compiler chooses the hash algorithm based on the target specification.

# `strip`

The tracking issue for this feature is: #72110.

Option `-Z strip=val` controls stripping of debuginfo and similar auxiliary data from binaries during linking.

Supported values for this option are:

- `none` - debuginfo and symbols (if they exist) are copied to the produced binary or separate files depending on the target (e.g. `.pdb` files in case of MSVC).
- `debuginfo` - debuginfo sections and debuginfo symbols from the symbol table section are stripped at link time and are not copied to the produced binary or separate files.
- `symbols` - same as `debuginfo`, but the rest of the symbol table section is stripped as well if the linker supports it.

# `tls_model`

The tracking issue for this feature is: None.

Option `-Z tls-model` controls TLS model used to generate code for accessing `#[thread_local]` `static` items.

Supported values for this option are:

- `global-dynamic` - General Dynamic TLS Model (alternatively called Global Dynamic) is the most general option usable in all circumstances, even if the TLS data is defined in a shared library loaded at runtime and is accessed from code outside of that library.
  This is the default for most targets.
- `local-dynamic` - model usable if the TLS data is only accessed from the shared library or executable it is defined in. The TLS data may be in a library loaded after startup (via `dlopen`).
- `initial-exec` - model usable if the TLS data is defined in the executable or in a shared library loaded at program startup. The TLS data must not be in a library loaded after startup (via `dlopen`).
- `local-exec` - model usable only if the TLS data is defined directly in the executable, but not in a shared library, and is accessed only from that executable.

`rustc` and LLVM may use a more optimized model than specified if they know that we are producing and executable rather than a library, or that the `static` item is private enough.

# Language features

# `aarch64_target_feature`

The tracking issue for this feature is: #44839

# `abi_amdgpu_kernel`

The tracking issue for this feature is: #51575

# `abi_avr_interrupt`

The tracking issue for this feature is: #69664

# `abi_efiapi`

The tracking issue for this feature is: #65815

# `abi_msp430_interrupt`

The tracking issue for this feature is: #38487

In the MSP430 architecture, interrupt handlers have a special calling convention. You can use the `"msp430-interrupt"` ABI to make the compiler apply the right calling convention to the interrupt handlers you define.

    #![feature(abi_msp430_interrupt)]
    #![no_std]

    // Place the interrupt handler at the appropriate memory address
    // (Alternatively, you can use `#[used]` and remove `pub` and `#[no_mangle]`)
    #[link_section = "__interrupt_vector_10"]
    #[no_mangle]
    pub static TIM0_VECTOR: extern "msp430-interrupt" fn() = tim0;

    // The interrupt handler
    extern "msp430-interrupt" fn tim0() {
        // ..
    }

    $ msp430-elf-objdump -CD ./target/msp430/release/app
    Disassembly of section __interrupt_vector_10:

    0000fff2 <TIM0_VECTOR>:
        fff2:       00 c0           interrupt service routine at 0xc000

    Disassembly of section .text:

    0000c000 <int::tim0>:
        c000:       00 13           reti

# `abi_ptx`

The tracking issue for this feature is: #38788

When emitting PTX code, all vanilla Rust functions (`fn`) get translated to "device" functions. These functions are not callable from the host via the CUDA API so a crate with only device functions is not too useful!

OTOH, "global" functions can be called by the host; you can think of them as the real public API of your crate. To produce a global function use the `"ptx-kernel"` ABI.

    #![feature(abi_ptx)]
    #![no_std]

    pub unsafe extern "ptx-kernel" fn global_function() {
        device_function();
    }

    pub fn device_function() {
        // ..
    }

    $ xargo rustc --target nvptx64-nvidia-cuda --release -- --emit=asm

    $ cat $(find -name '*.s')
    //
    // Generated by LLVM NVPTX Back-End
    //

    .version 3.2
    .target sm_20
    .address_size 64

            // .globl       _ZN6kernel15global_function17h46111ebe6516b382E

    .visible .entry _ZN6kernel15global_function17h46111ebe6516b382E()
    {

            ret;
    }

            // .globl       _ZN6kernel15device_function17hd6a0e4993bbf3f78E
    .visible .func _ZN6kernel15device_function17hd6a0e4993bbf3f78E()
    {

            ret;
    }

# `abi_thiscall`

The tracking issue for this feature is: #42202

The MSVC ABI on x86 Windows uses the `thiscall` calling convention for C++ instance methods by default; it is identical to the usual (C) calling convention on x86 Windows except that the first parameter of the method, the `this` pointer, is passed in the ECX register.

# `abi_unadjusted`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `abi_vectorcall`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `abi_x86_interrupt`

The tracking issue for this feature is: #40180

# `adx_target_feature`

The tracking issue for this feature is: #44839

# `alloc_error_handler`

The tracking issue for this feature is: #51540

# `allocator_internals`

This feature does not have a tracking issue, it is an unstable implementation detail of the `global_allocator` feature not intended for use outside the compiler.

# `allow_fail`

The tracking issue for this feature is: #46488

# `allow_internal_unsafe`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `allow_internal_unstable`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `arbitrary_enum_discriminant`

The tracking issue for this feature is: #60553

The `arbitrary_enum_discriminant` feature permits tuple-like and struct-like enum variants with `#[repr(<int-type>)]` to have explicit discriminants.

## Examples

    #![allow(unused)]
    #![feature(arbitrary_enum_discriminant)]

    fn main() {
    #[allow(dead_code)]
    #[repr(u8)]
    enum Enum {
        Unit = 3,
        Tuple(u16) = 2,
        Struct {
            a: u8,
            b: u16,
        } = 1,
    }

    impl Enum {
        fn tag(&self) -> u8 {
            unsafe { *(self as *const Self as *const u8) }
        }
    }

    assert_eq!(3, Enum::Unit.tag());
    assert_eq!(2, Enum::Tuple(5).tag());
    assert_eq!(1, Enum::Struct{a: 7, b: 11}.tag());
    }

# `arbitrary_self_types`

The tracking issue for this feature is: #44874

# `arm_target_feature`

The tracking issue for this feature is: #44839

# `associated_type_bounds`

The tracking issue for this feature is: #52662

# `associated_type_defaults`

The tracking issue for this feature is: #29661

# `async_closure`

The tracking issue for this feature is: #62290

# `avx512_target_feature`

The tracking issue for this feature is: #44839

# `bindings_after_at`

The tracking issue for this feature is: #65490

# `box_patterns`

The tracking issue for this feature is: #29641

See also `box_syntax`

Box patterns let you match on `Box<T>`s:

    #![feature(box_patterns)]

    fn main() {
        let b = Some(Box::new(5));
        match b {
            Some(box n) if n < 0 => {
                println!("Box contains negative number {}", n);
            },
            Some(box n) if n >= 0 => {
                println!("Box contains non-negative number {}", n);
            },
            None => {
                println!("No box");
            },
            _ => unreachable!()
        }
    }

# `box_syntax`

The tracking issue for this feature is: #49733

See also `box_patterns`

Currently the only stable way to create a `Box` is via the `Box::new` method. Also it is not possible in stable Rust to destructure a `Box` in a match pattern. The unstable `box` keyword can be used to create a `Box`. An example usage would be:

    #![feature(box_syntax)]

    fn main() {
        let b = box 5;
    }

# `c_variadic`

The tracking issue for this feature is: #44930

The `c_variadic` language feature enables C-variadic functions to be defined in Rust. The may be called both from within Rust and via FFI.

## Examples

    #![allow(unused)]
    #![feature(c_variadic)]

    fn main() {
    pub unsafe extern "C" fn add(n: usize, mut args: ...) -> usize {
        let mut sum = 0;
        for _ in 0..n {
            sum += args.arg::<usize>();
        }
        sum
    }
    }

# `cfg_sanitize`

The tracking issue for this feature is: #39699

The `cfg_sanitize` feature makes it possible to execute different code depending on whether a particular sanitizer is enabled or not.

## Examples

    #![allow(unused)]
    #![feature(cfg_sanitize)]

    fn main() {
    #[cfg(sanitize = "thread")]
    fn a() {
        // ...
    }

    #[cfg(not(sanitize = "thread"))]
    fn a() {
        // ...
    }

    fn b() {
        if cfg!(sanitize = "leak") {
            // ...
        } else {
            // ...
        }
    }
    }

# `cfg_target_has_atomic`

The tracking issue for this feature is: #32976

# `cfg_target_thread_local`

The tracking issue for this feature is: #29594

# `cfg_version`

The tracking issue for this feature is: #64796

The `cfg_version` feature makes it possible to execute different code depending on the compiler version.

## Examples

    #![allow(unused)]
    #![feature(cfg_version)]

    fn main() {
    #[cfg(version("1.42"))]
    fn a() {
        // ...
    }

    #[cfg(not(version("1.42")))]
    fn a() {
        // ...
    }

    fn b() {
        if cfg!(version("1.42")) {
            // ...
        } else {
            // ...
        }
    }
    }

# `cmpxchg16b_target_feature`

The tracking issue for this feature is: #44839

# `compiler_builtins`

This feature is internal to the Rust compiler and is not intended for general use.

# `const_eval_limit`

The tracking issue for this feature is: #67217

The `const_eval_limit` allows someone to limit the evaluation steps the CTFE undertakes to evaluate a `const fn`.

# `const_extern_fn`

The tracking issue for this feature is: #64926

# `const_fn`

The tracking issue for this feature is: #57563

The `const_fn` feature allows marking free functions and inherent methods as `const`, enabling them to be called in constants contexts, with constant arguments.

## Examples

    #![feature(const_fn)]

    const fn double(x: i32) -> i32 {
        x * 2
    }

    const FIVE: i32 = 5;
    const TEN: i32 = double(FIVE);

    fn main() {
        assert_eq!(5, FIVE);
        assert_eq!(10, TEN);
    }

# `const_fn_transmute`

The tracking issue for this feature is: #53605

# `const_fn_union`

The tracking issue for this feature is: #51909

# `const_generics`

The tracking issue for this feature is: #44580

# `const_in_array_repeat_expressions`

The tracking issue for this feature is: #49147

Relaxes the rules for repeat expressions, `[x; N]` such that `x` may also be `const` (strictly speaking rvalue promotable), in addition to `typeof(x): Copy`. The result of `[x; N]` where `x` is `const` is itself also `const`.

# `const_mut_refs`

The tracking issue for this feature is: #57349

# `const_panic`

The tracking issue for this feature is: #51999

# `const_precise_live_drops`

The tracking issue for this feature is: #73255

# `const_raw_ptr_deref`

The tracking issue for this feature is: #51911

# `const_raw_ptr_to_usize_cast`

The tracking issue for this feature is: #51910

# `const_trait_bound_opt_out`

The tracking issue for this feature is: #67794

# `const_trait_impl`

The tracking issue for this feature is: #67792

# `crate_visibility_modifier`

The tracking issue for this feature is: #53120

The `crate_visibility_modifier` feature allows the `crate` keyword to be used as a visibility modifier synonymous to `pub(crate)`, indicating that a type (function, &c.) is to be visible to the entire enclosing crate, but not to other crates.

    #![allow(unused)]
    #![feature(crate_visibility_modifier)]

    fn main() {
    crate struct Foo {
        bar: usize,
    }
    }

# `custom_inner_attributes`

The tracking issue for this feature is: #54726

# `custom_test_frameworks`

The tracking issue for this feature is: #50297

The `custom_test_frameworks` feature allows the use of `#[test_case]` and `#![test_runner]`. Any function, const, or static can be annotated with `#[test_case]` causing it to be aggregated (like `#[test]`) and be passed to the test runner determined by the `#![test_runner]` crate attribute.

    #![allow(unused)]
    #![feature(custom_test_frameworks)]
    #![test_runner(my_runner)]

    fn main() {
    fn my_runner(tests: &[&i32]) {
        for t in tests {
            if **t == 0 {
                println!("PASSED");
            } else {
                println!("FAILED");
            }
        }
    }

    #[test_case]
    const WILL_PASS: i32 = 0;

    #[test_case]
    const WILL_FAIL: i32 = 4;
    }

# `decl_macro`

The tracking issue for this feature is: #39412

# `default_type_parameter_fallback`

The tracking issue for this feature is: #27336

# `doc_alias`

The tracking issue for this feature is: #50146

You can add alias(es) to an item when using the `rustdoc` search through the `doc(alias)` attribute. Example:

    #![allow(unused)]
    #![feature(doc_alias)]

    fn main() {
    #[doc(alias = "x")]
    #[doc(alias = "big")]
    pub struct BigX;
    }

Then, when looking for it through the `rustdoc` search, if you enter "x" or "big", search will show the `BigX` struct first.

Note that this feature is currently hidden behind the `feature(doc_alias)` gate.

# `doc_cfg`

The tracking issue for this feature is: #43781

The `doc_cfg` feature allows an API be documented as only available in some specific platforms. This attribute has two effects:

1. In the annotated item's documentation, there will be a message saying "This is supported on (platform) only".

2. The item's doc-tests will only run on the specific platform.

In addition to allowing the use of the `#[doc(cfg)]` attribute, this feature enables the use of a special conditional compilation flag, `#[cfg(doc)]`, set whenever building documentation on your crate.

This feature was introduced as part of PR #43348 to allow the platform-specific parts of the standard library be documented.

    #![allow(unused)]
    #![feature(doc_cfg)]

    fn main() {
    #[cfg(any(windows, doc))]
    #[doc(cfg(windows))]
    /// The application's icon in the notification area (a.k.a. system tray).
    ///
    /// # Examples
    ///
    /// ```no_run
    /// extern crate my_awesome_ui_library;
    /// use my_awesome_ui_library::current_app;
    /// use my_awesome_ui_library::windows::notification;
    ///
    /// let icon = current_app().get::<notification::Icon>();
    /// icon.show();
    /// icon.show_message("Hello");
    /// ```
    pub struct Icon {
        // ...
    }
    }

# `doc_keyword`

The tracking issue for this feature is: #51315

# `doc_masked`

The tracking issue for this feature is: #44027

The `doc_masked` feature allows a crate to exclude types from a given crate from appearing in lists of trait implementations. The specifics of the feature are as follows:

1. When rustdoc encounters an `extern crate` statement annotated with a `#[doc(masked)]` attribute, it marks the crate as being masked.

2. When listing traits a given type implements, rustdoc ensures that traits from masked crates are not emitted into the documentation.

3. When listing types that implement a given trait, rustdoc ensures that types from masked crates are not emitted into the documentation.

This feature was introduced in PR #44026 to ensure that compiler-internal and implementation-specific types and traits were not included in the standard library's documentation. Such types would introduce broken links into the documentation.

# `dropck_eyepatch`

The tracking issue for this feature is: #34761

# `exclusive_range_pattern`

The tracking issue for this feature is: #37854

# `exhaustive_patterns`

The tracking issue for this feature is: #51085

# `extern_types`

The tracking issue for this feature is: #43467

# `external_doc`

The tracking issue for this feature is: #44732

The `external_doc` feature allows the use of the `include` parameter to the `#[doc]` attribute, to include external files in documentation. Use the attribute in place of, or in addition to, regular doc comments and `#[doc]` attributes, and `rustdoc` will load the given file when it renders documentation for your crate.

With the following files in the same directory:

`external-doc.md`:

    # My Awesome Type

    This is the documentation for this spectacular type.

`lib.rs`:

    #![feature(external_doc)]

    #[doc(include = "external-doc.md")]
    pub struct MyAwesomeType;

`rustdoc` will load the file `external-doc.md` and use it as the documentation for the `MyAwesomeType` struct.

When locating files, `rustdoc` will base paths in the `src/` directory, as if they were alongside the `lib.rs` for your crate. So if you want a `docs/` folder to live alongside the `src/` directory, start your paths with `../docs/` for `rustdoc` to properly find the file.

This feature was proposed in RFC #1990 and initially implemented in PR #44781.

# `f16c_target_feature`

The tracking issue for this feature is: #44839

# `ffi_const`

The `#[ffi_const]` attribute applies clang's `const` attribute to foreign functions declarations.

That is, `#[ffi_const]` functions shall have no effects except for its return value, which can only depend on the values of the function parameters, and is not affected by changes to the observable state of the program.

Applying the `#[ffi_const]` attribute to a function that violates these requirements is undefined behaviour.

This attribute enables Rust to perform common optimizations, like sub-expression elimination, and it can avoid emitting some calls in repeated invocations of the function with the same argument values regardless of other operations being performed in between these functions calls (as opposed to `#[ffi_pure]` functions).

## Pitfalls

A `#[ffi_const]` function can only read global memory that would not affect its return value for the whole execution of the program (e.g. immutable global memory). `#[ffi_const]` functions are referentially-transparent and therefore more strict than `#[ffi_pure]` functions.

A common pitfall involves applying the `#[ffi_const]` attribute to a function that reads memory through pointer arguments which do not necessarily point to immutable global memory.

A `#[ffi_const]` function that returns unit has no effect on the abstract machine's state, and a `#[ffi_const]` function cannot be `#[ffi_pure]`.

A `#[ffi_const]` function must not diverge, neither via a side effect (e.g. a call to `abort`) nor by infinite loops.

When translating C headers to Rust FFI, it is worth verifying for which targets the `const` attribute is enabled in those headers, and using the appropriate `cfg` macros in the Rust side to match those definitions. While the semantics of `const` are implemented identically by many C and C++ compilers, e.g., clang, GCC, ARM C/C++ compiler, IBM ILE C/C++, etc. they are not necessarily implemented in this way on all of them. It is therefore also worth verifying that the semantics of the C toolchain used to compile the binary being linked against are compatible with those of the `#[ffi_const]`.

# `ffi_pure`

The `#[ffi_pure]` attribute applies clang's `pure` attribute to foreign functions declarations.

That is, `#[ffi_pure]` functions shall have no effects except for its return value, which shall not change across two consecutive function calls with the same parameters.

Applying the `#[ffi_pure]` attribute to a function that violates these requirements is undefined behavior.

This attribute enables Rust to perform common optimizations, like sub-expression elimination and loop optimizations. Some common examples of pure functions are `strlen` or `memcmp`.

These optimizations are only applicable when the compiler can prove that no program state observable by the `#[ffi_pure]` function has changed between calls of the function, which could alter the result. See also the `#[ffi_const]` attribute, which provides stronger guarantees regarding the allowable behavior of a function, enabling further optimization.

## Pitfalls

A `#[ffi_pure]` function can read global memory through the function parameters (e.g. pointers), globals, etc. `#[ffi_pure]` functions are not referentially-transparent, and are therefore more relaxed than `#[ffi_const]` functions.

However, accesing global memory through volatile or atomic reads can violate the requirement that two consecutive function calls shall return the same value.

A `pure` function that returns unit has no effect on the abstract machine's state.

A `#[ffi_pure]` function must not diverge, neither via a side effect (e.g. a call to `abort`) nor by infinite loops.

When translating C headers to Rust FFI, it is worth verifying for which targets the `pure` attribute is enabled in those headers, and using the appropriate `cfg` macros in the Rust side to match those definitions. While the semantics of `pure` are implemented identically by many C and C++ compilers, e.g., clang, GCC, ARM C/C++ compiler, IBM ILE C/C++, etc. they are not necessarily implemented in this way on all of them. It is therefore also worth verifying that the semantics of the C toolchain used to compile the binary being linked against are compatible with those of the `#[ffi_pure]`.

# `ffi_returns_twice`

The tracking issue for this feature is: #58314

# `format_args_capture`

The tracking issue for this feature is: #67984

# `fundamental`

The tracking issue for this feature is: #29635

# `generators`

The tracking issue for this feature is: #43122

The `generators` feature gate in Rust allows you to define generator or coroutine literals. A generator is a "resumable function" that syntactically resembles a closure but compiles to much different semantics in the compiler itself. The primary feature of a generator is that it can be suspended during execution to be resumed at a later date. Generators use the `yield` keyword to "return", and then the caller can `resume` a generator to resume execution just after the `yield` keyword.

Generators are an extra-unstable feature in the compiler right now. Added in RFC 2033 they're mostly intended right now as a information/constraint gathering phase. The intent is that experimentation can happen on the nightly compiler before actual stabilization. A further RFC will be required to stabilize generators/coroutines and will likely contain at least a few small tweaks to the overall design.

A syntactical example of a generator is:

    #![feature(generators, generator_trait)]

    use std::ops::{Generator, GeneratorState};
    use std::pin::Pin;

    fn main() {
        let mut generator = || {
            yield 1;
            return "foo"
        };

        match Pin::new(&mut generator).resume(()) {
            GeneratorState::Yielded(1) => {}
            _ => panic!("unexpected value from resume"),
        }
        match Pin::new(&mut generator).resume(()) {
            GeneratorState::Complete("foo") => {}
            _ => panic!("unexpected value from resume"),
        }
    }

Generators are closure-like literals which can contain a `yield` statement. The `yield` statement takes an optional expression of a value to yield out of the generator. All generator literals implement the `Generator` trait in the `std::ops` module. The `Generator` trait has one main method, `resume`, which resumes execution of the generator at the previous suspension point.

An example of the control flow of generators is that the following example prints all numbers in order:

    #![feature(generators, generator_trait)]

    use std::ops::Generator;
    use std::pin::Pin;

    fn main() {
        let mut generator = || {
            println!("2");
            yield;
            println!("4");
        };

        println!("1");
        Pin::new(&mut generator).resume(());
        println!("3");
        Pin::new(&mut generator).resume(());
        println!("5");
    }

At this time the main intended use case of generators is an implementation primitive for async/await syntax, but generators will likely be extended to ergonomic implementations of iterators and other primitives in the future. Feedback on the design and usage is always appreciated!

### The `Generator` trait

The `Generator` trait in `std::ops` currently looks like:

    #![allow(unused)]
    fn main() {
    #![feature(arbitrary_self_types, generator_trait)]
    use std::ops::GeneratorState;
    use std::pin::Pin;

    pub trait Generator<R = ()> {
        type Yield;
        type Return;
        fn resume(self: Pin<&mut Self>, resume: R) -> GeneratorState<Self::Yield, Self::Return>;
    }
    }

The `Generator::Yield` type is the type of values that can be yielded with the `yield` statement. The `Generator::Return` type is the returned type of the generator. This is typically the last expression in a generator's definition or any value passed to `return` in a generator. The `resume` function is the entry point for executing the `Generator` itself.

The return value of `resume`, `GeneratorState`, looks like:

    #![allow(unused)]
    fn main() {
    pub enum GeneratorState<Y, R> {
        Yielded(Y),
        Complete(R),
    }
    }

The `Yielded` variant indicates that the generator can later be resumed. This corresponds to a `yield` point in a generator. The `Complete` variant indicates that the generator is complete and cannot be resumed again. Calling `resume` after a generator has returned `Complete` will likely result in a panic of the program.

### Closure-like semantics

The closure-like syntax for generators alludes to the fact that they also have closure-like semantics. Namely:

- When created, a generator executes no code. A closure literal does not actually execute any of the closure's code on construction, and similarly a generator literal does not execute any code inside the generator when constructed.

- Generators can capture outer variables by reference or by move, and this can be tweaked with the `move` keyword at the beginning of the closure. Like closures all generators will have an implicit environment which is inferred by the compiler. Outer variables can be moved into a generator for use as the generator progresses.

- Generator literals produce a value with a unique type which implements the `std::ops::Generator` trait. This allows actual execution of the generator through the `Generator::resume` method as well as also naming it in return types and such.

- Traits like `Send` and `Sync` are automatically implemented for a `Generator` depending on the captured variables of the environment. Unlike closures, generators also depend on variables live across suspension points. This means that although the ambient environment may be `Send` or `Sync`, the generator itself may not be due to internal variables live across `yield` points being not-`Send` or not-`Sync`. Note that generators do not implement traits like `Copy` or `Clone` automatically.

- Whenever a generator is dropped it will drop all captured environment variables.

### Generators as state machines

In the compiler, generators are currently compiled as state machines. Each `yield` expression will correspond to a different state that stores all live variables over that suspension point. Resumption of a generator will dispatch on the current state and then execute internally until a `yield` is reached, at which point all state is saved off in the generator and a value is returned.

Let's take a look at an example to see what's going on here:

    #![feature(generators, generator_trait)]

    use std::ops::Generator;
    use std::pin::Pin;

    fn main() {
        let ret = "foo";
        let mut generator = move || {
            yield 1;
            return ret
        };

        Pin::new(&mut generator).resume(());
        Pin::new(&mut generator).resume(());
    }

This generator literal will compile down to something similar to:

    #![feature(arbitrary_self_types, generators, generator_trait)]

    use std::ops::{Generator, GeneratorState};
    use std::pin::Pin;

    fn main() {
        let ret = "foo";
        let mut generator = {
            enum __Generator {
                Start(&'static str),
                Yield1(&'static str),
                Done,
            }

            impl Generator for __Generator {
                type Yield = i32;
                type Return = &'static str;

                fn resume(mut self: Pin<&mut Self>, resume: ()) -> GeneratorState<i32, &'static str> {
                    use std::mem;
                    match mem::replace(&mut *self, __Generator::Done) {
                        __Generator::Start(s) => {
                            *self = __Generator::Yield1(s);
                            GeneratorState::Yielded(1)
                        }

                        __Generator::Yield1(s) => {
                            *self = __Generator::Done;
                            GeneratorState::Complete(s)
                        }

                        __Generator::Done => {
                            panic!("generator resumed after completion")
                        }
                    }
                }
            }

            __Generator::Start(ret)
        };

        Pin::new(&mut generator).resume(());
        Pin::new(&mut generator).resume(());
    }

Notably here we can see that the compiler is generating a fresh type, `__Generator` in this case. This type has a number of states (represented here as an `enum`) corresponding to each of the conceptual states of the generator. At the beginning we're closing over our outer variable `foo` and then that variable is also live over the `yield` point, so it's stored in both states.

When the generator starts it'll immediately yield 1, but it saves off its state just before it does so indicating that it has reached the yield point. Upon resuming again we'll execute the `return ret` which returns the `Complete` state.

Here we can also note that the `Done` state, if resumed, panics immediately as it's invalid to resume a completed generator. It's also worth noting that this is just a rough desugaring, not a normative specification for what the compiler does.

# `generic_associated_types`

The tracking issue for this feature is: #44265

# `half_open_range_patterns`

The tracking issue for this feature is: #67264

# `hexagon_target_feature`

The tracking issue for this feature is: #44839

# `impl_trait_in_bindings`

The tracking issue for this feature is: #63065

The `impl_trait_in_bindings` feature gate lets you use `impl Trait` syntax in `let`, `static`, and `const` bindings.

A simple example is:

    #![feature(impl_trait_in_bindings)]

    use std::fmt::Debug;

    fn main() {
        let a: impl Debug + Clone = 42;
        let b = a.clone();
        println!("{:?}", b); // prints `42`
    }

Note however that because the types of `a` and `b` are opaque in the above example, calling inherent methods or methods outside of the specified traits (e.g., `a.abs()` or `b.abs()`) is not allowed, and yields an error.

# `in_band_lifetimes`

The tracking issue for this feature is: #44524

# `infer_static_outlives_requirements`

The tracking issue for this feature is: #54185

The `infer_static_outlives_requirements` feature indicates that certain `'static` outlives requirements can be inferred by the compiler rather than stating them explicitly.

Note: It is an accompanying feature to `infer_outlives_requirements`, which must be enabled to infer outlives requirements.

For example, currently generic struct definitions that contain references, require where-clauses of the form T: 'static. By using this feature the outlives predicates will be inferred, although they may still be written explicitly.

    struct Foo<U> where U: 'static { // <-- currently required
        bar: Bar<U>
    }
    struct Bar<T: 'static> {
        x: T,
    }

## Examples:

    #![feature(infer_outlives_requirements)]
    #![feature(infer_static_outlives_requirements)]

    #[rustc_outlives]
    // Implicitly infer U: 'static
    struct Foo<U> {
        bar: Bar<U>
    }
    struct Bar<T: 'static> {
        x: T,
    }

# `intrinsics`

The tracking issue for this feature is: None.

Intrinsics are never intended to be stable directly, but intrinsics are often exported in some sort of stable manner. Prefer using the stable interfaces to the intrinsic directly when you can.

These are imported as if they were FFI functions, with the special `rust-intrinsic` ABI. For example, if one was in a freestanding context, but wished to be able to `transmute` between types, and perform efficient pointer arithmetic, one would import those functions via a declaration like

    #![feature(intrinsics)]
    fn main() {}

    extern "rust-intrinsic" {
        fn transmute<T, U>(x: T) -> U;

        fn offset<T>(dst: *const T, offset: isize) -> *const T;
    }

As with any other FFI functions, these are always `unsafe` to call.

# `label_break_value`

The tracking issue for this feature is: #48594

# `lang_items`

The tracking issue for this feature is: None.

The `rustc` compiler has certain pluggable operations, that is, functionality that isn't hard-coded into the language, but is implemented in libraries, with a special marker to tell the compiler it exists. The marker is the attribute `#[lang = "..."]` and there are various different values of `...`, i.e. various different 'lang items'.

For example, `Box` pointers require two lang items, one for allocation and one for deallocation. A freestanding program that uses the `Box` sugar for dynamic allocations via `malloc` and `free`:

    #![feature(lang_items, box_syntax, start, libc, core_intrinsics)]
    #![no_std]
    use core::intrinsics;
    use core::panic::PanicInfo;

    extern crate libc;

    #[lang = "owned_box"]
    pub struct Box<T>(*mut T);

    #[lang = "exchange_malloc"]
    unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
        let p = libc::malloc(size as libc::size_t) as *mut u8;

        // Check if `malloc` failed:
        if p as usize == 0 {
            intrinsics::abort();
        }

        p
    }

    #[lang = "box_free"]
    unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
        libc::free(ptr as *mut libc::c_void)
    }

    #[start]
    fn main(_argc: isize, _argv: *const *const u8) -> isize {
        let _x = box 1;

        0
    }

    #[lang = "eh_personality"] extern fn rust_eh_personality() {}
    #[lang = "panic_impl"] extern fn rust_begin_panic(info: &PanicInfo) -> ! { unsafe { intrinsics::abort() } }
    #[no_mangle] pub extern fn rust_eh_register_frames () {}
    #[no_mangle] pub extern fn rust_eh_unregister_frames () {}

Note the use of `abort`: the `exchange_malloc` lang item is assumed to return a valid pointer, and so needs to do the check internally.

Other features provided by lang items include:

- overloadable operators via traits: the traits corresponding to the `==`, `<`, dereferencing (`*`) and `+` (etc.) operators are all marked with lang items; those specific four are `eq`, `ord`, `deref`, and `add` respectively.
- stack unwinding and general failure; the `eh_personality`, `panic` and `panic_bounds_checks` lang items.
- the traits in `std::marker` used to indicate types of various kinds; lang items `send`, `sync` and `copy`.
- the marker types and variance indicators found in `std::marker`; lang items `covariant_type`, `contravariant_lifetime`, etc.

Lang items are loaded lazily by the compiler; e.g. if one never uses `Box` then there is no need to define functions for `exchange_malloc` and `box_free`. `rustc` will emit an error when an item is needed but not found in the current crate or any that it depends on.

Most lang items are defined by `libcore`, but if you're trying to build an executable without the standard library, you'll run into the need for lang items. The rest of this page focuses on this use-case, even though lang items are a bit broader than that.

### Using libc

In order to build a `#[no_std]` executable we will need libc as a dependency. We can specify this using our `Cargo.toml` file:

    [dependencies]
    libc = { version = "0.2.14", default-features = false }

Note that the default features have been disabled. This is a critical step - the default features of libc include the standard library and so must be disabled.

### Writing an executable without stdlib

Controlling the entry point is possible in two ways: the `#[start]` attribute, or overriding the default shim for the C `main` function with your own.

The function marked `#[start]` is passed the command line parameters in the same format as C:

    #![feature(lang_items, core_intrinsics)]
    #![feature(start)]
    #![no_std]
    use core::intrinsics;
    use core::panic::PanicInfo;

    // Pull in the system libc library for what crt0.o likely requires.
    extern crate libc;

    // Entry point for this program.
    #[start]
    fn start(_argc: isize, _argv: *const *const u8) -> isize {
        0
    }

    // These functions are used by the compiler, but not
    // for a bare-bones hello world. These are normally
    // provided by libstd.
    #[lang = "eh_personality"]
    #[no_mangle]
    pub extern fn rust_eh_personality() {
    }

    #[lang = "panic_impl"]
    #[no_mangle]
    pub extern fn rust_begin_panic(info: &PanicInfo) -> ! {
        unsafe { intrinsics::abort() }
    }

To override the compiler-inserted `main` shim, one has to disable it with `#![no_main]` and then create the appropriate symbol with the correct ABI and the correct name, which requires overriding the compiler's name mangling too:

    #![feature(lang_items, core_intrinsics)]
    #![feature(start)]
    #![no_std]
    #![no_main]
    use core::intrinsics;
    use core::panic::PanicInfo;

    // Pull in the system libc library for what crt0.o likely requires.
    extern crate libc;

    // Entry point for this program.
    #[no_mangle] // ensure that this symbol is called `main` in the output
    pub extern fn main(_argc: i32, _argv: *const *const u8) -> i32 {
        0
    }

    // These functions are used by the compiler, but not
    // for a bare-bones hello world. These are normally
    // provided by libstd.
    #[lang = "eh_personality"]
    #[no_mangle]
    pub extern fn rust_eh_personality() {
    }

    #[lang = "panic_impl"]
    #[no_mangle]
    pub extern fn rust_begin_panic(info: &PanicInfo) -> ! {
        unsafe { intrinsics::abort() }
    }

In many cases, you may need to manually link to the `compiler_builtins` crate when building a `no_std` binary. You may observe this via linker error messages such as "`undefined reference to `__rust_probestack'`".

## More about the language items

The compiler currently makes a few assumptions about symbols which are available in the executable to call. Normally these functions are provided by the standard library, but without it you must define your own. These symbols are called "language items", and they each have an internal name, and then a signature that an implementation must conform to.

The first of these functions, `rust_eh_personality`, is used by the failure mechanisms of the compiler. This is often mapped to GCC's personality function (see the libstd implementation for more information), but crates which do not trigger a panic can be assured that this function is never called. The language item's name is `eh_personality`.

The second function, `rust_begin_panic`, is also used by the failure mechanisms of the compiler. When a panic happens, this controls the message that's displayed on the screen. While the language item's name is `panic_impl`, the symbol name is `rust_begin_panic`.

Finally, a `eh_catch_typeinfo` static is needed for certain targets which implement Rust panics on top of C++ exceptions.

## List of all language items

This is a list of all language items in Rust along with where they are located in the source code.

- Primitives
  - `i8`: `libcore/num/mod.rs`
  - `i16`: `libcore/num/mod.rs`
  - `i32`: `libcore/num/mod.rs`
  - `i64`: `libcore/num/mod.rs`
  - `i128`: `libcore/num/mod.rs`
  - `isize`: `libcore/num/mod.rs`
  - `u8`: `libcore/num/mod.rs`
  - `u16`: `libcore/num/mod.rs`
  - `u32`: `libcore/num/mod.rs`
  - `u64`: `libcore/num/mod.rs`
  - `u128`: `libcore/num/mod.rs`
  - `usize`: `libcore/num/mod.rs`
  - `f32`: `libstd/f32.rs`
  - `f64`: `libstd/f64.rs`
  - `char`: `libcore/char.rs`
  - `slice`: `liballoc/slice.rs`
  - `str`: `liballoc/str.rs`
  - `const_ptr`: `libcore/ptr.rs`
  - `mut_ptr`: `libcore/ptr.rs`
  - `unsafe_cell`: `libcore/cell.rs`
- Runtime
  - `start`: `libstd/rt.rs`
  - `eh_personality`: `libpanic_unwind/emcc.rs` (EMCC)
  - `eh_personality`: `libpanic_unwind/gcc.rs` (GNU)
  - `eh_personality`: `libpanic_unwind/seh.rs` (SEH)
  - `eh_catch_typeinfo`: `libpanic_unwind/emcc.rs` (EMCC)
  - `panic`: `libcore/panicking.rs`
  - `panic_bounds_check`: `libcore/panicking.rs`
  - `panic_impl`: `libcore/panicking.rs`
  - `panic_impl`: `libstd/panicking.rs`
- Allocations
  - `owned_box`: `liballoc/boxed.rs`
  - `exchange_malloc`: `liballoc/heap.rs`
  - `box_free`: `liballoc/heap.rs`
- Operands
  - `not`: `libcore/ops/bit.rs`
  - `bitand`: `libcore/ops/bit.rs`
  - `bitor`: `libcore/ops/bit.rs`
  - `bitxor`: `libcore/ops/bit.rs`
  - `shl`: `libcore/ops/bit.rs`
  - `shr`: `libcore/ops/bit.rs`
  - `bitand_assign`: `libcore/ops/bit.rs`
  - `bitor_assign`: `libcore/ops/bit.rs`
  - `bitxor_assign`: `libcore/ops/bit.rs`
  - `shl_assign`: `libcore/ops/bit.rs`
  - `shr_assign`: `libcore/ops/bit.rs`
  - `deref`: `libcore/ops/deref.rs`
  - `deref_mut`: `libcore/ops/deref.rs`
  - `index`: `libcore/ops/index.rs`
  - `index_mut`: `libcore/ops/index.rs`
  - `add`: `libcore/ops/arith.rs`
  - `sub`: `libcore/ops/arith.rs`
  - `mul`: `libcore/ops/arith.rs`
  - `div`: `libcore/ops/arith.rs`
  - `rem`: `libcore/ops/arith.rs`
  - `neg`: `libcore/ops/arith.rs`
  - `add_assign`: `libcore/ops/arith.rs`
  - `sub_assign`: `libcore/ops/arith.rs`
  - `mul_assign`: `libcore/ops/arith.rs`
  - `div_assign`: `libcore/ops/arith.rs`
  - `rem_assign`: `libcore/ops/arith.rs`
  - `eq`: `libcore/cmp.rs`
  - `ord`: `libcore/cmp.rs`
- Functions
  - `fn`: `libcore/ops/function.rs`
  - `fn_mut`: `libcore/ops/function.rs`
  - `fn_once`: `libcore/ops/function.rs`
  - `generator_state`: `libcore/ops/generator.rs`
  - `generator`: `libcore/ops/generator.rs`
- Other
  - `coerce_unsized`: `libcore/ops/unsize.rs`
  - `drop`: `libcore/ops/drop.rs`
  - `drop_in_place`: `libcore/ptr.rs`
  - `clone`: `libcore/clone.rs`
  - `copy`: `libcore/marker.rs`
  - `send`: `libcore/marker.rs`
  - `sized`: `libcore/marker.rs`
  - `unsize`: `libcore/marker.rs`
  - `sync`: `libcore/marker.rs`
  - `phantom_data`: `libcore/marker.rs`
  - `discriminant_kind`: `libcore/marker.rs`
  - `freeze`: `libcore/marker.rs`
  - `debug_trait`: `libcore/fmt/mod.rs`
  - `non_zero`: `libcore/nonzero.rs`
  - `arc`: `liballoc/sync.rs`
  - `rc`: `liballoc/rc.rs`

# `lazy_normalization_consts`

The tracking issue for this feature is: #72219

# `let_chains`

The tracking issue for this feature is: #53667

# `link_args`

The tracking issue for this feature is: #29596

You can tell `rustc` how to customize linking, and that is via the `link_args` attribute. This attribute is applied to `extern` blocks and specifies raw flags which need to get passed to the linker when producing an artifact. An example usage would be:

    #![feature(link_args)]

    #[link_args = "-foo -bar -baz"]
    extern {}
    fn main() {}

Note that this feature is currently hidden behind the `feature(link_args)` gate because this is not a sanctioned way of performing linking. Right now `rustc` shells out to the system linker (`gcc` on most systems, `link.exe` on MSVC), so it makes sense to provide extra command line arguments, but this will not always be the case. In the future `rustc` may use LLVM directly to link native libraries, in which case `link_args` will have no meaning. You can achieve the same effect as the `link_args` attribute with the `-C link-args` argument to `rustc`.

It is highly recommended to not use this attribute, and rather use the more formal `#[link(...)]` attribute on `extern` blocks instead.

# `link_cfg`

This feature is internal to the Rust compiler and is not intended for general use.

# `link_llvm_intrinsics`

The tracking issue for this feature is: #29602

# `linkage`

The tracking issue for this feature is: #29603

# `lint_reasons`

The tracking issue for this feature is: #54503

# `main`

The tracking issue for this feature is: #29634

# `marker_trait_attr`

The tracking issue for this feature is: #29864

Normally, Rust keeps you from adding trait implementations that could overlap with each other, as it would be ambiguous which to use. This feature, however, carves out an exception to that rule: a trait can opt-in to having overlapping implementations, at the cost that those implementations are not allowed to override anything (and thus the trait itself cannot have any associated items, as they're pointless when they'd need to do the same thing for every type anyway).

    #![allow(unused)]
    #![feature(marker_trait_attr)]

    fn main() {
    #[marker] trait CheapToClone: Clone {}

    impl<T: Copy> CheapToClone for T {}

    // These could potentially overlap with the blanket implementation above,
    // so are only allowed because CheapToClone is a marker trait.
    impl<T: CheapToClone, U: CheapToClone> CheapToClone for (T, U) {}
    impl<T: CheapToClone> CheapToClone for std::ops::Range<T> {}

    fn cheap_clone<T: CheapToClone>(t: T) -> T {
        t.clone()
    }
    }

This is expected to replace the unstable `overlapping_marker_traits` feature, which applied to all empty traits (without needing an opt-in).

# `member_constraints`

The tracking issue for this feature is: #61997

The `member_constraints` feature gate lets you use `impl Trait` syntax with multiple unrelated lifetime parameters.

A simple example is:

    #![feature(member_constraints)]

    trait Trait<'a, 'b> { }
    impl<T> Trait<'_, '_> for T {}

    fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> impl Trait<'a, 'b> {
      (x, y)
    }

    fn main() { }

Without the `member_constraints` feature gate, the above example is an error because both `'a` and `'b` appear in the impl Trait bounds, but neither outlives the other.

# `min_specialization`

The tracking issue for this feature is: #31844

# `mips_target_feature`

The tracking issue for this feature is: #44839

# `mmx_target_feature`

The tracking issue for this feature is: #44839

# `movbe_target_feature`

The tracking issue for this feature is: #44839

# `move_ref_pattern`

The tracking issue for this feature is: #68354

# `naked_functions`

The tracking issue for this feature is: #32408

# `needs_panic_runtime`

The tracking issue for this feature is: #32837

# `negative_impls`

The tracking issue for this feature is #68318.

With the feature gate `negative_impls`, you can write negative impls as well as positive ones:

    #![allow(unused)]
    #![feature(negative_impls)]
    fn main() {
    trait DerefMut { }
    impl<T: ?Sized> !DerefMut for &T { }
    }

Negative impls indicate a semver guarantee that the given trait will not be implemented for the given types. Negative impls play an additional purpose for auto traits, described below.

Negative impls have the following characteristics:

- They do not have any items.
- They must obey the orphan rules as if they were a positive impl.
- They cannot "overlap" with any positive impls.

## Semver interaction

It is a breaking change to remove a negative impl. Negative impls are a commitment not to implement the given trait for the named types.

## Orphan and overlap rules

Negative impls must obey the same orphan rules as a positive impl. This implies you cannot add a negative impl for types defined in upstream crates and so forth.

Similarly, negative impls cannot overlap with positive impls, again using the same "overlap" check that we ordinarily use to determine if two impls overlap. (Note that positive impls typically cannot overlap with one another either, except as permitted by specialization.)

## Interaction with auto traits

Declaring a negative impl `impl !SomeAutoTrait for SomeType` for an auto-trait serves two purposes:

- as with any trait, it declares that `SomeType` will never implement `SomeAutoTrait`;
- it disables the automatic `SomeType: SomeAutoTrait` impl that would otherwise have been generated.

Note that, at present, there is no way to indicate that a given type does not implement an auto trait but that it may do so in the future. For ordinary types, this is done by simply not declaring any impl at all, but that is not an option for auto traits. A workaround is that one could embed a marker type as one of the fields, where the marker type is `!AutoTrait`.

## Immediate uses

Negative impls are used to declare that `&T: !DerefMut` and `&mut T: !Clone`, as required to fix the soundness of `Pin` described in #66544.

This serves two purposes:

- For proving the correctness of unsafe code, we can use that impl as evidence that no `DerefMut` or `Clone` impl exists.
- It prevents downstream crates from creating such impls.

# `never_type`

The tracking issue for this feature is: #35121

# `never_type_fallback`

The tracking issue for this feature is: #65992

# `nll`

The tracking issue for this feature is: #43234

# `no_core`

The tracking issue for this feature is: #29639

# `no_niche`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `no_sanitize`

The tracking issue for this feature is: #39699

The `no_sanitize` attribute can be used to selectively disable sanitizer instrumentation in an annotated function. This might be useful to: avoid instrumentation overhead in a performance critical function, or avoid instrumenting code that contains constructs unsupported by given sanitizer.

The precise effect of this annotation depends on particular sanitizer in use. For example, with `no_sanitize(thread)`, the thread sanitizer will no longer instrument non-atomic store / load operations, but it will instrument atomic operations to avoid reporting false positives and provide meaning full stack traces.

## Examples

    #![allow(unused)]
    #![feature(no_sanitize)]

    fn main() {
    #[no_sanitize(address)]
    fn foo() {
      // ...
    }
    }

# `non_ascii_idents`

The tracking issue for this feature is: #55467

The `non_ascii_idents` feature adds support for non-ASCII identifiers.

## Examples

    #![allow(unused)]
    #![feature(non_ascii_idents)]

    fn main() {
    const ε: f64 = 0.00001f64;
    const Π: f64 = 3.14f64;
    }

## Changes to the language reference

> Lexer:
> IDENTIFIER :
> XID_start XID_continue*
> | `_` XID_continue+

An identifier is any nonempty Unicode string of the following form:

Either

- The first character has property `XID_start`
- The remaining characters have property `XID_continue`

Or

- The first character is `_`
- The identifier is more than one character, `_` alone is not an identifier
- The remaining characters have property `XID_continue`

that does not occur in the set of strict keywords.

> Note: `XID_start` and `XID_continue` as character properties cover the character ranges used to form the more familiar C and Java language-family identifiers.

# `object_safe_for_dispatch`

The tracking issue for this feature is: #43561

# `omit_gdb_pretty_printer_section`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `optimize_attribute`

The tracking issue for this feature is: #54882

# `optin_builtin_traits`

The tracking issue for this feature is #13231

The `optin_builtin_traits` feature gate allows you to define auto traits.

Auto traits, like `Send` or `Sync` in the standard library, are marker traits that are automatically implemented for every type, unless the type, or a type it contains, has explicitly opted out via a negative impl. (Negative impls are separately controlled by the `negative_impls` feature.)

    impl !Trait for Type

Example:

    #![feature(negative_impls)]
    #![feature(optin_builtin_traits)]

    auto trait Valid {}

    struct True;
    struct False;

    impl !Valid for False {}

    struct MaybeValid<T>(T);

    fn must_be_valid<T: Valid>(_t: T) { }

    fn main() {
        // works
        must_be_valid( MaybeValid(True) );

        // compiler error - trait bound not satisfied
        // must_be_valid( MaybeValid(False) );
    }

## Automatic trait implementations

When a type is declared as an `auto trait`, we will automatically create impls for every struct/enum/union, unless an explicit impl is provided. These automatic impls contain a where clause for each field of the form `T: AutoTrait`, where `T` is the type of the field and `AutoTrait` is the auto trait in question. As an example, consider the struct `List` and the auto trait `Send`:

    #![allow(unused)]
    fn main() {
    struct List<T> {
      data: T,
      next: Option<Box<List<T>>>,
    }
    }

Presuming that there is no explicit impl of `Send` for `List`, the compiler will supply an automatic impl of the form:

    #![allow(unused)]
    fn main() {
    struct List<T> {
      data: T,
      next: Option<Box<List<T>>>,
    }

    unsafe impl<T> Send for List<T>
    where
      T: Send, // from the field `data`
      Option<Box<List<T>>>: Send, // from the field `next`
    { }
    }

Explicit impls may be either positive or negative. They take the form:

    impl<...> AutoTrait for StructName<..> { }
    impl<...> !AutoTrait for StructName<..> { }

## Coinduction: Auto traits permit cyclic matching

Unlike ordinary trait matching, auto traits are coinductive. This means, in short, that cycles which occur in trait matching are considered ok. As an example, consider the recursive struct `List` introduced in the previous section. In attempting to determine whether `List: Send`, we would wind up in a cycle: to apply the impl, we must show that `Option<Box<List>>: Send`, which will in turn require `Box<List>: Send` and then finally `List: Send` again. Under ordinary trait matching, this cycle would be an error, but for an auto trait it is considered a successful match.

## Items

Auto traits cannot have any trait items, such as methods or associated types. This ensures that we can generate default implementations.

## Supertraits

Auto traits cannot have supertraits. This is for soundness reasons, as the interaction of coinduction with implied bounds is difficult to reconcile.

# `or_patterns`

The tracking issue for this feature is: #54883

The `or_pattern` language feature allows `|` to be arbitrarily nested within a pattern, for example, `Some(A(0) | B(1 | 2))` becomes a valid pattern.

## Examples

    #![feature(or_patterns)]

    pub enum Foo {
        Bar,
        Baz,
        Quux,
    }

    pub fn example(maybe_foo: Option<Foo>) {
        match maybe_foo {
            Some(Foo::Bar | Foo::Baz) => {
                println!("The value contained `Bar` or `Baz`");
            }
            Some(_) => {
                println!("The value did not contain `Bar` or `Baz`");
            }
            None => {
                println!("The value was `None`");
            }
        }
    }

# `panic_runtime`

The tracking issue for this feature is: #32837

# `platform_intrinsics`

The tracking issue for this feature is: #27731

# `plugin`

The tracking issue for this feature is: #29597

This feature is part of "compiler plugins." It will often be used with the `plugin_registrar` and `rustc_private` features.

`rustc` can load compiler plugins, which are user-provided libraries that extend the compiler's behavior with new lint checks, etc.

A plugin is a dynamic library crate with a designated registrar function that registers extensions with `rustc`. Other crates can load these extensions using the crate attribute `#![plugin(...)]`. See the `rustc_driver::plugin` documentation for more about the mechanics of defining and loading a plugin.

In the vast majority of cases, a plugin should only be used through `#![plugin]` and not through an `extern crate` item. Linking a plugin would pull in all of librustc_ast and librustc as dependencies of your crate. This is generally unwanted unless you are building another plugin.

The usual practice is to put compiler plugins in their own crate, separate from any `macro_rules!` macros or ordinary Rust code meant to be used by consumers of a library.

# Lint plugins

Plugins can extend Rust's lint infrastructure with additional checks for code style, safety, etc. Now let's write a plugin `lint_plugin_test.rs` that warns about any item named `lintme`.

    #![feature(plugin_registrar)]
    #![feature(box_syntax, rustc_private)]

    extern crate rustc_ast;

    // Load rustc as a plugin to get macros
    #[macro_use]
    extern crate rustc;
    extern crate rustc_driver;

    use rustc::lint::{EarlyContext, LintContext, LintPass, EarlyLintPass,
                      EarlyLintPassObject, LintArray};
    use rustc_driver::plugin::Registry;
    use rustc_ast::ast;

    declare_lint!(TEST_LINT, Warn, "Warn about items named 'lintme'");

    struct Pass;

    impl LintPass for Pass {
        fn get_lints(&self) -> LintArray {
            lint_array!(TEST_LINT)
        }
    }

    impl EarlyLintPass for Pass {
        fn check_item(&mut self, cx: &EarlyContext, it: &ast::Item) {
            if it.ident.as_str() == "lintme" {
                cx.span_lint(TEST_LINT, it.span, "item is named 'lintme'");
            }
        }
    }

    #[plugin_registrar]
    pub fn plugin_registrar(reg: &mut Registry) {
        reg.register_early_lint_pass(box Pass as EarlyLintPassObject);
    }

Then code like

    #![plugin(lint_plugin_test)]

    fn lintme() { }

will produce a compiler warning:

    foo.rs:4:1: 4:16 warning: item is named 'lintme', #[warn(test_lint)] on by default
    foo.rs:4 fn lintme() { }
             ^~~~~~~~~~~~~~~

The components of a lint plugin are:

- one or more `declare_lint!` invocations, which define static `Lint` structs;

- a struct holding any state needed by the lint pass (here, none);

- a `LintPass` implementation defining how to check each syntax element. A single `LintPass` may call `span_lint` for several different `Lint`s, but should register them all through the `get_lints` method.

Lint passes are syntax traversals, but they run at a late stage of compilation where type information is available. `rustc`'s built-in lints mostly use the same infrastructure as lint plugins, and provide examples of how to access type information.

Lints defined by plugins are controlled by the usual attributes and compiler flags, e.g. `#[allow(test_lint)]` or `-A test-lint`. These identifiers are derived from the first argument to `declare_lint!`, with appropriate case and punctuation conversion.

You can run `rustc -W help foo.rs` to see a list of lints known to `rustc`, including those provided by plugins loaded by `foo.rs`.

# `plugin_registrar`

The tracking issue for this feature is: #29597

This feature is part of "compiler plugins." It will often be used with the `plugin` and `rustc_private` features as well. For more details, see their docs.

# `powerpc_target_feature`

The tracking issue for this feature is: #44839

# `precise_pointer_size_matching`

The tracking issue for this feature is: #56354

# `prelude_import`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `proc_macro_hygiene`

The tracking issue for this feature is: #54727

# `profiler_runtime`

The tracking issue for this feature is: #42524.

# `raw_dylib`

The tracking issue for this feature is: #58713

# `raw_ref_op`

The tracking issue for this feature is: #64490

# `register_attr`

The tracking issue for this feature is: #66080

# `register_tool`

The tracking issue for this feature is: #66079

# `repr_simd`

The tracking issue for this feature is: #27731

# `repr128`

The tracking issue for this feature is: #56071

The `repr128` feature adds support for `#[repr(u128)]` on `enum`s.

    #![allow(unused)]
    #![feature(repr128)]

    fn main() {
    #[repr(u128)]
    enum Foo {
        Bar(u64),
    }
    }

# `riscv_target_feature`

The tracking issue for this feature is: #44839

# `rtm_target_feature`

The tracking issue for this feature is: #44839

# `rustc_attrs`

This feature has no tracking issue, and is therefore internal to the compiler, not being intended for general use.

Note: `rustc_attrs` enables many rustc-internal attributes and this page only discuss a few of them.

The `rustc_attrs` feature allows debugging rustc type layouts by using `#[rustc_layout(...)]` to debug layout at compile time (it even works with `cargo check`) as an alternative to `rustc -Z print-type-sizes` that is way more verbose.

Options provided by `#[rustc_layout(...)]` are `debug`, `size`, `abi`. Note that it only work best with sized type without generics.

## Examples

    #![feature(rustc_attrs)]

    #[rustc_layout(abi, size)]
    pub enum X {
        Y(u8, u8, u8),
        Z(isize),
    }

When that is compiled, the compiler will error with something like

    error: abi: Aggregate { sized: true }
     --> src/lib.rs:4:1
      |
    4 | / pub enum T {
    5 | |     Y(u8, u8, u8),
    6 | |     Z(isize),
    7 | | }
      | |_^

    error: size: Size { raw: 16 }
     --> src/lib.rs:4:1
      |
    4 | / pub enum T {
    5 | |     Y(u8, u8, u8),
    6 | |     Z(isize),
    7 | | }
      | |_^

    error: aborting due to 2 previous errors

# `rustc_private`

The tracking issue for this feature is: #27812

# `simd_ffi`

The tracking issue for this feature is: #27731

# `specialization`

The tracking issue for this feature is: #31844

# `sse4a_target_feature`

The tracking issue for this feature is: #44839

# `staged_api`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `start`

The tracking issue for this feature is: #29633

# `static_nobundle`

The tracking issue for this feature is: #37403

# `stmt_expr_attributes`

The tracking issue for this feature is: #15701

# `structural_match`

The tracking issue for this feature is: #31434

# `target_feature_11`

The tracking issue for this feature is: #69098

# `tbm_target_feature`

The tracking issue for this feature is: #44839

# `test_2018_feature`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `thread_local`

The tracking issue for this feature is: #29594

# `trait_alias`

The tracking issue for this feature is: #41517

The `trait_alias` feature adds support for trait aliases. These allow aliases to be created for one or more traits (currently just a single regular trait plus any number of auto-traits), and used wherever traits would normally be used as either bounds or trait objects.

    #![feature(trait_alias)]

    trait Foo = std::fmt::Debug + Send;
    trait Bar = Foo + Sync;

    // Use trait alias as bound on type parameter.
    fn foo<T: Foo>(v: &T) {
        println!("{:?}", v);
    }

    pub fn main() {
        foo(&1);

        // Use trait alias for trait objects.
        let a: &Bar = &123;
        println!("{:?}", a);
        let b = Box::new(456) as Box<dyn Foo>;
        println!("{:?}", b);
    }

# `transparent_unions`

The tracking issue for this feature is #60405

The `transparent_unions` feature allows you mark `union`s as `#[repr(transparent)]`. A `union` may be `#[repr(transparent)]` in exactly the same conditions in which a `struct` may be `#[repr(transparent)]` (generally, this means the `union` must have exactly one non-zero-sized field). Some concrete illustrations follow.

    #![allow(unused)]
    #![feature(transparent_unions)]

    fn main() {
    // This union has the same representation as `f32`.
    #[repr(transparent)]
    union SingleFieldUnion {
        field: f32,
    }

    // This union has the same representation as `usize`.
    #[repr(transparent)]
    union MultiFieldUnion {
        field: usize,
        nothing: (),
    }
    }

For consistency with transparent `struct`s, `union`s must have exactly one non-zero-sized field. If all fields are zero-sized, the `union` must not be `#[repr(transparent)]`:

    #![allow(unused)]
    #![feature(transparent_unions)]

    fn main() {
    // This (non-transparent) union is already valid in stable Rust:
    pub union GoodUnion {
        pub nothing: (),
    }

    // Error: transparent union needs exactly one non-zero-sized field, but has 0
    // #[repr(transparent)]
    // pub union BadUnion {
    //     pub nothing: (),
    // }
    }

The one exception is if the `union` is generic over `T` and has a field of type `T`, it may be `#[repr(transparent)]` even if `T` is a zero-sized type:

    #![allow(unused)]
    #![feature(transparent_unions)]

    fn main() {
    // This union has the same representation as `T`.
    #[repr(transparent)]
    pub union GenericUnion<T: Copy> { // Unions with non-`Copy` fields are unstable.
        pub field: T,
        pub nothing: (),
    }

    // This is okay even though `()` is a zero-sized type.
    pub const THIS_IS_OKAY: GenericUnion<()> = GenericUnion { field: () };
    }

Like transarent `struct`s, a transparent `union` of type `U` has the same layout, size, and ABI as its single non-ZST field. If it is generic over a type `T`, and all its fields are ZSTs except for exactly one field of type `T`, then it has the same layout and ABI as `T` (even if `T` is a ZST when monomorphized).

Like transparent `struct`s, transparent `union`s are FFI-safe if and only if their underlying representation type is also FFI-safe.

A `union` may not be eligible for the same nonnull-style optimizations that a `struct` or `enum` (with the same fields) are eligible for. Adding `#[repr(transparent)]` to `union` does not change this. To give a more concrete example, it is unspecified whether `size_of::<T>()` is equal to `size_of::<Option<T>>()`, where `T` is a `union` (regardless of whether or not it is transparent). The Rust compiler is free to perform this optimization if possible, but is not required to, and different compiler versions may differ in their application of these optimizations.

# `trivial_bounds`

The tracking issue for this feature is: #48214

# `try_blocks`

The tracking issue for this feature is: #31436

The `try_blocks` feature adds support for `try` blocks. A `try` block creates a new scope one can use the `?` operator in.

    #![allow(unused)]
    #![feature(try_blocks)]

    fn main() {
    use std::num::ParseIntError;

    let result: Result<i32, ParseIntError> = try {
        "1".parse::<i32>()?
            + "2".parse::<i32>()?
            + "3".parse::<i32>()?
    };
    assert_eq!(result, Ok(6));

    let result: Result<i32, ParseIntError> = try {
        "1".parse::<i32>()?
            + "foo".parse::<i32>()?
            + "3".parse::<i32>()?
    };
    assert!(result.is_err());
    }

# `type_alias_impl_trait`

The tracking issue for this feature is: #63063

# `type_ascription`

The tracking issue for this feature is: #23416

# `unboxed_closures`

The tracking issue for this feature is #29625

See Also: `fn_traits`

The `unboxed_closures` feature allows you to write functions using the `"rust-call"` ABI, required for implementing the `Fn*` family of traits. `"rust-call"` functions must have exactly one (non self) argument, a tuple representing the argument list.

    #![feature(unboxed_closures)]

    extern "rust-call" fn add_args(args: (u32, u32)) -> u32 {
        args.0 + args.1
    }

    fn main() {}

# `unsafe_block_in_unsafe_fn`

The tracking issue for this feature is: #71668

# `unsized_locals`

The tracking issue for this feature is: #48055

This implements RFC1909. When turned on, you can have unsized arguments and locals:

    #![feature(unsized_locals)]

    use std::any::Any;

    fn main() {
        let x: Box<dyn Any> = Box::new(42);
        let x: dyn Any = *x;
        //  ^ unsized local variable
        //               ^^ unsized temporary
        foo(x);
    }

    fn foo(_: dyn Any) {}
    //     ^^^^^^ unsized argument

The RFC still forbids the following unsized expressions:

    #![feature(unsized_locals)]

    use std::any::Any;

    struct MyStruct<T: ?Sized> {
        content: T,
    }

    struct MyTupleStruct<T: ?Sized>(T);

    fn answer() -> Box<dyn Any> {
        Box::new(42)
    }

    fn main() {
        // You CANNOT have unsized statics.
        static X: dyn Any = *answer();  // ERROR
        const Y: dyn Any = *answer();  // ERROR

        // You CANNOT have struct initialized unsized.
        MyStruct { content: *answer() };  // ERROR
        MyTupleStruct(*answer());  // ERROR
        (42, *answer());  // ERROR

        // You CANNOT have unsized return types.
        fn my_function() -> dyn Any { *answer() }  // ERROR

        // You CAN have unsized local variables...
        let mut x: dyn Any = *answer();  // OK
        // ...but you CANNOT reassign to them.
        x = *answer();  // ERROR

        // You CANNOT even initialize them separately.
        let y: dyn Any;  // OK
        y = *answer();  // ERROR

        // Not mentioned in the RFC, but by-move captured variables are also Sized.
        let x: dyn Any = *answer();
        (move || {  // ERROR
            let y = x;
        })();

        // You CAN create a closure with unsized arguments,
        // but you CANNOT call it.
        // This is an implementation detail and may be changed in the future.
        let f = |x: dyn Any| {};
        f(*answer());  // ERROR
    }

## By-value trait objects

With this feature, you can have by-value `self` arguments without `Self: Sized` bounds.

    #![feature(unsized_locals)]

    trait Foo {
        fn foo(self) {}
    }

    impl<T: ?Sized> Foo for T {}

    fn main() {
        let slice: Box<[i32]> = Box::new([1, 2, 3]);
        <[i32] as Foo>::foo(*slice);
    }

And `Foo` will also be object-safe.

    #![feature(unsized_locals)]

    trait Foo {
        fn foo(self) {}
    }

    impl<T: ?Sized> Foo for T {}

    fn main () {
        let slice: Box<dyn Foo> = Box::new([1, 2, 3]);
        // doesn't compile yet
        <dyn Foo as Foo>::foo(*slice);
    }

One of the objectives of this feature is to allow `Box<dyn FnOnce>`.

## Variable length arrays

The RFC also describes an extension to the array literal syntax: `[e; dyn n]`. In the syntax, `n` isn't necessarily a constant expression. The array is dynamically allocated on the stack and has the type of `[T]`, instead of `[T; n]`.

    #![feature(unsized_locals)]

    fn mergesort<T: Ord>(a: &mut [T]) {
        let mut tmp = [T; dyn a.len()];
        // ...
    }

    fn main() {
        let mut a = [3, 1, 5, 6];
        mergesort(&mut a);
        assert_eq!(a, [1, 3, 5, 6]);
    }

VLAs are not implemented yet. The syntax isn't final, either. We may need an alternative syntax for Rust 2015 because, in Rust 2015, expressions like `[e; dyn(1)]` would be ambiguous. One possible alternative proposed in the RFC is `[e; n]`: if `n` captures one or more local variables, then it is considered as `[e; dyn n]`.

## Advisory on stack usage

It's advised not to casually use the `#![feature(unsized_locals)]` feature. Typical use-cases are:

- When you need a by-value trait objects.
- When you really need a fast allocation of small temporary arrays.

Another pitfall is repetitive allocation and temporaries. Currently the compiler simply extends the stack frame every time it encounters an unsized assignment. So for example, the code

    #![feature(unsized_locals)]

    fn main() {
        let x: Box<[i32]> = Box::new([1, 2, 3, 4, 5]);
        let _x = {{{{{{{{{{*x}}}}}}}}}};
    }

and the code

    #![feature(unsized_locals)]

    fn main() {
        for _ in 0..10 {
            let x: Box<[i32]> = Box::new([1, 2, 3, 4, 5]);
            let _x = *x;
        }
    }

will unnecessarily extend the stack frame.

# `unsized_tuple_coercion`

The tracking issue for this feature is: #42877

This is a part of RFC0401. According to the RFC, there should be an implementation like this:

    impl<..., T, U: ?Sized> Unsized<(..., U)> for (..., T) where T: Unsized<U> {}

This implementation is currently gated behind `#[feature(unsized_tuple_coercion)]` to avoid insta-stability. Therefore you can use it like this:

    #![feature(unsized_tuple_coercion)]

    fn main() {
        let x : ([i32; 3], [i32; 3]) = ([1, 2, 3], [4, 5, 6]);
        let y : &([i32; 3], [i32]) = &x;
        assert_eq!(y.1[0], 4);
    }

# `untagged_unions`

The tracking issue for this feature is: #55149

# `unwind_attributes`

The tracking issue for this feature is: #58760

# `wasm_target_feature`

The tracking issue for this feature is: #44839

# Library Features

# `alloc_error_hook`

The tracking issue for this feature is: #51245

# `alloc_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `alloc_layout_extra`

The tracking issue for this feature is: #55724

# `alloc_prelude`

The tracking issue for this feature is: #58935

# `allocator_api`

The tracking issue for this feature is #32838

Sometimes you want the memory for one collection to use a different allocator than the memory for another collection. In this case, replacing the global allocator is not a workable option. Instead, you need to pass in an instance of an `AllocRef` to each collection for which you want a custom allocator.

TBD

# `arc_mutate_strong_count`

The tracking issue for this feature is: #71983

# `array_error_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `array_value_iter`

The tracking issue for this feature is: #65798

# `asm`

The tracking issue for this feature is: #72016

For extremely low-level manipulations and performance reasons, one might wish to control the CPU directly. Rust supports using inline assembly to do this via the `asm!` macro.

# Guide-level explanation

Rust provides support for inline assembly via the `asm!` macro. It can be used to embed handwritten assembly in the assembly output generated by the compiler. Generally this should not be necessary, but might be where the required performance or timing cannot be otherwise achieved. Accessing low level hardware primitives, e.g. in kernel code, may also demand this functionality.

> Note: the examples here are given in x86/x86-64 assembly, but ARM, AArch64 and RISC-V are also supported.

## Basic usage

Let us start with the simplest possible example:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    unsafe {
        asm!("nop");
    }
    }

This will insert a NOP (no operation) instruction into the assembly generated by the compiler. Note that all `asm!` invocations have to be inside an `unsafe` block, as they could insert arbitrary instructions and break various invariants. The instructions to be inserted are listed in the first argument of the `asm!` macro as a string literal.

## Inputs and outputs

Now inserting an instruction that does nothing is rather boring. Let us do something that actually acts on data:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    let x: u64;
    unsafe {
        asm!("mov {}, 5", out(reg) x);
    }
    assert_eq!(x, 5);
    }

This will write the value `5` into the `u64` variable `x`. You can see that the string literal we use to specify instructions is actually a template string. It is governed by the same rules as Rust format strings. The arguments that are inserted into the template however look a bit different then you may be familiar with. First we need to specify if the variable is an input or an output of the inline assembly. In this case it is an output. We declared this by writing `out`. We also need to specify in what kind of register the assembly expects the variable. In this case we put it in an arbitrary general purpose register by specifying `reg`. The compiler will choose an appropriate register to insert into the template and will read the variable from there after the inline assembly finishes executing.

Let us see another example that also uses an input:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    let i: u64 = 3;
    let o: u64;
    unsafe {
        asm!(
            "mov {0}, {1}",
            "add {0}, {number}",
            out(reg) o,
            in(reg) i,
            number = const 5,
        );
    }
    assert_eq!(o, 8);
    }

This will add `5` to the input in variable `i` and write the result to variable `o`. The particular way this assembly does this is first copying the value from `i` to the output, and then adding `5` to it.

The example shows a few things:

First, we can see that `asm!` allows multiple template string arguments; each one is treated as a separate line of assembly code, as if they were all joined together with newlines between them. This makes it easy to format assembly code.

Second, we can see that inputs are declared by writing `in` instead of `out`.

Third, one of our operands has a type we haven't seen yet, `const`. This tells the compiler to expand this argument to value directly inside the assembly template. This is only possible for constants and literals.

Fourth, we can see that we can specify an argument number, or name as in any format string. For inline assembly templates this is particularly useful as arguments are often used more than once. For more complex inline assembly using this facility is generally recommended, as it improves readability, and allows reordering instructions without changing the argument order.

We can further refine the above example to avoid the `mov` instruction:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    let mut x: u64 = 3;
    unsafe {
        asm!("add {0}, {number}", inout(reg) x, number = const 5);
    }
    assert_eq!(x, 8);
    }

We can see that `inout` is used to specify an argument that is both input and output. This is different from specifying an input and output separately in that it is guaranteed to assign both to the same register.

It is also possible to specify different variables for the input and output parts of an `inout` operand:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    let x: u64 = 3;
    let y: u64;
    unsafe {
        asm!("add {0}, {number}", inout(reg) x => y, number = const 5);
    }
    assert_eq!(y, 8);
    }

## Late output operands

The Rust compiler is conservative with its allocation of operands. It is assumed that an `out` can be written at any time, and can therefore not share its location with any other argument. However, to guarantee optimal performance it is important to use as few registers as possible, so they won't have to be saved and reloaded around the inline assembly block. To achieve this Rust provides a `lateout` specifier. This can be used on any output that is written only after all inputs have been consumed. There is also a `inlateout` variant of this specifier.

Here is an example where `inlateout` cannot be used:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    let mut a: u64 = 4;
    let b: u64 = 4;
    let c: u64 = 4;
    unsafe {
        asm!(
            "add {0}, {1}",
            "add {0}, {2}",
            inout(reg) a,
            in(reg) b,
            in(reg) c,
        );
    }
    assert_eq!(a, 12);
    }

Here the compiler is free to allocate the same register for inputs `b` and `c` since it knows they have the same value. However it must allocate a separate register for `a` since it uses `inout` and not `inlateout`. If `inlateout` was used, then `a` and `c` could be allocated to the same register, in which case the first instruction to overwrite the value of `c` and cause the assembly code to produce the wrong result.

However the following example can use `inlateout` since the output is only modified after all input registers have been read:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    let mut a: u64 = 4;
    let b: u64 = 4;
    unsafe {
        asm!("add {0}, {1}", inlateout(reg) a, in(reg) b);
    }
    assert_eq!(a, 8);
    }

As you can see, this assembly fragment will still work correctly if `a` and `b` are assigned to the same register.

## Explicit register operands

Some instructions require that the operands be in a specific register. Therefore, Rust inline assembly provides some more specific constraint specifiers. While `reg` is generally available on any architecture, these are highly architecture specific. E.g. for x86 the general purpose registers `eax`, `ebx`, `ecx`, `edx`, `ebp`, `esi`, and `edi` among others can be addressed by their name.

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    let cmd = 0xd1;
    unsafe {
        asm!("out 0x64, eax", in("eax") cmd);
    }
    }

In this example we call the `out` instruction to output the content of the `cmd` variable to port `0x64`. Since the `out` instruction only accepts `eax` (and its sub registers) as operand we had to use the `eax` constraint specifier.

Note that unlike other operand types, explicit register operands cannot be used in the template string: you can't use `{}` and should write the register name directly instead. Also, they must appear at the end of the operand list after all other operand types.

Consider this example which uses the x86 `mul` instruction:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    fn mul(a: u64, b: u64) -> u128 {
        let lo: u64;
        let hi: u64;

        unsafe {
            asm!(
                // The x86 mul instruction takes rax as an implicit input and writes
                // the 128-bit result of the multiplication to rax:rdx.
                "mul {}",
                in(reg) a,
                inlateout("rax") b => lo,
                lateout("rdx") hi
            );
        }

        ((hi as u128) << 64) + lo as u128
    }
    }

This uses the `mul` instruction to multiply two 64-bit inputs with a 128-bit result. The only explicit operand is a register, that we fill from the variable `a`. The second operand is implicit, and must be the `rax` register, which we fill from the variable `b`. The lower 64 bits of the result are stored in `rax` from which we fill the variable `lo`. The higher 64 bits are stored in `rdx` from which we fill the variable `hi`.

## Clobbered registers

In many cases inline assembly will modify state that is not needed as an output. Usually this is either because we have to use a scratch register in the assembly, or instructions modify state that we don't need to further examine. This state is generally referred to as being "clobbered". We need to tell the compiler about this since it may need to save and restore this state around the inline assembly block.

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    let ebx: u32;
    let ecx: u32;

    unsafe {
        asm!(
            "cpuid",
            // EAX 4 selects the "Deterministic Cache Parameters" CPUID leaf
            inout("eax") 4 => _,
            // ECX 0 selects the L0 cache information.
            inout("ecx") 0 => ecx,
            lateout("ebx") ebx,
            lateout("edx") _,
        );
    }

    println!(
        "L1 Cache: {}",
        ((ebx >> 22) + 1) * (((ebx >> 12) & 0x3ff) + 1) * ((ebx & 0xfff) + 1) * (ecx + 1)
    );
    }

In the example above we use the `cpuid` instruction to get the L1 cache size. This instruction writes to `eax`, `ebx`, `ecx`, and `edx`, but for the cache size we only care about the contents of `ebx` and `ecx`.

However we still need to tell the compiler that `eax` and `edx` have been modified so that it can save any values that were in these registers before the asm. This is done by declaring these as outputs but with `_` instead of a variable name, which indicates that the output value is to be discarded.

This can also be used with a general register class (e.g. `reg`) to obtain a scratch register for use inside the asm code:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    // Multiply x by 6 using shifts and adds
    let mut x: u64 = 4;
    unsafe {
        asm!(
            "mov {tmp}, {x}",
            "shl {tmp}, 1",
            "shl {x}, 2",
            "add {x}, {tmp}",
            x = inout(reg) x,
            tmp = out(reg) _,
        );
    }
    assert_eq!(x, 4 * 6);
    }

## Symbol operands

A special operand type, `sym`, allows you to use the symbol name of a `fn` or `static` in inline assembly code. This allows you to call a function or access a global variable without needing to keep its address in a register.

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    extern "C" fn foo(arg: i32) {
        println!("arg = {}", arg);
    }

    fn call_foo(arg: i32) {
        unsafe {
            asm!(
                "call {}",
                sym foo,
                // 1st argument in rdi, which is caller-saved
                inout("rdi") arg => _,
                // All caller-saved registers must be marked as clobberred
                out("rax") _, out("rcx") _, out("rdx") _, out("rsi") _,
                out("r8") _, out("r9") _, out("r10") _, out("r11") _,
                out("xmm0") _, out("xmm1") _, out("xmm2") _, out("xmm3") _,
                out("xmm4") _, out("xmm5") _, out("xmm6") _, out("xmm7") _,
                out("xmm8") _, out("xmm9") _, out("xmm10") _, out("xmm11") _,
                out("xmm12") _, out("xmm13") _, out("xmm14") _, out("xmm15") _,
            )
        }
    }
    }

Note that the `fn` or `static` item does not need to be public or `#[no_mangle]`: the compiler will automatically insert the appropriate mangled symbol name into the assembly code.

## Register template modifiers

In some cases, fine control is needed over the way a register name is formatted when inserted into the template string. This is needed when an architecture's assembly language has several names for the same register, each typically being a "view" over a subset of the register (e.g. the low 32 bits of a 64-bit register).

By default the compiler will always choose the name that refers to the full register size (e.g. `rax` on x86-64, `eax` on x86, etc).

This default can be overriden by using modifiers on the template string operands, just like you would with format strings:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    let mut x: u16 = 0xab;

    unsafe {
        asm!("mov {0:h}, {0:l}", inout(reg_abcd) x);
    }

    assert_eq!(x, 0xabab);
    }

In this example, we use the `reg_abcd` register class to restrict the register allocator to the 4 legacy x86 register (`ax`, `bx`, `cx`, `dx`) of which the first two bytes can be addressed independently.

Let us assume that the register allocator has chosen to allocate `x` in the `ax` register. The `h` modifier will emit the register name for the high byte of that register and the `l` modifier will emit the register name for the low byte. The asm code will therefore be expanded as `mov ah, al` which copies the low byte of the value into the high byte.

If you use a smaller data type (e.g. `u16`) with an operand and forget the use template modifiers, the compiler will emit a warning and suggest the correct modifier to use.

## Options

By default, an inline assembly block is treated the same way as an external FFI function call with a custom calling convention: it may read/write memory, have observable side effects, etc. However in many cases, it is desirable to give the compiler more information about what the assembly code is actually doing so that it can optimize better.

Let's take our previous example of an `add` instruction:

    #![allow(unused)]
    fn main() {
    #![feature(asm)]
    let mut a: u64 = 4;
    let b: u64 = 4;
    unsafe {
        asm!(
            "add {0}, {1}",
            inlateout(reg) a, in(reg) b,
            options(pure, nomem, nostack),
        );
    }
    assert_eq!(a, 8);
    }

Options can be provided as an optional final argument to the `asm!` macro. We specified three options here:

- `pure` means that the asm code has no observable side effects and that its output depends only on its inputs. This allows the compiler optimizer to call the inline asm fewer times or even eliminate it entirely.
- `nomem` means that the asm code does not read or write to memory. By default the compiler will assume that inline assembly can read or write any memory address that is accessible to it (e.g. through a pointer passed as an operand, or a global).
- `nostack` means that the asm code does not push any data onto the stack. This allows the compiler to use optimizations such as the stack red zone on x86-64 to avoid stack pointer adjustments.

These allow the compiler to better optimize code using `asm!`, for example by eliminating pure `asm!` blocks whose outputs are not needed.

See the reference for the full list of available options and their effects.

# Reference-level explanation

Inline assembler is implemented as an unsafe macro `asm!()`. The first argument to this macro is a template string literal used to build the final assembly. The following arguments specify input and output operands. When required, options are specified as the final argument.

The following ABNF specifies the general syntax:

    dir_spec := "in" / "out" / "lateout" / "inout" / "inlateout"
    reg_spec := <register class> / "<explicit register>"
    operand_expr := expr / "_" / expr "=>" expr / expr "=>" "_"
    reg_operand := dir_spec "(" reg_spec ")" operand_expr
    operand := reg_operand / "const" const_expr / "sym" path
    option := "pure" / "nomem" / "readonly" / "preserves_flags" / "noreturn" / "att_syntax"
    options := "options(" option *["," option] [","] ")"
    asm := "asm!(" format_string *("," format_string) *("," [ident "="] operand) ["," options] [","] ")"

The macro will initially be supported only on ARM, AArch64, Hexagon, x86, x86-64 and RISC-V targets. Support for more targets may be added in the future. The compiler will emit an error if `asm!` is used on an unsupported target.

## Template string arguments

The assembler template uses the same syntax as format strings (i.e. placeholders are specified by curly braces). The corresponding arguments are accessed in order, by index, or by name. However, implicit named arguments (introduced by RFC #2795) are not supported.

An `asm!` invocation may have one or more template string arguments; an `asm!` with multiple template string arguments is treated as if all the strings were concatenated with a `\n` between them. The expected usage is for each template string argument to correspond to a line of assembly code. All template string arguments must appear before any other arguments.

As with format strings, named arguments must appear after positional arguments. Explicit register operands must appear at the end of the operand list, after named arguments if any.

Explicit register operands cannot be used by placeholders in the template string. All other named and positional operands must appear at least once in the template string, otherwise a compiler error is generated.

The exact assembly code syntax is target-specific and opaque to the compiler except for the way operands are substituted into the template string to form the code passed to the assembler.

The 5 targets specified in this RFC (x86, ARM, AArch64, RISC-V, Hexagon) all use the assembly code syntax of the GNU assembler (GAS). On x86, the `.intel_syntax noprefix` mode of GAS is used by default. On ARM, the `.syntax unified` mode is used. These targets impose an additional restriction on the assembly code: any assembler state (e.g. the current section which can be changed with `.section`) must be restored to its original value at the end of the asm string. Assembly code that does not conform to the GAS syntax will result in assembler-specific behavior.

## Operand type

Several types of operands are supported:

- `in(<reg>) <expr>`
  - `<reg>` can refer to a register class or an explicit register. The allocated register name is substituted into the asm template string.
  - The allocated register will contain the value of `<expr>` at the start of the asm code.
  - The allocated register must contain the same value at the end of the asm code (except if a `lateout` is allocated to the same register).
- `out(<reg>) <expr>`
  - `<reg>` can refer to a register class or an explicit register. The allocated register name is substituted into the asm template string.
  - The allocated register will contain an undefined value at the start of the asm code.
  - `<expr>` must be a (possibly uninitialized) place expression, to which the contents of the allocated register is written to at the end of the asm code.
  - An underscore (`_`) may be specified instead of an expression, which will cause the contents of the register to be discarded at the end of the asm code (effectively acting as a clobber).
- `lateout(<reg>) <expr>`
  - Identical to `out` except that the register allocator can reuse a register allocated to an `in`.
  - You should only write to the register after all inputs are read, otherwise you may clobber an input.
- `inout(<reg>) <expr>`
  - `<reg>` can refer to a register class or an explicit register. The allocated register name is substituted into the asm template string.
  - The allocated register will contain the value of `<expr>` at the start of the asm code.
  - `<expr>` must be a mutable initialized place expression, to which the contents of the allocated register is written to at the end of the asm code.
- `inout(<reg>) <in expr> => <out expr>`
  - Same as `inout` except that the initial value of the register is taken from the value of `<in expr>`.
  - `<out expr>` must be a (possibly uninitialized) place expression, to which the contents of the allocated register is written to at the end of the asm code.
  - An underscore (`_`) may be specified instead of an expression for `<out expr>`, which will cause the contents of the register to be discarded at the end of the asm code (effectively acting as a clobber).
  - `<in expr>` and `<out expr>` may have different types.
- `inlateout(<reg>) <expr>` / `inlateout(<reg>) <in expr> => <out expr>`
  - Identical to `inout` except that the register allocator can reuse a register allocated to an `in` (this can happen if the compiler knows the `in` has the same initial value as the `inlateout`).
  - You should only write to the register after all inputs are read, otherwise you may clobber an input.
- `const <expr>`
  - `<expr>` must be an integer or floating-point constant expression.
  - The value of the expression is formatted as a string and substituted directly into the asm template string.
- `sym <path>`
  - `<path>` must refer to a `fn` or `static`.
  - A mangled symbol name referring to the item is substituted into the asm template string.
  - The substituted string does not include any modifiers (e.g. GOT, PLT, relocations, etc).
  - `<path>` is allowed to point to a `#[thread_local]` static, in which case the asm code can combine the symbol with relocations (e.g. `@plt`, `@TPOFF`) to read from thread-local data.

Operand expressions are evaluated from left to right, just like function call arguments. After the `asm!` has executed, outputs are written to in left to right order. This is significant if two outputs point to the same place: that place will contain the value of the rightmost output.

## Register operands

Input and output operands can be specified either as an explicit register or as a register class from which the register allocator can select a register. Explicit registers are specified as string literals (e.g. `"eax"`) while register classes are specified as identifiers (e.g. `reg`). Using string literals for register names enables support for architectures that use special characters in register names, such as MIPS (`$0`, `$1`, etc).

Note that explicit registers treat register aliases (e.g. `r14` vs `lr` on ARM) and smaller views of a register (e.g. `eax` vs `rax`) as equivalent to the base register. It is a compile-time error to use the same explicit register for two input operands or two output operands. Additionally, it is also a compile-time error to use overlapping registers (e.g. ARM VFP) in input operands or in output operands.

Only the following types are allowed as operands for inline assembly:

- Integers (signed and unsigned)
- Floating-point numbers
- Pointers (thin only)
- Function pointers
- SIMD vectors (structs defined with `#[repr(simd)]` and which implement `Copy`). This includes architecture-specific vector types defined in `std::arch` such as `__m128` (x86) or `int8x16_t` (ARM).

Here is the list of currently supported register classes:

Architecture | Register class | Registers | LLVM constraint code
--- | --- | --- | ---
x86 | `reg` | `ax`, `bx`, `cx`, `dx`, `si`, `di`, `r[8-15]` (x86-64 only) | `r`
x86 | `reg_abcd` | `ax`, `bx`, `cx`, `dx` | `Q`
x86-32 | `reg_byte` | `al`, `bl`, `cl`, `dl`, `ah`, `bh`, `ch`, `dh` | `q`
x86-64 | `reg_byte` | `al`, `bl`, `cl`, `dl`, `sil`, `dil`, `r[8-15]b`, `ah`*, `bh`*, `ch`*, `dh`* | `q`
x86 | `xmm_reg` | `xmm[0-7]` (x86) `xmm[0-15]` (x86-64) | `x`
x86 | `ymm_reg` | `ymm[0-7]` (x86) `ymm[0-15]` (x86-64) | `x`
x86 | `zmm_reg` | `zmm[0-7]` (x86) `zmm[0-31]` (x86-64) | `v`
x86 | `kreg` | `k[1-7]` | `Yk`
AArch64 | `reg` | `x[0-28]`, `x30` | `r`
AArch64 | `vreg` | `v[0-31]` | `w`
AArch64 | `vreg_low16` | `v[0-15]` | `x`
ARM | `reg` | `r[0-5]` `r7`*, `r[8-10]`, `r11`*, `r12`, `r14` | `r`
ARM (Thumb) | `reg_thumb` | `r[0-r7]` | `l`
ARM (ARM) | `reg_thumb` | `r[0-r10]`, `r12`, `r14` | `l`
ARM | `sreg` | `s[0-31]` | `t`
ARM | `sreg_low16` | `s[0-15]` | `x`
ARM | `dreg` | `d[0-31]` | `w`
ARM | `dreg_low16` | `d[0-15]` | `t`
ARM | `dreg_low8` | `d[0-8]` | `x`
ARM | `qreg` | `q[0-15]` | `w`
ARM | `qreg_low8` | `q[0-7]` | `t`
ARM | `qreg_low4` | `q[0-3]` | `x`
NVPTX | `reg16` | None* | `h`
NVPTX | `reg32` | None* | `r`
NVPTX | `reg64` | None* | `l`
RISC-V | `reg` | `x1`, `x[5-7]`, `x[9-15]`, `x[16-31]` (non-RV32E) | `r`
RISC-V | `freg` | `f[0-31]` | `f`
Hexagon | `reg` | `r[0-28]` | `r`

> Note: On x86 we treat `reg_byte` differently from `reg` because the compiler can allocate `al` and `ah` separately whereas `reg` reserves the whole register.

> Note #2: On x86-64 the high byte registers (e.g. `ah`) are only available when used as an explicit register. Specifying the `reg_byte` register class for an operand will always allocate a low byte register.

> Note #3: NVPTX doesn't have a fixed register set, so named registers are not supported.

> Note #4: On ARM the frame pointer is either `r7` or `r11` depending on the platform.

Additional register classes may be added in the future based on demand (e.g. MMX, x87, etc).

Each register class has constraints on which value types they can be used with. This is necessary because the way a value is loaded into a register depends on its type. For example, on big-endian systems, loading a `i32x4` and a `i8x16` into a SIMD register may result in different register contents even if the byte-wise memory representation of both values is identical. The availability of supported types for a particular register class may depend on what target features are currently enabled.

Architecture | Register class | Target feature | Allowed types
--- | --- | --- | ---
x86-32 | `reg` | None | `i16`, `i32`, `f32`
x86-64 | `reg` | None | `i16`, `i32`, `f32`, `i64`, `f64`
x86 | `reg_byte` | None | `i8`
x86 | `xmm_reg` | `sse` | `i32`, `f32`, `i64`, `f64`, `i8x16`, `i16x8`, `i32x4`, `i64x2`, `f32x4`, `f64x2`
x86 | `ymm_reg` | `avx` | `i32`, `f32`, `i64`, `f64`, `i8x16`, `i16x8`, `i32x4`, `i64x2`, `f32x4`, `f64x2` `i8x32`, `i16x16`, `i32x8`, `i64x4`, `f32x8`, `f64x4`
x86 | `zmm_reg` | `avx512f` | `i32`, `f32`, `i64`, `f64`, `i8x16`, `i16x8`, `i32x4`, `i64x2`, `f32x4`, `f64x2` `i8x32`, `i16x16`, `i32x8`, `i64x4`, `f32x8`, `f64x4` `i8x64`, `i16x32`, `i32x16`, `i64x8`, `f32x16`, `f64x8`
x86 | `kreg` | `axv512f` | `i8`, `i16`
x86 | `kreg` | `axv512bw` | `i32`, `i64`
AArch64 | `reg` | None | `i8`, `i16`, `i32`, `f32`, `i64`, `f64`
AArch64 | `vreg` | `fp` | `i8`, `i16`, `i32`, `f32`, `i64`, `f64`, `i8x8`, `i16x4`, `i32x2`, `i64x1`, `f32x2`, `f64x1`, `i8x16`, `i16x8`, `i32x4`, `i64x2`, `f32x4`, `f64x2`
ARM | `reg` | None | `i8`, `i16`, `i32`, `f32`
ARM | `sreg` | `vfp2` | `i32`, `f32`
ARM | `dreg` | `vfp2` | `i64`, `f64`, `i8x8`, `i16x4`, `i32x2`, `i64x1`, `f32x2`
ARM | `qreg` | `neon` | `i8x16`, `i16x8`, `i32x4`, `i64x2`, `f32x4`
NVPTX | `reg16` | None | `i8`, `i16`
NVPTX | `reg32` | None | `i8`, `i16`, `i32`, `f32`
NVPTX | `reg64` | None | `i8`, `i16`, `i32`, `f32`, `i64`, `f64`
RISC-V32 | `reg` | None | `i8`, `i16`, `i32`, `f32`
RISC-V64 | `reg` | None | `i8`, `i16`, `i32`, `f32`, `i64`, `f64`
RISC-V | `freg` | `f` | `f32`
RISC-V | `freg` | `d` | `f64`
Hexagon | `reg` | None | `i8`, `i16`, `i32`, `f32`

> Note: For the purposes of the above table pointers, function pointers and `isize`/`usize` are treated as the equivalent integer type (`i16`/`i32`/`i64` depending on the target).

If a value is of a smaller size than the register it is allocated in then the upper bits of that register will have an undefined value for inputs and will be ignored for outputs. The only exception is the `freg` register class on RISC-V where `f32` values are NaN-boxed in a `f64` as required by the RISC-V architecture.

When separate input and output expressions are specified for an `inout` operand, both expressions must have the same type. The only exception is if both operands are pointers or integers, in which case they are only required to have the same size. This restriction exists because the register allocators in LLVM and GCC sometimes cannot handle tied operands with different types.

## Register names

Some registers have multiple names. These are all treated by the compiler as identical to the base register name. Here is the list of all supported register aliases:

Architecture | Base register | Aliases
--- | --- | ---
x86 | `ax` | `eax`, `rax`
x86 | `bx` | `ebx`, `rbx`
x86 | `cx` | `ecx`, `rcx`
x86 | `dx` | `edx`, `rdx`
x86 | `si` | `esi`, `rsi`
x86 | `di` | `edi`, `rdi`
x86 | `bp` | `bpl`, `ebp`, `rbp`
x86 | `sp` | `spl`, `esp`, `rsp`
x86 | `ip` | `eip`, `rip`
x86 | `st(0)` | `st`
x86 | `r[8-15]` | `r[8-15]b`, `r[8-15]w`, `r[8-15]d`
x86 | `xmm[0-31]` | `ymm[0-31]`, `zmm[0-31]`
AArch64 | `x[0-30]` | `w[0-30]`
AArch64 | `x29` | `fp`
AArch64 | `x30` | `lr`
AArch64 | `sp` | `wsp`
AArch64 | `xzr` | `wzr`
AArch64 | `v[0-31]` | `b[0-31]`, `h[0-31]`, `s[0-31]`, `d[0-31]`, `q[0-31]`
ARM | `r[0-3]` | `a[1-4]`
ARM | `r[4-9]` | `v[1-6]`
ARM | `r9` | `rfp`
ARM | `r10` | `sl`
ARM | `r11` | `fp`
ARM | `r12` | `ip`
ARM | `r13` | `sp`
ARM | `r14` | `lr`
ARM | `r15` | `pc`
RISC-V | `x0` | `zero`
RISC-V | `x1` | `ra`
RISC-V | `x2` | `sp`
RISC-V | `x3` | `gp`
RISC-V | `x4` | `tp`
RISC-V | `x[5-7]` | `t[0-2]`
RISC-V | `x8` | `fp`, `s0`
RISC-V | `x9` | `s1`
RISC-V | `x[10-17]` | `a[0-7]`
RISC-V | `x[18-27]` | `s[2-11]`
RISC-V | `x[28-31]` | `t[3-6]`
RISC-V | `f[0-7]` | `ft[0-7]`
RISC-V | `f[8-9]` | `fs[0-1]`
RISC-V | `f[10-17]` | `fa[0-7]`
RISC-V | `f[18-27]` | `fs[2-11]`
RISC-V | `f[28-31]` | `ft[8-11]`
Hexagon | `r29` | `sp`
Hexagon | `r30` | `fr`
Hexagon | `r31` | `lr`

Some registers cannot be used for input or output operands:

Architecture | Unsupported register | Reason
--- | --- | ---
All | `sp` | The stack pointer must be restored to its original value at the end of an asm code block.
All | `bp` (x86), `x29` (AArch64), `x8` (RISC-V), `fr` (Hexagon) | The frame pointer cannot be used as an input or output.
ARM | `r7` or `r11` | On ARM the frame pointer can be either `r7` or `r11` depending on the target. The frame pointer cannot be used as an input or output.
ARM | `r6` | `r6` is used internally by LLVM as a base pointer and therefore cannot be used as an input or output.
x86 | `k0` | This is a constant zero register which can't be modified.
x86 | `ip` | This is the program counter, not a real register.
x86 | `mm[0-7]` | MMX registers are not currently supported (but may be in the future).
x86 | `st([0-7])` | x87 registers are not currently supported (but may be in the future).
AArch64 | `xzr` | This is a constant zero register which can't be modified.
ARM | `pc` | This is the program counter, not a real register.
RISC-V | `x0` | This is a constant zero register which can't be modified.
RISC-V | `gp`, `tp` | These registers are reserved and cannot be used as inputs or outputs.
Hexagon | `lr` | This is the link register which cannot be used as an input or output.

## Template modifiers

The placeholders can be augmented by modifiers which are specified after the `:` in the curly braces. These modifiers do not affect register allocation, but change the way operands are formatted when inserted into the template string. Only one modifier is allowed per template placeholder.

The supported modifiers are a subset of LLVM's (and GCC's) asm template argument modifiers, but do not use the same letter codes.

Architecture | Register class | Modifier | Example output | LLVM modifier
--- | --- | --- | --- | ---
x86-32 | `reg` | None | `eax` | `k`
x86-64 | `reg` | None | `rax` | `q`
x86-32 | `reg_abcd` | `l` | `al` | `b`
x86-64 | `reg` | `l` | `al` | `b`
x86 | `reg_abcd` | `h` | `ah` | `h`
x86 | `reg` | `x` | `ax` | `w`
x86 | `reg` | `e` | `eax` | `k`
x86-64 | `reg` | `r` | `rax` | `q`
x86 | `reg_byte` | None | `al` / `ah` | None
x86 | `xmm_reg` | None | `xmm0` | `x`
x86 | `ymm_reg` | None | `ymm0` | `t`
x86 | `zmm_reg` | None | `zmm0` | `g`
x86 | `*mm_reg` | `x` | `xmm0` | `x`
x86 | `*mm_reg` | `y` | `ymm0` | `t`
x86 | `*mm_reg` | `z` | `zmm0` | `g`
x86 | `kreg` | None | `k1` | None
AArch64 | `reg` | None | `x0` | `x`
AArch64 | `reg` | `w` | `w0` | `w`
AArch64 | `reg` | `x` | `x0` | `x`
AArch64 | `vreg` | None | `v0` | None
AArch64 | `vreg` | `v` | `v0` | None
AArch64 | `vreg` | `b` | `b0` | `b`
AArch64 | `vreg` | `h` | `h0` | `h`
AArch64 | `vreg` | `s` | `s0` | `s`
AArch64 | `vreg` | `d` | `d0` | `d`
AArch64 | `vreg` | `q` | `q0` | `q`
ARM | `reg` | None | `r0` | None
ARM | `sreg` | None | `s0` | None
ARM | `dreg` | None | `d0` | `P`
ARM | `qreg` | None | `q0` | `q`
ARM | `qreg` | `e` / `f` | `d0` / `d1` | `e` / `f`
NVPTX | `reg16` | None | `rs0` | None
NVPTX | `reg32` | None | `r0` | None
NVPTX | `reg64` | None | `rd0` | None
RISC-V | `reg` | None | `x1` | None
RISC-V | `freg` | None | `f0` | None
Hexagon | `reg` | None | `r0` | None

> Notes:

> - on ARM `e` / `f`: this prints the low or high doubleword register name of a NEON quad (128-bit) register.
> - on x86: our behavior for `reg` with no modifiers differs from what GCC does. GCC will infer the modifier based on the operand value type, while we default to the full register size.
> - on x86 `xmm_reg`: the `x`, `t` and `g` LLVM modifiers are not yet implemented in LLVM (they are supported by GCC only), but this should be a simple change.

As stated in the previous section, passing an input value smaller than the register width will result in the upper bits of the register containing undefined values. This is not a problem if the inline asm only accesses the lower bits of the register, which can be done by using a template modifier to use a subregister name in the asm code (e.g. `ax` instead of `rax`). Since this an easy pitfall, the compiler will suggest a template modifier to use where appropriate given the input type. If all references to an operand already have modifiers then the warning is suppressed for that operand.

## Options

Flags are used to further influence the behavior of the inline assembly block. Currently the following options are defined:

- `pure`: The `asm` block has no side effects, and its outputs depend only on its direct inputs (i.e. the values themselves, not what they point to) or values read from memory (unless the `nomem` options is also set). This allows the compiler to execute the `asm` block fewer times than specified in the program (e.g. by hoisting it out of a loop) or even eliminate it entirely if the outputs are not used.
- `nomem`: The `asm` blocks does not read or write to any memory. This allows the compiler to cache the values of modified global variables in registers across the `asm` block since it knows that they are not read or written to by the `asm`.
- `readonly`: The `asm` block does not write to any memory. This allows the compiler to cache the values of unmodified global variables in registers across the `asm` block since it knows that they are not written to by the `asm`.
- `preserves_flags`: The `asm` block does not modify the flags register (defined in the rules below). This allows the compiler to avoid recomputing the condition flags after the `asm` block.
- `noreturn`: The `asm` block never returns, and its return type is defined as `!` (never). Behavior is undefined if execution falls through past the end of the asm code. A `noreturn` asm block behaves just like a function which doesn't return; notably, local variables in scope are not dropped before it is invoked.
- `nostack`: The `asm` block does not push data to the stack, or write to the stack red-zone (if supported by the target). If this option is not used then the stack pointer is guaranteed to be suitably aligned (according to the target ABI) for a function call.
- `att_syntax`: This option is only valid on x86, and causes the assembler to use the `.att_syntax prefix` mode of the GNU assembler. Register operands are substituted in with a leading `%`.

The compiler performs some additional checks on options:

- The `nomem` and `readonly` options are mutually exclusive: it is a compile-time error to specify both.
- The `pure` option must be combined with either the `nomem` or `readonly` options, otherwise a compile-time error is emitted.
- It is a compile-time error to specify `pure` on an asm block with no outputs or only discarded outputs (`_`).
- It is a compile-time error to specify `noreturn` on an asm block with outputs.

## Rules for inline assembly

- Any registers not specified as inputs will contain an undefined value on entry to the asm block.
  - An "undefined value" in the context of inline assembly means that the register can (non-deterministically) have any one of the possible values allowed by the architecture. Notably it is not the same as an LLVM `undef` which can have a different value every time you read it (since such a concept does not exist in assembly code).
- Any registers not specified as outputs must have the same value upon exiting the asm block as they had on entry, otherwise behavior is undefined.
  - This only applies to registers which can be specified as an input or output. Other registers follow target-specific rules.
  - Note that a `lateout` may be allocated to the same register as an `in`, in which case this rule does not apply. Code should not rely on this however since it depends on the results of register allocation.
- Behavior is undefined if execution unwinds out of an asm block.
  - This also applies if the assembly code calls a function which then unwinds.
- The set of memory locations that assembly code is allowed the read and write are the same as those allowed for an FFI function.
  - Refer to the unsafe code guidelines for the exact rules.
  - If the `readonly` option is set, then only memory reads are allowed.
  - If the `nomem` option is set then no reads or writes to memory are allowed.
  - These rules do not apply to memory which is private to the asm code, such as stack space allocated within the asm block.
- The compiler cannot assume that the instructions in the asm are the ones that will actually end up executed.
  - This effectively means that the compiler must treat the `asm!` as a black box and only take the interface specification into account, not the instructions themselves.
  - Runtime code patching is allowed, via target-specific mechanisms (outside the scope of this RFC).
- Unless the `nostack` option is set, asm code is allowed to use stack space below the stack pointer.
  - On entry to the asm block the stack pointer is guaranteed to be suitably aligned (according to the target ABI) for a function call.
  - You are responsible for making sure you don't overflow the stack (e.g. use stack probing to ensure you hit a guard page).
  - You should adjust the stack pointer when allocating stack memory as required by the target ABI.
  - The stack pointer must be restored to its original value before leaving the asm block.
- If the `noreturn` option is set then behavior is undefined if execution falls through to the end of the asm block.
- If the `pure` option is set then behavior is undefined if the `asm` has side-effects other than its direct outputs. Behavior is also undefined if two executions of the `asm` code with the same inputs result in different outputs.
  - When used with the `nomem` option, "inputs" are just the direct inputs of the `asm!`.
  - When used with the `readonly` option, "inputs" comprise the direct inputs of the `asm!` and any memory that the `asm!` block is allowed to read.
- These flags registers must be restored upon exiting the asm block if the `preserves_flags` option is set:
  - x86
    - Status flags in `EFLAGS` (CF, PF, AF, ZF, SF, OF).
    - Floating-point status word (all).
    - Floating-point exception flags in `MXCSR` (PE, UE, OE, ZE, DE, IE).
  - ARM
    - Condition flags in `CPSR` (N, Z, C, V)
    - Saturation flag in `CPSR` (Q)
    - Greater than or equal flags in `CPSR` (GE).
    - Condition flags in `FPSCR` (N, Z, C, V)
    - Saturation flag in `FPSCR` (QC)
    - Floating-point exception flags in `FPSCR` (IDC, IXC, UFC, OFC, DZC, IOC).
  - AArch64
    - Condition flags (`NZCV` register).
    - Floating-point status (`FPSR` register).
  - RISC-V
    - Floating-point exception flags in `fcsr` (`fflags`).
- On x86, the direction flag (DF in `EFLAGS`) is clear on entry to an asm block and must be clear on exit.
  - Behavior is undefined if the direction flag is set on exiting an asm block.
- The requirement of restoring the stack pointer and non-output registers to their original value only applies when exiting an `asm!` block.
  - This means that `asm!` blocks that never return (even if not marked `noreturn`) don't need to preserve these registers.
  - When returning to a different `asm!` block than you entered (e.g. for context switching), these registers must contain the value they had upon entering the `asm!` block that you are exiting.
    - You cannot exit an `asm!` block that has not been entered. Neither can you exit an `asm!` block that has already been exited.
    - You are responsible for switching any target-specific state (e.g. thread-local storage, stack bounds).
    - The set of memory locations that you may access is the intersection of those allowed by the `asm!` blocks you entered and exited.
- You cannot assume that an `asm!` block will appear exactly once in the output binary. The compiler is allowed to instantiate multiple copies of the `asm!` block, for example when the function containing it is inlined in multiple places.
  - As a consequence, you should only use local labels inside inline assembly code. Defining symbols in assembly code may lead to assembler and/or linker errors due to duplicate symbol definitions.

> Note: As a general rule, the flags covered by `preserves_flags` are those which are not preserved when performing a function call.

# `assoc_char_consts`

The tracking issue for this feature is: #71763

# `assoc_char_funcs`

The tracking issue for this feature is: #71763

# `atomic_mut_ptr`

The tracking issue for this feature is: #66893

# `backtrace`

The tracking issue for this feature is: #53487

# `binary_heap_drain_sorted`

The tracking issue for this feature is: #59278

# `binary_heap_into_iter_sorted`

The tracking issue for this feature is: #59278

# `binary_heap_retain`

The tracking issue for this feature is: #71503

# `bool_to_option`

The tracking issue for this feature is: #64260

# `bound_cloned`

The tracking issue for this feature is: #61356

# `box_into_boxed_slice`

The tracking issue for this feature is: #71582

# `box_into_pin`

The tracking issue for this feature is: #62370

# `box_into_raw_non_null`

The tracking issue for this feature is: #47336

# `btree_drain_filter`

The tracking issue for this feature is: #70530

# `bufreader_seek_relative`

The tracking issue for this feature is: #31100

# `c_void_variant`

This feature is internal to the Rust compiler and is not intended for general use.

# `can_vector`

The tracking issue for this feature is: #69941

# `cell_leak`

The tracking issue for this feature is: #69099

# `cell_update`

The tracking issue for this feature is: #50186

# `cfg_accessible`

The tracking issue for this feature is: #64797

# `char_error_internals`

This feature is internal to the Rust compiler and is not intended for general use.

# `char_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `clamp`

The tracking issue for this feature is: #44095

# `cmp_min_max_by`

The tracking issue for this feature is: #64460

# `coerce_unsized`

The tracking issue for this feature is: #27732

# `concat_idents`

The tracking issue for this feature is: #29599

The `concat_idents` feature adds a macro for concatenating multiple identifiers into one identifier.

## Examples

    #![feature(concat_idents)]

    fn main() {
        fn foobar() -> u32 { 23 }
        let f = concat_idents!(foo, bar);
        assert_eq!(f(), 23);
    }

# `const_alloc_layout`

The tracking issue for this feature is: #67521

# `const_ascii_ctype_on_intrinsics`

The tracking issue for this feature is: #68983

# `const_btree_new`

The tracking issue for this feature is: #71835

# `const_caller_location`

The tracking issue for this feature is: #47809

# `const_checked_int_methods`

The tracking issue for this feature is: #53718

# `const_cstr_unchecked`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `const_cttz`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `const_discriminant`

The tracking issue for this feature is: #69821

# `const_euclidean_int_methods`

The tracking issue for this feature is: #53718

# `const_generic_impls_guard`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `const_int_pow`

The tracking issue for this feature is: #53718

# `const_int_sign`

The tracking issue for this feature is: #53718

# `const_int_unchecked_arith`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `const_likely`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `const_nonnull_slice_from_raw_parts`

The tracking issue for this feature is: #71941

# `const_nonzero_int_methods`

The tracking issue for this feature is: #53718

# `const_overflowing_int_methods`

The tracking issue for this feature is: #53718

# `const_pref_align_of`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `const_ptr_offset`

The tracking issue for this feature is: #71499

# `const_ptr_offset_from`

The tracking issue for this feature is: #41079

# `const_raw_ptr_comparison`

The tracking issue for this feature is: #53020

# `const_result`

The tracking issue for this feature is: #67520

# `const_saturating_int_methods`

The tracking issue for this feature is: #53718

# `const_slice_from_raw_parts`

The tracking issue for this feature is: #67456

# `const_slice_ptr_len`

The tracking issue for this feature is: #71146

# `const_transmute`

The tracking issue for this feature is: #53605

# `const_type_id`

The tracking issue for this feature is: #41875

# `const_type_name`

The tracking issue for this feature is: #63084

# `const_wrapping_int_methods`

The tracking issue for this feature is: #53718

# `constctlz`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `container_error_extra`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `convert_float_to_int`

The tracking issue for this feature is: #67057

# `core_intrinsics`

This feature is internal to the Rust compiler and is not intended for general use.

# `core_panic`

This feature is internal to the Rust compiler and is not intended for general use.

# `core_private_bignum`

This feature is internal to the Rust compiler and is not intended for general use.

# `core_private_diy_float`

This feature is internal to the Rust compiler and is not intended for general use.

# `cow_is_borrowed`

The tracking issue for this feature is: #65143

# `cstring_from_vec_with_nul`

The tracking issue for this feature is: #73179

# `deadline_api`

The tracking issue for this feature is: #46316

# `debug_non_exhaustive`

The tracking issue for this feature is: #67364

# `dec2flt`

This feature is internal to the Rust compiler and is not intended for general use.

# `default_free_fn`

The tracking issue for this feature is: #73014

Adds a free `default()` function to the `std::default` module. This function just forwards to [`Default::default()`], but may remove repetition of the word "default" from the call site.

Here is an example:

    #![feature(default_free_fn)]
    use std::default::default;

    #[derive(Default)]
    struct AppConfig {
        foo: FooConfig,
        bar: BarConfig,
    }

    #[derive(Default)]
    struct FooConfig {
        foo: i32,
    }

    #[derive(Default)]
    struct BarConfig {
        bar: f32,
        baz: u8,
    }

    fn main() {
        let options = AppConfig {
            foo: default(),
            bar: BarConfig {
                bar: 10.1,
                ..default()
            },
        };
    }

# `deque_make_contiguous`

The tracking issue for this feature is: #70929

# `deque_range`

The tracking issue for this feature is: #74217

# `derive_clone_copy`

This feature is internal to the Rust compiler and is not intended for general use.

# `derive_eq`

This feature is internal to the Rust compiler and is not intended for general use.

# `discriminant_kind`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `dispatch_from_dyn`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `div_duration`

The tracking issue for this feature is: #63139

# `drain_filter`

The tracking issue for this feature is: #43244

# `duration_constants`

The tracking issue for this feature is: #57391

# `duration_zero`

The tracking issue for this feature is: #73544

# `entry_insert`

The tracking issue for this feature is: #65225

# `error_iter`

The tracking issue for this feature is: #58520

# `error_type_id`

The tracking issue for this feature is: #60784

# `exact_size_is_empty`

The tracking issue for this feature is: #35428

# `extend_one`

The tracking issue for this feature is: #72631

# `fd`

This feature is internal to the Rust compiler and is not intended for general use.

# `fd_read`

This feature is internal to the Rust compiler and is not intended for general use.

# `fixed_size_array`

The tracking issue for this feature is: #27778

# `flt2dec`

This feature is internal to the Rust compiler and is not intended for general use.

# `fmt_internals`

This feature is internal to the Rust compiler and is not intended for general use.

# `fn_traits`

The tracking issue for this feature is #29625

See Also: `unboxed_closures`

The `fn_traits` feature allows for implementation of the `Fn*` traits for creating custom closure-like types.

    #![feature(unboxed_closures)]
    #![feature(fn_traits)]

    struct Adder {
        a: u32
    }

    impl FnOnce<(u32, )> for Adder {
        type Output = u32;
        extern "rust-call" fn call_once(self, b: (u32, )) -> Self::Output {
            self.a + b.0
        }
    }

    fn main() {
        let adder = Adder { a: 3 };
        assert_eq!(adder(2), 5);
    }

# `foo`

The tracking issue for this feature is: #1234

# `forget_unsized`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `format_args_nl`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `future_poll_fn`

The tracking issue for this feature is: #72302

# `future_readiness_fns`

The tracking issue for this feature is: #70921

# `gen_future`

The tracking issue for this feature is: #50547

# `generator_trait`

The tracking issue for this feature is: #43122

# `get_mut_unchecked`

The tracking issue for this feature is: #63292

# `global_asm`

The tracking issue for this feature is: #35119

The `global_asm!` macro allows the programmer to write arbitrary assembly outside the scope of a function body, passing it through `rustc` and `llvm` to the assembler. The macro is a no-frills interface to LLVM's concept of module-level inline assembly. That is, all caveats applicable to LLVM's module-level inline assembly apply to `global_asm!`.

`global_asm!` fills a role not currently satisfied by either `asm!` or `#[naked]` functions. The programmer has all features of the assembler at their disposal. The linker will expect to resolve any symbols defined in the inline assembly, modulo any symbols marked as external. It also means syntax for directives and assembly follow the conventions of the assembler in your toolchain.

A simple usage looks like this:

    #![feature(global_asm)]
    you also need relevant target_arch cfgs
    global_asm!(include_str!("something_neato.s"));

And a more complicated usage looks like this:

    #![feature(global_asm)]
    #![cfg(any(target_arch = "x86", target_arch = "x86_64"))]

    pub mod sally {
        global_asm!(r#"
            .global foo
          foo:
            jmp baz
        "#);

        #[no_mangle]
        pub unsafe extern "C" fn baz() {}
    }

    // the symbols `foo` and `bar` are global, no matter where
    // `global_asm!` was used.
    extern "C" {
        fn foo();
        fn bar();
    }

    pub mod harry {
        global_asm!(r#"
            .global bar
          bar:
            jmp quux
        "#);

        #[no_mangle]
        pub unsafe extern "C" fn quux() {}
    }

You may use `global_asm!` multiple times, anywhere in your crate, in whatever way suits you. The effect is as if you concatenated all usages and placed the larger, single usage in the crate root.

If you don't need quite as much power and flexibility as `global_asm!` provides, and you don't mind restricting your inline assembly to `fn` bodies only, you might try the asm feature instead.

# `hash_raw_entry`

The tracking issue for this feature is: #56167

# `hash_set_entry`

The tracking issue for this feature is: #60896

# `hashmap_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `inner_deref`

The tracking issue for this feature is: #50264

# `int_error_internals`

This feature is internal to the Rust compiler and is not intended for general use.

# `int_error_matching`

The tracking issue for this feature is: #22639

# `integer_atomics`

The tracking issue for this feature is: #32976

# `internal_uninit_const`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `into_future`

The tracking issue for this feature is: #67644

# `io_slice_advance`

The tracking issue for this feature is: #62726

# `ip`

The tracking issue for this feature is: #27709

# `is_sorted`

The tracking issue for this feature is: #53485

Add the methods `is_sorted`, `is_sorted_by` and `is_sorted_by_key` to `[T]`; add the methods `is_sorted`, `is_sorted_by` and `is_sorted_by_key` to `Iterator`.

# `iter_is_partitioned`

The tracking issue for this feature is: #62544

# `iter_map_while`

The tracking issue for this feature is: #68537

# `iter_order_by`

The tracking issue for this feature is: #64295

# `iter_partition_in_place`

The tracking issue for this feature is: #62543

# `iterator_fold_self`

The tracking issue for this feature is: #68125

# `layout_for_ptr`

The tracking issue for this feature is: #69835

# `liballoc_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `libstd_io_internals`

This feature is internal to the Rust compiler and is not intended for general use.

# `libstd_sys_internals`

This feature is internal to the Rust compiler and is not intended for general use.

# `libstd_thread_internals`

This feature is internal to the Rust compiler and is not intended for general use.

# `linked_list_cursors`

The tracking issue for this feature is: #58533

# `linked_list_extras`

The tracking issue for this feature is: #27794

# `linked_list_prepend`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `linked_list_remove`

The tracking issue for this feature is: #69210

# `llvm_asm`

The tracking issue for this feature is: #70173

For extremely low-level manipulations and performance reasons, one might wish to control the CPU directly. Rust supports using inline assembly to do this via the `llvm_asm!` macro.

    llvm_asm!(assembly template
       : output operands
       : input operands
       : clobbers
       : options
       );

Any use of `llvm_asm` is feature gated (requires `#![feature(llvm_asm)]` on the crate to allow) and of course requires an `unsafe` block.

> Note: the examples here are given in x86/x86-64 assembly, but all platforms are supported.

## Assembly template

The `assembly template` is the only required parameter and must be a literal string (i.e. `""`)

    #![feature(llvm_asm)]

    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    fn foo() {
        unsafe {
            llvm_asm!("NOP");
        }
    }

    // Other platforms:
    #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    fn foo() { /* ... */ }

    fn main() {
        // ...
        foo();
        // ...
    }

(The `feature(llvm_asm)` and `#[cfg]`s are omitted from now on.)

Output operands, input operands, clobbers and options are all optional but you must add the right number of `:` if you skip them:

    #![feature(llvm_asm)]
    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    fn main() { unsafe {
    llvm_asm!("xor %eax, %eax"
        :
        :
        : "eax"
       );
    } }
    #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    fn main() {}

Whitespace also doesn't matter:

    #![feature(llvm_asm)]
    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    fn main() { unsafe {
    llvm_asm!("xor %eax, %eax" ::: "eax");
    } }
    #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    fn main() {}

## Operands

Input and output operands follow the same format: `: "constraints1"(expr1), "constraints2"(expr2), ..."`. Output operand expressions must be mutable place, or not yet assigned:

    #![feature(llvm_asm)]
    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    fn add(a: i32, b: i32) -> i32 {
        let c: i32;
        unsafe {
            llvm_asm!("add $2, $0"
                 : "=r"(c)
                 : "0"(a), "r"(b)
                 );
        }
        c
    }
    #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    fn add(a: i32, b: i32) -> i32 { a + b }

    fn main() {
        assert_eq!(add(3, 14159), 14162)
    }

If you would like to use real operands in this position, however, you are required to put curly braces `{}` around the register that you want, and you are required to put the specific size of the operand. This is useful for very low level programming, where which register you use is important:

    #![allow(unused)]
    fn main() {
    #![feature(llvm_asm)]
    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    unsafe fn read_byte_in(port: u16) -> u8 {
    let result: u8;
    llvm_asm!("in %dx, %al" : "={al}"(result) : "{dx}"(port));
    result
    }
    }

## Clobbers

Some instructions modify registers which might otherwise have held different values so we use the clobbers list to indicate to the compiler not to assume any values loaded into those registers will stay valid.

    #![feature(llvm_asm)]
    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    fn main() { unsafe {
    // Put the value 0x200 in eax:
    llvm_asm!("mov $$0x200, %eax" : /* no outputs */ : /* no inputs */ : "eax");
    } }
    #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    fn main() {}

Input and output registers need not be listed since that information is already communicated by the given constraints. Otherwise, any other registers used either implicitly or explicitly should be listed.

If the assembly changes the condition code register `cc` should be specified as one of the clobbers. Similarly, if the assembly modifies memory, `memory` should also be specified.

## Options

The last section, `options` is specific to Rust. The format is comma separated literal strings (i.e. `:"foo", "bar", "baz"`). It's used to specify some extra info about the inline assembly:

Current valid options are:

1. volatile - specifying this is analogous to `__asm__ __volatile__ (...)` in gcc/clang.
2. alignstack - certain instructions expect the stack to be aligned a certain way (i.e. SSE) and specifying this indicates to the compiler to insert its usual stack alignment code
3. intel - use intel syntax instead of the default AT&T.

    #![feature(llvm_asm)]
    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    fn main() {
    let result: i32;
    unsafe {
       llvm_asm!("mov eax, 2" : "={eax}"(result) : : : "intel")
    }
    println!("eax is currently {}", result);
    }
    #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    fn main() {}

## More Information

The current implementation of the `llvm_asm!` macro is a direct binding to LLVM's inline assembler expressions, so be sure to check out their documentation as well for more information about clobbers, constraints, etc.

If you need more power and don't mind losing some of the niceties of `llvm_asm!`, check out global_asm.

# `log_syntax`

The tracking issue for this feature is: #29598

# `map_entry_replace`

The tracking issue for this feature is: #44286

# `map_first_last`

The tracking issue for this feature is: #62924

# `maybe_uninit_extra`

The tracking issue for this feature is: #63567

# `maybe_uninit_ref`

The tracking issue for this feature is: #63568

# `maybe_uninit_slice`

The tracking issue for this feature is: #63569

# `maybe_uninit_slice_assume_init`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `maybe_uninit_uninit_array`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `new_uninit`

The tracking issue for this feature is: #63291

# `nonnull_slice_from_raw_parts`

The tracking issue for this feature is: #71941

# `once_poison`

The tracking issue for this feature is: #33577

# `option_expect_none`

The tracking issue for this feature is: #62633

# `option_result_contains`

The tracking issue for this feature is: #62358

# `option_unwrap_none`

The tracking issue for this feature is: #62633

# `option_zip`

The tracking issue for this feature is: #70086

# `or_insert_with_key`

The tracking issue for this feature is: #71024

# `osstring_ascii`

The tracking issue for this feature is: #70516

# `panic_abort`

The tracking issue for this feature is: #32837

# `panic_info_message`

The tracking issue for this feature is: #66745

# `panic_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `panic_unwind`

The tracking issue for this feature is: #32837

# `partition_point`

The tracking issue for this feature is: #73831

# `pattern`

The tracking issue for this feature is: #27721

# `peekable_next_if`

The tracking issue for this feature is: #72480

# `poll_map`

The tracking issue for this feature is: #63514

# `print_internals`

This feature is internal to the Rust compiler and is not intended for general use.

# `proc_macro_def_site`

The tracking issue for this feature is: #54724

# `proc_macro_diagnostic`

The tracking issue for this feature is: #54140

# `proc_macro_internals`

The tracking issue for this feature is: #27812

# `proc_macro_is_available`

The tracking issue for this feature is: #71436

# `proc_macro_quote`

The tracking issue for this feature is: #54722

# `proc_macro_raw_ident`

The tracking issue for this feature is: #54723

# `proc_macro_span`

The tracking issue for this feature is: #54725

# `process_exitcode_placeholder`

The tracking issue for this feature is: #48711

# `process_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `profiler_runtime_lib`

This feature is internal to the Rust compiler and is not intended for general use.

# `ptr_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `ptr_offset_from`

The tracking issue for this feature is: #41079

# `ptr_wrapping_offset_from`

The tracking issue for this feature is: #41079

# `range_is_empty`

The tracking issue for this feature is: #48111

# `raw`

The tracking issue for this feature is: #27751

# `raw_ref_macros`

The tracking issue for this feature is: #73394

# `raw_vec_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `rc_into_raw_non_null`

The tracking issue for this feature is: #47336

# `read_initializer`

The tracking issue for this feature is: #42788

# `receiver_trait`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `refcell_take`

The tracking issue for this feature is: #71395

# `renamed_spin_loop`

The tracking issue for this feature is: #55002

# `result_cloned`

The tracking issue for this feature is: #63168

# `result_contains_err`

The tracking issue for this feature is: #62358

# `result_copied`

The tracking issue for this feature is: #63168

# `result_flattening`

The tracking issue for this feature is: #70142

# `rt`

This feature is internal to the Rust compiler and is not intended for general use.

# `seek_convenience`

The tracking issue for this feature is: #59359

# `set_stdio`

This feature is internal to the Rust compiler and is not intended for general use.

# `sgx_platform`

The tracking issue for this feature is: #56975

# `shrink_to`

The tracking issue for this feature is: #56431

# `slice_concat_ext`

The tracking issue for this feature is: #27747

# `slice_concat_trait`

The tracking issue for this feature is: #27747

# `slice_fill`

The tracking issue for this feature is: #70758

# `slice_index_methods`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `slice_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `slice_iter_mut_as_slice`

The tracking issue for this feature is: #58957

# `slice_partition_at_index`

The tracking issue for this feature is: #55300

# `slice_partition_dedup`

The tracking issue for this feature is: #54279

# `slice_ptr_len`

The tracking issue for this feature is: #71146

# `slice_ptr_range`

The tracking issue for this feature is: #65807

# `slice_strip`

The tracking issue for this feature is: #73413

# `sort_internals`

This feature is internal to the Rust compiler and is not intended for general use.

# `split_inclusive`

The tracking issue for this feature is: #72360

# `std_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `stdsimd`

The tracking issue for this feature is: #48556

# `step_trait`

The tracking issue for this feature is: #42168

# `step_trait_ext`

The tracking issue for this feature is: #42168

# `str_internals`

This feature is internal to the Rust compiler and is not intended for general use.

# `tau_constant`

The tracking issue for this feature is: #66770

# `termination_trait_lib`

The tracking issue for this feature is: #43301

# `test`

The tracking issue for this feature is: None.

The internals of the `test` crate are unstable, behind the `test` flag. The most widely used part of the `test` crate are benchmark tests, which can test the performance of your code. Let's make our `src/lib.rs` look like this (comments elided):

    #![feature(test)]

    extern crate test;

    pub fn add_two(a: i32) -> i32 {
        a + 2
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        use test::Bencher;

        #[test]
        fn it_works() {
            assert_eq!(4, add_two(2));
        }

        #[bench]
        fn bench_add_two(b: &mut Bencher) {
            b.iter(|| add_two(2));
        }
    }

Note the `test` feature gate, which enables this unstable feature.

We've imported the `test` crate, which contains our benchmarking support. We have a new function as well, with the `bench` attribute. Unlike regular tests, which take no arguments, benchmark tests take a `&mut Bencher`. This `Bencher` provides an `iter` method, which takes a closure. This closure contains the code we'd like to benchmark.

We can run benchmark tests with `cargo bench`:

    $ cargo bench
       Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
         Running target/release/adder-91b3e234d4ed382a

    running 2 tests
    test tests::it_works ... ignored
    test tests::bench_add_two ... bench:         1 ns/iter (+/- 0)

    test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured

Our non-benchmark test was ignored. You may have noticed that `cargo bench` takes a bit longer than `cargo test`. This is because Rust runs our benchmark a number of times, and then takes the average. Because we're doing so little work in this example, we have a `1 ns/iter (+/- 0)`, but this would show the variance if there was one.

Advice on writing benchmarks:

- Move setup code outside the `iter` loop; only put the part you want to measure inside
- Make the code do "the same thing" on each iteration; do not accumulate or change state
- Make the outer function idempotent too; the benchmark runner is likely to run it many times
- Make the inner `iter` loop short and fast so benchmark runs are fast and the calibrator can adjust the run-length at fine resolution
- Make the code in the `iter` loop do something simple, to assist in pinpointing performance improvements (or regressions)

## Gotcha: optimizations

There's another tricky part to writing benchmarks: benchmarks compiled with optimizations activated can be dramatically changed by the optimizer so that the benchmark is no longer benchmarking what one expects. For example, the compiler might recognize that some calculation has no external effects and remove it entirely.

    #![feature(test)]

    extern crate test;
    use test::Bencher;

    #[bench]
    fn bench_xor_1000_ints(b: &mut Bencher) {
        b.iter(|| {
            (0..1000).fold(0, |old, new| old ^ new);
        });
    }

gives the following results

    running 1 test
    test bench_xor_1000_ints ... bench:         0 ns/iter (+/- 0)

    test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

The benchmarking runner offers two ways to avoid this. Either, the closure that the `iter` method receives can return an arbitrary value which forces the optimizer to consider the result used and ensures it cannot remove the computation entirely. This could be done for the example above by adjusting the `b.iter` call to

    #![allow(unused)]
    fn main() {
    struct X;
    impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
    b.iter(|| {
        // Note lack of `;` (could also use an explicit `return`).
        (0..1000).fold(0, |old, new| old ^ new)
    });
    }

Or, the other option is to call the generic `test::black_box` function, which is an opaque "black box" to the optimizer and so forces it to consider any argument as used.

    #![feature(test)]

    extern crate test;

    fn main() {
    struct X;
    impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
    b.iter(|| {
        let n = test::black_box(1000);

        (0..n).fold(0, |a, b| a ^ b)
    })
    }

Neither of these read or modify the value, and are very cheap for small values. Larger values can be passed indirectly to reduce overhead (e.g. `black_box(&huge_struct)`).

Performing either of the above changes gives the following benchmarking results

    running 1 test
    test bench_xor_1000_ints ... bench:       131 ns/iter (+/- 3)

    test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

However, the optimizer can still modify a testcase in an undesirable manner even when using either of the above.

# `thread_id_value`

The tracking issue for this feature is: #67939

# `thread_local_internals`

This feature is internal to the Rust compiler and is not intended for general use.

# `thread_spawn_unchecked`

The tracking issue for this feature is: #55132

# `tidy_test_never_used_anywhere_else`

This feature is internal to the Rust compiler and is not intended for general use.

# `toowned_clone_into`

The tracking issue for this feature is: #41263

# `total_cmp`

The tracking issue for this feature is: #72599

# `trace_macros`

The tracking issue for this feature is #29598.

With `trace_macros` you can trace the expansion of macros in your code.

## Examples

    #![feature(trace_macros)]

    fn main() {
        trace_macros!(true);
        println!("Hello, Rust!");
        trace_macros!(false);
    }

The `cargo build` output:

    note: trace_macro
     --> src/main.rs:5:5
      |
    5 |     println!("Hello, Rust!");
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = note: expanding `println! { "Hello, Rust!" }`
      = note: to `print ! ( concat ! ( "Hello, Rust!" , "\n" ) )`
      = note: expanding `print! { concat ! ( "Hello, Rust!" , "\n" ) }`
      = note: to `$crate :: io :: _print ( format_args ! ( concat ! ( "Hello, Rust!" , "\n" ) )
              )`

        Finished dev [unoptimized + debuginfo] target(s) in 0.60 secs

# `trusted_len`

The tracking issue for this feature is: #37572

# `try_find`

The tracking issue for this feature is: #63178

# `try_reserve`

The tracking issue for this feature is: #48043

# `try_trait`

The tracking issue for this feature is: #42327

This introduces a new trait `Try` for extending the `?` operator to types other than `Result` (a part of RFC 1859). The trait provides the canonical way to view a type in terms of a success/failure dichotomy. This will allow `?` to supplant the `try_opt!` macro on `Option` and the `try_ready!` macro on `Poll`, among other things.

Here's an example implementation of the trait:

    /// A distinct type to represent the `None` value of an `Option`.
    ///
    /// This enables using the `?` operator on `Option`; it's rarely useful alone.
    #[derive(Debug)]
    #[unstable(feature = "try_trait", issue = "42327")]
    pub struct None { _priv: () }

    #[unstable(feature = "try_trait", issue = "42327")]
    impl<T> ops::Try for Option<T>  {
        type Ok = T;
        type Error = None;

        fn into_result(self) -> Result<T, None> {
            self.ok_or(None { _priv: () })
        }

        fn from_ok(v: T) -> Self {
            Some(v)
        }

        fn from_error(_: None) -> Self {
            None
        }
    }

Note the `Error` associated type here is a new marker. The `?` operator allows interconversion between different `Try` implementers only when the error type can be converted `Into` the error type of the enclosing function (or catch block). Having a distinct error type (as opposed to just `()`, or similar) restricts this to where it's semantically meaningful.

# `type_name_of_val`

The tracking issue for this feature is: #66359

# `unchecked_math`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `unicode_internals`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `unsafe_cell_raw_get`

The tracking issue for this feature is: #66358

# `unsize`

The tracking issue for this feature is: #27732

# `unwrap_infallible`

The tracking issue for this feature is: #61695

# `update_panic_count`

This feature is internal to the Rust compiler and is not intended for general use.

# `variant_count`

The tracking issue for this feature is: #73662

# `vec_into_raw_parts`

The tracking issue for this feature is: #65816

# `vec_leak`

The tracking issue for this feature is: #62195

# `vec_remove_item`

The tracking issue for this feature is: #40062

# `vec_resize_default`

The tracking issue for this feature is: #41758

# `wake_trait`

The tracking issue for this feature is: #69912

# `wasi_ext`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `windows_by_handle`

The tracking issue for this feature is: #63010

# `windows_c`

This feature is internal to the Rust compiler and is not intended for general use.

# `windows_file_type_ext`

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

# `windows_handle`

This feature is internal to the Rust compiler and is not intended for general use.

# `windows_net`

This feature is internal to the Rust compiler and is not intended for general use.

# `windows_stdio`

This feature is internal to the Rust compiler and is not intended for general use.

# `with_options`

The tracking issue for this feature is: #65439

# `wrapping_int_impl`

The tracking issue for this feature is: #32463

# `wrapping_next_power_of_two`

The tracking issue for this feature is: #32463

# `write_all_vectored`

The tracking issue for this feature is: #70436