mod.rs.html -- source
Description: Source to the Rust file `library/core/src/macros/mod.rs`.
Keywords: rust, rustlang, rust-lang

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    #[doc(include = "panic.md")]
    #[macro_export]
    #[allow_internal_unstable(core_panic, const_caller_location)]
    #[stable(feature = "core", since = "1.6.0")]
    macro_rules! panic {
        () => (
            $crate::panic!("explicit panic")
        );
        ($msg:literal) => (
            $crate::panicking::panic($msg)
        );
        ($msg:expr) => (
            $crate::panic!("{}", $crate::convert::identity::<&str>($msg))
        );
        ($msg:expr,) => (
            $crate::panic!($msg)
        );
        ($fmt:expr, $($arg:tt)+) => (
            $crate::panicking::panic_fmt($crate::format_args!($fmt, $($arg)+))
        );
    }

    /// Asserts that two expressions are equal to each other (using [`PartialEq`]).
    ///
    /// On panic, this macro will print the values of the expressions with their
    /// debug representations.
    ///
    /// Like [`assert!`], this macro has a second form, where a custom
    /// panic message can be provided.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = 3;
    /// let b = 1 + 2;
    /// assert_eq!(a, b);
    ///
    /// assert_eq!(a, b, "we are testing addition with {} and {}", a, b);
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    macro_rules! assert_eq {
        ($left:expr, $right:expr) => ({
            match (&$left, &$right) {
                (left_val, right_val) => {
                    if !(*left_val == *right_val) {
                        // The reborrows below are intentional. Without them, the stack slot for the
                        // borrow is initialized even before the values are compared, leading to a
                        // noticeable slow down.
                        panic!(r#"assertion failed: `(left == right)`
      left: `{:?}`,
     right: `{:?}`"#, &*left_val, &*right_val)
                    }
                }
            }
        });
        ($left:expr, $right:expr,) => ({
            $crate::assert_eq!($left, $right)
        });
        ($left:expr, $right:expr, $($arg:tt)+) => ({
            match (&($left), &($right)) {
                (left_val, right_val) => {
                    if !(*left_val == *right_val) {
                        // The reborrows below are intentional. Without them, the stack slot for the
                        // borrow is initialized even before the values are compared, leading to a
                        // noticeable slow down.
                        panic!(r#"assertion failed: `(left == right)`
      left: `{:?}`,
     right: `{:?}`: {}"#, &*left_val, &*right_val,
                               $crate::format_args!($($arg)+))
                    }
                }
            }
        });
    }

    /// Asserts that two expressions are not equal to each other (using [`PartialEq`]).
    ///
    /// On panic, this macro will print the values of the expressions with their
    /// debug representations.
    ///
    /// Like [`assert!`], this macro has a second form, where a custom
    /// panic message can be provided.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = 3;
    /// let b = 2;
    /// assert_ne!(a, b);
    ///
    /// assert_ne!(a, b, "we are testing that the values are not equal");
    /// ```
    #[macro_export]
    #[stable(feature = "assert_ne", since = "1.13.0")]
    macro_rules! assert_ne {
        ($left:expr, $right:expr) => ({
            match (&$left, &$right) {
                (left_val, right_val) => {
                    if *left_val == *right_val {
                        // The reborrows below are intentional. Without them, the stack slot for the
                        // borrow is initialized even before the values are compared, leading to a
                        // noticeable slow down.
                        panic!(r#"assertion failed: `(left != right)`
      left: `{:?}`,
     right: `{:?}`"#, &*left_val, &*right_val)
                    }
                }
            }
        });
        ($left:expr, $right:expr,) => {
            $crate::assert_ne!($left, $right)
        };
        ($left:expr, $right:expr, $($arg:tt)+) => ({
            match (&($left), &($right)) {
                (left_val, right_val) => {
                    if *left_val == *right_val {
                        // The reborrows below are intentional. Without them, the stack slot for the
                        // borrow is initialized even before the values are compared, leading to a
                        // noticeable slow down.
                        panic!(r#"assertion failed: `(left != right)`
      left: `{:?}`,
     right: `{:?}`: {}"#, &*left_val, &*right_val,
                               $crate::format_args!($($arg)+))
                    }
                }
            }
        });
    }

    /// Asserts that a boolean expression is `true` at runtime.
    ///
    /// This will invoke the [`panic!`] macro if the provided expression cannot be
    /// evaluated to `true` at runtime.
    ///
    /// Like [`assert!`], this macro also has a second version, where a custom panic
    /// message can be provided.
    ///
    /// # Uses
    ///
    /// Unlike [`assert!`], `debug_assert!` statements are only enabled in non
    /// optimized builds by default. An optimized build will not execute
    /// `debug_assert!` statements unless `-C debug-assertions` is passed to the
    /// compiler. This makes `debug_assert!` useful for checks that are too
    /// expensive to be present in a release build but may be helpful during
    /// development. The result of expanding `debug_assert!` is always type checked.
    ///
    /// An unchecked assertion allows a program in an inconsistent state to keep
    /// running, which might have unexpected consequences but does not introduce
    /// unsafety as long as this only happens in safe code. The performance cost
    /// of assertions, however, is not measurable in general. Replacing [`assert!`]
    /// with `debug_assert!` is thus only encouraged after thorough profiling, and
    /// more importantly, only in safe code!
    ///
    /// # Examples
    ///
    /// ```
    /// // the panic message for these assertions is the stringified value of the
    /// // expression given.
    /// debug_assert!(true);
    ///
    /// fn some_expensive_computation() -> bool { true } // a very simple function
    /// debug_assert!(some_expensive_computation());
    ///
    /// // assert with a custom message
    /// let x = true;
    /// debug_assert!(x, "x wasn't true!");
    ///
    /// let a = 3; let b = 27;
    /// debug_assert!(a + b == 30, "a = {}, b = {}", a, b);
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    macro_rules! debug_assert {
        ($($arg:tt)*) => (if $crate::cfg!(debug_assertions) { $crate::assert!($($arg)*); })
    }

    /// Asserts that two expressions are equal to each other.
    ///
    /// On panic, this macro will print the values of the expressions with their
    /// debug representations.
    ///
    /// Unlike [`assert_eq!`], `debug_assert_eq!` statements are only enabled in non
    /// optimized builds by default. An optimized build will not execute
    /// `debug_assert_eq!` statements unless `-C debug-assertions` is passed to the
    /// compiler. This makes `debug_assert_eq!` useful for checks that are too
    /// expensive to be present in a release build but may be helpful during
    /// development. The result of expanding `debug_assert_eq!` is always type checked.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = 3;
    /// let b = 1 + 2;
    /// debug_assert_eq!(a, b);
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    macro_rules! debug_assert_eq {
        ($($arg:tt)*) => (if $crate::cfg!(debug_assertions) { $crate::assert_eq!($($arg)*); })
    }

    /// Asserts that two expressions are not equal to each other.
    ///
    /// On panic, this macro will print the values of the expressions with their
    /// debug representations.
    ///
    /// Unlike [`assert_ne!`], `debug_assert_ne!` statements are only enabled in non
    /// optimized builds by default. An optimized build will not execute
    /// `debug_assert_ne!` statements unless `-C debug-assertions` is passed to the
    /// compiler. This makes `debug_assert_ne!` useful for checks that are too
    /// expensive to be present in a release build but may be helpful during
    /// development. The result of expanding `debug_assert_ne!` is always type checked.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = 3;
    /// let b = 2;
    /// debug_assert_ne!(a, b);
    /// ```
    #[macro_export]
    #[stable(feature = "assert_ne", since = "1.13.0")]
    macro_rules! debug_assert_ne {
        ($($arg:tt)*) => (if $crate::cfg!(debug_assertions) { $crate::assert_ne!($($arg)*); })
    }

    /// Returns whether the given expression matches any of the given patterns.
    ///
    /// Like in a `match` expression, the pattern can be optionally followed by `if`
    /// and a guard expression that has access to names bound by the pattern.
    ///
    /// # Examples
    ///
    /// ```
    /// let foo = 'f';
    /// assert!(matches!(foo, 'A'..='Z' | 'a'..='z'));
    ///
    /// let bar = Some(4);
    /// assert!(matches!(bar, Some(x) if x > 2));
    /// ```
    #[macro_export]
    #[stable(feature = "matches_macro", since = "1.42.0")]
    macro_rules! matches {
        ($expression:expr, $( $pattern:pat )|+ $( if $guard: expr )? $(,)?) => {
            match $expression {
                $( $pattern )|+ $( if $guard )? => true,
                _ => false
            }
        }
    }

    /// Unwraps a result or propagates its error.
    ///
    /// The `?` operator was added to replace `try!` and should be used instead.
    /// Furthermore, `try` is a reserved word in Rust 2018, so if you must use
    /// it, you will need to use the [raw-identifier syntax][ris]: `r#try`.
    ///
    /// [ris]: https://doc.rust-lang.org/nightly/rust-by-example/compatibility/raw_identifiers.html
    ///
    /// `try!` matches the given [`Result`]. In case of the `Ok` variant, the
    /// expression has the value of the wrapped value.
    ///
    /// In case of the `Err` variant, it retrieves the inner error. `try!` then
    /// performs conversion using `From`. This provides automatic conversion
    /// between specialized errors and more general ones. The resulting
    /// error is then immediately returned.
    ///
    /// Because of the early return, `try!` can only be used in functions that
    /// return [`Result`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io;
    /// use std::fs::File;
    /// use std::io::prelude::*;
    ///
    /// enum MyError {
    ///     FileWriteError
    /// }
    ///
    /// impl From<io::Error> for MyError {
    ///     fn from(e: io::Error) -> MyError {
    ///         MyError::FileWriteError
    ///     }
    /// }
    ///
    /// // The preferred method of quick returning Errors
    /// fn write_to_file_question() -> Result<(), MyError> {
    ///     let mut file = File::create("my_best_friends.txt")?;
    ///     file.write_all(b"This is a list of my best friends.")?;
    ///     Ok(())
    /// }
    ///
    /// // The previous method of quick returning Errors
    /// fn write_to_file_using_try() -> Result<(), MyError> {
    ///     let mut file = r#try!(File::create("my_best_friends.txt"));
    ///     r#try!(file.write_all(b"This is a list of my best friends."));
    ///     Ok(())
    /// }
    ///
    /// // This is equivalent to:
    /// fn write_to_file_using_match() -> Result<(), MyError> {
    ///     let mut file = r#try!(File::create("my_best_friends.txt"));
    ///     match file.write_all(b"This is a list of my best friends.") {
    ///         Ok(v) => v,
    ///         Err(e) => return Err(From::from(e)),
    ///     }
    ///     Ok(())
    /// }
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(since = "1.39.0", reason = "use the `?` operator instead")]
    #[doc(alias = "?")]
    macro_rules! r#try {
        ($expr:expr) => {
            match $expr {
                $crate::result::Result::Ok(val) => val,
                $crate::result::Result::Err(err) => {
                    return $crate::result::Result::Err($crate::convert::From::from(err));
                }
            }
        };
        ($expr:expr,) => {
            $crate::r#try!($expr)
        };
    }

    /// Writes formatted data into a buffer.
    ///
    /// This macro accepts a format string, a list of arguments, and a 'writer'. Arguments will be
    /// formatted according to the specified format string and the result will be passed to the writer.
    /// The writer may be any value with a `write_fmt` method; generally this comes from an
    /// implementation of either the [`fmt::Write`] or the [`io::Write`] trait. The macro
    /// returns whatever the `write_fmt` method returns; commonly a [`fmt::Result`], or an
    /// [`io::Result`].
    ///
    /// See [`std::fmt`] for more information on the format string syntax.
    ///
    /// [`std::fmt`]: crate::fmt
    /// [`fmt::Write`]: crate::fmt::Write
    /// [`io::Write`]: ../std/io/trait.Write.html
    /// [`fmt::Result`]: crate::fmt::Result
    /// [`io::Result`]: ../std/io/type.Result.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::Write;
    ///
    /// fn main() -> std::io::Result<()> {
    ///     let mut w = Vec::new();
    ///     write!(&mut w, "test")?;
    ///     write!(&mut w, "formatted {}", "arguments")?;
    ///
    ///     assert_eq!(w, b"testformatted arguments");
    ///     Ok(())
    /// }
    /// ```
    ///
    /// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
    /// implementing either, as objects do not typically implement both. However, the module must
    /// import the traits qualified so their names do not conflict:
    ///
    /// ```
    /// use std::fmt::Write as FmtWrite;
    /// use std::io::Write as IoWrite;
    ///
    /// fn main() -> Result<(), Box<dyn std::error::Error>> {
    ///     let mut s = String::new();
    ///     let mut v = Vec::new();
    ///
    ///     write!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
    ///     write!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
    ///     assert_eq!(v, b"s = \"abc 123\"");
    ///     Ok(())
    /// }
    /// ```
    ///
    /// Note: This macro can be used in `no_std` setups as well.
    /// In a `no_std` setup you are responsible for the implementation details of the components.
    ///
    /// ```no_run
    /// # extern crate core;
    /// use core::fmt::Write;
    ///
    /// struct Example;
    ///
    /// impl Write for Example {
    ///     fn write_str(&mut self, _s: &str) -> core::fmt::Result {
    ///          unimplemented!();
    ///     }
    /// }
    ///
    /// let mut m = Example{};
    /// write!(&mut m, "Hello World").expect("Not written");
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    macro_rules! write {
        ($dst:expr, $($arg:tt)*) => ($dst.write_fmt($crate::format_args!($($arg)*)))
    }

    /// Write formatted data into a buffer, with a newline appended.
    ///
    /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
    /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
    ///
    /// For more information, see [`write!`]. For information on the format string syntax, see
    /// [`std::fmt`].
    ///
    /// [`std::fmt`]: crate::fmt
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::{Write, Result};
    ///
    /// fn main() -> Result<()> {
    ///     let mut w = Vec::new();
    ///     writeln!(&mut w)?;
    ///     writeln!(&mut w, "test")?;
    ///     writeln!(&mut w, "formatted {}", "arguments")?;
    ///
    ///     assert_eq!(&w[..], "\ntest\nformatted arguments\n".as_bytes());
    ///     Ok(())
    /// }
    /// ```
    ///
    /// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
    /// implementing either, as objects do not typically implement both. However, the module must
    /// import the traits qualified so their names do not conflict:
    ///
    /// ```
    /// use std::fmt::Write as FmtWrite;
    /// use std::io::Write as IoWrite;
    ///
    /// fn main() -> Result<(), Box<dyn std::error::Error>> {
    ///     let mut s = String::new();
    ///     let mut v = Vec::new();
    ///
    ///     writeln!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
    ///     writeln!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
    ///     assert_eq!(v, b"s = \"abc 123\\n\"\n");
    ///     Ok(())
    /// }
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[allow_internal_unstable(format_args_nl)]
    macro_rules! writeln {
        ($dst:expr) => (
            $crate::write!($dst, "\n")
        );
        ($dst:expr,) => (
            $crate::writeln!($dst)
        );
        ($dst:expr, $($arg:tt)*) => (
            $dst.write_fmt($crate::format_args_nl!($($arg)*))
        );
    }

    /// Indicates unreachable code.
    ///
    /// This is useful any time that the compiler can't determine that some code is unreachable. For
    /// example:
    ///
    /// * Match arms with guard conditions.
    /// * Loops that dynamically terminate.
    /// * Iterators that dynamically terminate.
    ///
    /// If the determination that the code is unreachable proves incorrect, the
    /// program immediately terminates with a [`panic!`].
    ///
    /// The unsafe counterpart of this macro is the [`unreachable_unchecked`] function, which
    /// will cause undefined behavior if the code is reached.
    ///
    /// [`unreachable_unchecked`]: crate::hint::unreachable_unchecked
    ///
    /// # Panics
    ///
    /// This will always [`panic!`]
    ///
    /// # Examples
    ///
    /// Match arms:
    ///
    /// ```
    /// # #[allow(dead_code)]
    /// fn foo(x: Option<i32>) {
    ///     match x {
    ///         Some(n) if n >= 0 => println!("Some(Non-negative)"),
    ///         Some(n) if n <  0 => println!("Some(Negative)"),
    ///         Some(_)           => unreachable!(), // compile error if commented out
    ///         None              => println!("None")
    ///     }
    /// }
    /// ```
    ///
    /// Iterators:
    ///
    /// ```
    /// # #[allow(dead_code)]
    /// fn divide_by_three(x: u32) -> u32 { // one of the poorest implementations of x/3
    ///     for i in 0.. {
    ///         if 3*i < i { panic!("u32 overflow"); }
    ///         if x < 3*i { return i-1; }
    ///     }
    ///     unreachable!();
    /// }
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    macro_rules! unreachable {
        () => ({
            panic!("internal error: entered unreachable code")
        });
        ($msg:expr) => ({
            $crate::unreachable!("{}", $msg)
        });
        ($msg:expr,) => ({
            $crate::unreachable!($msg)
        });
        ($fmt:expr, $($arg:tt)*) => ({
            panic!($crate::concat!("internal error: entered unreachable code: ", $fmt), $($arg)*)
        });
    }

    /// Indicates unimplemented code by panicking with a message of "not implemented".
    ///
    /// This allows your code to type-check, which is useful if you are prototyping or
    /// implementing a trait that requires multiple methods which you don't plan of using all of.
    ///
    /// The difference between `unimplemented!` and [`todo!`](macro.todo.html) is that while `todo!`
    /// conveys an intent of implementing the functionality later and the message is "not yet
    /// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
    /// Also some IDEs will mark `todo!`s.
    ///
    /// # Panics
    ///
    /// This will always [panic!](macro.panic.html) because `unimplemented!` is just a
    /// shorthand for `panic!` with a fixed, specific message.
    ///
    /// Like `panic!`, this macro has a second form for displaying custom values.
    ///
    /// # Examples
    ///
    /// Say we have a trait `Foo`:
    ///
    /// ```
    /// trait Foo {
    ///     fn bar(&self) -> u8;
    ///     fn baz(&self);
    ///     fn qux(&self) -> Result<u64, ()>;
    /// }
    /// ```
    ///
    /// We want to implement `Foo` for 'MyStruct', but for some reason it only makes sense
    /// to implement the `bar()` function. `baz()` and `qux()` will still need to be defined
    /// in our implementation of `Foo`, but we can use `unimplemented!` in their definitions
    /// to allow our code to compile.
    ///
    /// We still want to have our program stop running if the unimplemented methods are
    /// reached.
    ///
    /// ```
    /// # trait Foo {
    /// #     fn bar(&self) -> u8;
    /// #     fn baz(&self);
    /// #     fn qux(&self) -> Result<u64, ()>;
    /// # }
    /// struct MyStruct;
    ///
    /// impl Foo for MyStruct {
    ///     fn bar(&self) -> u8 {
    ///         1 + 1
    ///     }
    ///
    ///     fn baz(&self) {
    ///         // It makes no sense to `baz` a `MyStruct`, so we have no logic here
    ///         // at all.
    ///         // This will display "thread 'main' panicked at 'not implemented'".
    ///         unimplemented!();
    ///     }
    ///
    ///     fn qux(&self) -> Result<u64, ()> {
    ///         // We have some logic here,
    ///         // We can add a message to unimplemented! to display our omission.
    ///         // This will display:
    ///         // "thread 'main' panicked at 'not implemented: MyStruct isn't quxable'".
    ///         unimplemented!("MyStruct isn't quxable");
    ///     }
    /// }
    ///
    /// fn main() {
    ///     let s = MyStruct;
    ///     s.bar();
    /// }
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    macro_rules! unimplemented {
        () => (panic!("not implemented"));
        ($($arg:tt)+) => (panic!("not implemented: {}", $crate::format_args!($($arg)+)));
    }

    /// Indicates unfinished code.
    ///
    /// This can be useful if you are prototyping and are just looking to have your
    /// code typecheck.
    ///
    /// The difference between [`unimplemented!`] and `todo!` is that while `todo!` conveys
    /// an intent of implementing the functionality later and the message is "not yet
    /// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
    /// Also some IDEs will mark `todo!`s.
    ///
    /// # Panics
    ///
    /// This will always [panic!](macro.panic.html)
    ///
    /// # Examples
    ///
    /// Here's an example of some in-progress code. We have a trait `Foo`:
    ///
    /// ```
    /// trait Foo {
    ///     fn bar(&self);
    ///     fn baz(&self);
    /// }
    /// ```
    ///
    /// We want to implement `Foo` on one of our types, but we also want to work on
    /// just `bar()` first. In order for our code to compile, we need to implement
    /// `baz()`, so we can use `todo!`:
    ///
    /// ```
    /// # trait Foo {
    /// #     fn bar(&self);
    /// #     fn baz(&self);
    /// # }
    /// struct MyStruct;
    ///
    /// impl Foo for MyStruct {
    ///     fn bar(&self) {
    ///         // implementation goes here
    ///     }
    ///
    ///     fn baz(&self) {
    ///         // let's not worry about implementing baz() for now
    ///         todo!();
    ///     }
    /// }
    ///
    /// fn main() {
    ///     let s = MyStruct;
    ///     s.bar();
    ///
    ///     // we aren't even using baz(), so this is fine.
    /// }
    /// ```
    #[macro_export]
    #[stable(feature = "todo_macro", since = "1.40.0")]
    macro_rules! todo {
        () => (panic!("not yet implemented"));
        ($($arg:tt)+) => (panic!("not yet implemented: {}", $crate::format_args!($($arg)+)));
    }

    /// Definitions of built-in macros.
    ///
    /// Most of the macro properties (stability, visibility, etc.) are taken from the source code here,
    /// with exception of expansion functions transforming macro inputs into outputs,
    /// those functions are provided by the compiler.
    pub(crate) mod builtin {

        /// Causes compilation to fail with the given error message when encountered.
        ///
        /// This macro should be used when a crate uses a conditional compilation strategy to provide
        /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
        /// but emits an error during *compilation* rather than at *runtime*.
        ///
        /// # Examples
        ///
        /// Two such examples are macros and `#[cfg]` environments.
        ///
        /// Emit better compiler error if a macro is passed invalid values. Without the final branch,
        /// the compiler would still emit an error, but the error's message would not mention the two
        /// valid values.
        ///
        /// ```compile_fail
        /// macro_rules! give_me_foo_or_bar {
        ///     (foo) => {};
        ///     (bar) => {};
        ///     ($x:ident) => {
        ///         compile_error!("This macro only accepts `foo` or `bar`");
        ///     }
        /// }
        ///
        /// give_me_foo_or_bar!(neither);
        /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
        /// ```
        ///
        /// Emit compiler error if one of a number of features isn't available.
        ///
        /// ```compile_fail
        /// #[cfg(not(any(feature = "foo", feature = "bar")))]
        /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.");
        /// ```
        #[stable(feature = "compile_error_macro", since = "1.20.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! compile_error {
            ($msg:expr) => {{ /* compiler built-in */ }};
            ($msg:expr,) => {{ /* compiler built-in */ }};
        }

        /// Constructs parameters for the other string-formatting macros.
        ///
        /// This macro functions by taking a formatting string literal containing
        /// `{}` for each additional argument passed. `format_args!` prepares the
        /// additional parameters to ensure the output can be interpreted as a string
        /// and canonicalizes the arguments into a single type. Any value that implements
        /// the [`Display`] trait can be passed to `format_args!`, as can any
        /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
        ///
        /// This macro produces a value of type [`fmt::Arguments`]. This value can be
        /// passed to the macros within [`std::fmt`] for performing useful redirection.
        /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
        /// proxied through this one. `format_args!`, unlike its derived macros, avoids
        /// heap allocations.
        ///
        /// You can use the [`fmt::Arguments`] value that `format_args!` returns
        /// in `Debug` and `Display` contexts as seen below. The example also shows
        /// that `Debug` and `Display` format to the same thing: the interpolated
        /// format string in `format_args!`.
        ///
        /// ```rust
        /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
        /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
        /// assert_eq!("1 foo 2", display);
        /// assert_eq!(display, debug);
        /// ```
        ///
        /// For more information, see the documentation in [`std::fmt`].
        ///
        /// [`Display`]: crate::fmt::Display
        /// [`Debug`]: crate::fmt::Debug
        /// [`fmt::Arguments`]: crate::fmt::Arguments
        /// [`std::fmt`]: crate::fmt
        /// [`format!`]: ../std/macro.format.html
        /// [`println!`]: ../std/macro.println.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// let s = fmt::format(format_args!("hello {}", "world"));
        /// assert_eq!(s, format!("hello {}", "world"));
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[allow_internal_unstable(fmt_internals)]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! format_args {
            ($fmt:expr) => {{ /* compiler built-in */ }};
            ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
        }

        /// Same as `format_args`, but adds a newline in the end.
        #[unstable(
            feature = "format_args_nl",
            issue = "none",
            reason = "`format_args_nl` is only for internal \
                      language use and is subject to change"
        )]
        #[allow_internal_unstable(fmt_internals)]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! format_args_nl {
            ($fmt:expr) => {{ /* compiler built-in */ }};
            ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
        }

        /// Inspects an environment variable at compile time.
        ///
        /// This macro will expand to the value of the named environment variable at
        /// compile time, yielding an expression of type `&'static str`.
        ///
        /// If the environment variable is not defined, then a compilation error
        /// will be emitted. To not emit a compile error, use the [`option_env!`]
        /// macro instead.
        ///
        /// # Examples
        ///
        /// ```
        /// let path: &'static str = env!("PATH");
        /// println!("the $PATH variable at the time of compiling was: {}", path);
        /// ```
        ///
        /// You can customize the error message by passing a string as the second
        /// parameter:
        ///
        /// ```compile_fail
        /// let doc: &'static str = env!("documentation", "what's that?!");
        /// ```
        ///
        /// If the `documentation` environment variable is not defined, you'll get
        /// the following error:
        ///
        /// ```text
        /// error: what's that?!
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! env {
            ($name:expr) => {{ /* compiler built-in */ }};
            ($name:expr,) => {{ /* compiler built-in */ }};
        }

        /// Optionally inspects an environment variable at compile time.
        ///
        /// If the named environment variable is present at compile time, this will
        /// expand into an expression of type `Option<&'static str>` whose value is
        /// `Some` of the value of the environment variable. If the environment
        /// variable is not present, then this will expand to `None`. See
        /// [`Option<T>`][Option] for more information on this type.
        ///
        /// A compile time error is never emitted when using this macro regardless
        /// of whether the environment variable is present or not.
        ///
        /// # Examples
        ///
        /// ```
        /// let key: Option<&'static str> = option_env!("SECRET_KEY");
        /// println!("the secret key might be: {:?}", key);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! option_env {
            ($name:expr) => {{ /* compiler built-in */ }};
            ($name:expr,) => {{ /* compiler built-in */ }};
        }

        /// Concatenates identifiers into one identifier.
        ///
        /// This macro takes any number of comma-separated identifiers, and
        /// concatenates them all into one, yielding an expression which is a new
        /// identifier. Note that hygiene makes it such that this macro cannot
        /// capture local variables. Also, as a general rule, macros are only
        /// allowed in item, statement or expression position. That means while
        /// you may use this macro for referring to existing variables, functions or
        /// modules etc, you cannot define a new one with it.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(concat_idents)]
        ///
        /// # fn main() {
        /// fn foobar() -> u32 { 23 }
        ///
        /// let f = concat_idents!(foo, bar);
        /// println!("{}", f());
        ///
        /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
        /// # }
        /// ```
        #[unstable(
            feature = "concat_idents",
            issue = "29599",
            reason = "`concat_idents` is not stable enough for use and is subject to change"
        )]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! concat_idents {
            ($($e:ident),+) => {{ /* compiler built-in */ }};
            ($($e:ident,)+) => {{ /* compiler built-in */ }};
        }

        /// Concatenates literals into a static string slice.
        ///
        /// This macro takes any number of comma-separated literals, yielding an
        /// expression of type `&'static str` which represents all of the literals
        /// concatenated left-to-right.
        ///
        /// Integer and floating point literals are stringified in order to be
        /// concatenated.
        ///
        /// # Examples
        ///
        /// ```
        /// let s = concat!("test", 10, 'b', true);
        /// assert_eq!(s, "test10btrue");
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! concat {
            ($($e:expr),*) => {{ /* compiler built-in */ }};
            ($($e:expr,)*) => {{ /* compiler built-in */ }};
        }

        /// Expands to the line number on which it was invoked.
        ///
        /// With [`column!`] and [`file!`], these macros provide debugging information for
        /// developers about the location within the source.
        ///
        /// The expanded expression has type `u32` and is 1-based, so the first line
        /// in each file evaluates to 1, the second to 2, etc. This is consistent
        /// with error messages by common compilers or popular editors.
        /// The returned line is *not necessarily* the line of the `line!` invocation itself,
        /// but rather the first macro invocation leading up to the invocation
        /// of the `line!` macro.
        ///
        /// # Examples
        ///
        /// ```
        /// let current_line = line!();
        /// println!("defined on line: {}", current_line);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! line {
            () => {
                /* compiler built-in */
            };
        }

        /// Expands to the column number at which it was invoked.
        ///
        /// With [`line!`] and [`file!`], these macros provide debugging information for
        /// developers about the location within the source.
        ///
        /// The expanded expression has type `u32` and is 1-based, so the first column
        /// in each line evaluates to 1, the second to 2, etc. This is consistent
        /// with error messages by common compilers or popular editors.
        /// The returned column is *not necessarily* the line of the `column!` invocation itself,
        /// but rather the first macro invocation leading up to the invocation
        /// of the `column!` macro.
        ///
        /// # Examples
        ///
        /// ```
        /// let current_col = column!();
        /// println!("defined on column: {}", current_col);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! column {
            () => {
                /* compiler built-in */
            };
        }

        /// Expands to the file name in which it was invoked.
        ///
        /// With [`line!`] and [`column!`], these macros provide debugging information for
        /// developers about the location within the source.
        ///
        /// The expanded expression has type `&'static str`, and the returned file
        /// is not the invocation of the `file!` macro itself, but rather the
        /// first macro invocation leading up to the invocation of the `file!`
        /// macro.
        ///
        /// # Examples
        ///
        /// ```
        /// let this_file = file!();
        /// println!("defined in file: {}", this_file);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! file {
            () => {
                /* compiler built-in */
            };
        }

        /// Stringifies its arguments.
        ///
        /// This macro will yield an expression of type `&'static str` which is the
        /// stringification of all the tokens passed to the macro. No restrictions
        /// are placed on the syntax of the macro invocation itself.
        ///
        /// Note that the expanded results of the input tokens may change in the
        /// future. You should be careful if you rely on the output.
        ///
        /// # Examples
        ///
        /// ```
        /// let one_plus_one = stringify!(1 + 1);
        /// assert_eq!(one_plus_one, "1 + 1");
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! stringify {
            ($($t:tt)*) => {
                /* compiler built-in */
            };
        }

        /// Includes a UTF-8 encoded file as a string.
        ///
        /// The file is located relative to the current file (similarly to how
        /// modules are found). The provided path is interpreted in a platform-specific
        /// way at compile time. So, for instance, an invocation with a Windows path
        /// containing backslashes `\` would not compile correctly on Unix.
        ///
        /// This macro will yield an expression of type `&'static str` which is the
        /// contents of the file.
        ///
        /// # Examples
        ///
        /// Assume there are two files in the same directory with the following
        /// contents:
        ///
        /// File 'spanish.in':
        ///
        /// ```text
        /// adiós
        /// ```
        ///
        /// File 'main.rs':
        ///
        /// ```ignore (cannot-doctest-external-file-dependency)
        /// fn main() {
        ///     let my_str = include_str!("spanish.in");
        ///     assert_eq!(my_str, "adiós\n");
        ///     print!("{}", my_str);
        /// }
        /// ```
        ///
        /// Compiling 'main.rs' and running the resulting binary will print "adiós".
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! include_str {
            ($file:expr) => {{ /* compiler built-in */ }};
            ($file:expr,) => {{ /* compiler built-in */ }};
        }

        /// Includes a file as a reference to a byte array.
        ///
        /// The file is located relative to the current file (similarly to how
        /// modules are found). The provided path is interpreted in a platform-specific
        /// way at compile time. So, for instance, an invocation with a Windows path
        /// containing backslashes `\` would not compile correctly on Unix.
        ///
        /// This macro will yield an expression of type `&'static [u8; N]` which is
        /// the contents of the file.
        ///
        /// # Examples
        ///
        /// Assume there are two files in the same directory with the following
        /// contents:
        ///
        /// File 'spanish.in':
        ///
        /// ```text
        /// adiós
        /// ```
        ///
        /// File 'main.rs':
        ///
        /// ```ignore (cannot-doctest-external-file-dependency)
        /// fn main() {
        ///     let bytes = include_bytes!("spanish.in");
        ///     assert_eq!(bytes, b"adi\xc3\xb3s\n");
        ///     print!("{}", String::from_utf8_lossy(bytes));
        /// }
        /// ```
        ///
        /// Compiling 'main.rs' and running the resulting binary will print "adiós".
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! include_bytes {
            ($file:expr) => {{ /* compiler built-in */ }};
            ($file:expr,) => {{ /* compiler built-in */ }};
        }

        /// Expands to a string that represents the current module path.
        ///
        /// The current module path can be thought of as the hierarchy of modules
        /// leading back up to the crate root. The first component of the path
        /// returned is the name of the crate currently being compiled.
        ///
        /// # Examples
        ///
        /// ```
        /// mod test {
        ///     pub fn foo() {
        ///         assert!(module_path!().ends_with("test"));
        ///     }
        /// }
        ///
        /// test::foo();
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! module_path {
            () => {
                /* compiler built-in */
            };
        }

        /// Evaluates boolean combinations of configuration flags at compile-time.
        ///
        /// In addition to the `#[cfg]` attribute, this macro is provided to allow
        /// boolean expression evaluation of configuration flags. This frequently
        /// leads to less duplicated code.
        ///
        /// The syntax given to this macro is the same syntax as the [`cfg`]
        /// attribute.
        ///
        /// `cfg!`, unlike `#[cfg]`, does not remove any code and only evaluates to true or false. For
        /// example, all blocks in an if/else expression need to be valid when `cfg!` is used for
        /// the condition, regardless of what `cfg!` is evaluating.
        ///
        /// [`cfg`]: ../reference/conditional-compilation.html#the-cfg-attribute
        ///
        /// # Examples
        ///
        /// ```
        /// let my_directory = if cfg!(windows) {
        ///     "windows-specific-directory"
        /// } else {
        ///     "unix-directory"
        /// };
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! cfg {
            ($($cfg:tt)*) => {
                /* compiler built-in */
            };
        }

        /// Parses a file as an expression or an item according to the context.
        ///
        /// The file is located relative to the current file (similarly to how
        /// modules are found). The provided path is interpreted in a platform-specific
        /// way at compile time. So, for instance, an invocation with a Windows path
        /// containing backslashes `\` would not compile correctly on Unix.
        ///
        /// Using this macro is often a bad idea, because if the file is
        /// parsed as an expression, it is going to be placed in the
        /// surrounding code unhygienically. This could result in variables
        /// or functions being different from what the file expected if
        /// there are variables or functions that have the same name in
        /// the current file.
        ///
        /// # Examples
        ///
        /// Assume there are two files in the same directory with the following
        /// contents:
        ///
        /// File 'monkeys.in':
        ///
        /// ```ignore (only-for-syntax-highlight)
        /// ['🙈', '🙊', '🙉']
        ///     .iter()
        ///     .cycle()
        ///     .take(6)
        ///     .collect::<String>()
        /// ```
        ///
        /// File 'main.rs':
        ///
        /// ```ignore (cannot-doctest-external-file-dependency)
        /// fn main() {
        ///     let my_string = include!("monkeys.in");
        ///     assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
        ///     println!("{}", my_string);
        /// }
        /// ```
        ///
        /// Compiling 'main.rs' and running the resulting binary will print
        /// "🙈🙊🙉🙈🙊🙉".
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! include {
            ($file:expr) => {{ /* compiler built-in */ }};
            ($file:expr,) => {{ /* compiler built-in */ }};
        }

        /// Asserts that a boolean expression is `true` at runtime.
        ///
        /// This will invoke the [`panic!`] macro if the provided expression cannot be
        /// evaluated to `true` at runtime.
        ///
        /// # Uses
        ///
        /// Assertions are always checked in both debug and release builds, and cannot
        /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
        /// release builds by default.
        ///
        /// Unsafe code may rely on `assert!` to enforce run-time invariants that, if
        /// violated could lead to unsafety.
        ///
        /// Other use-cases of `assert!` include testing and enforcing run-time
        /// invariants in safe code (whose violation cannot result in unsafety).
        ///
        /// # Custom Messages
        ///
        /// This macro has a second form, where a custom panic message can
        /// be provided with or without arguments for formatting. See [`std::fmt`]
        /// for syntax for this form.
        ///
        /// [`std::fmt`]: crate::fmt
        ///
        /// # Examples
        ///
        /// ```
        /// // the panic message for these assertions is the stringified value of the
        /// // expression given.
        /// assert!(true);
        ///
        /// fn some_computation() -> bool { true } // a very simple function
        ///
        /// assert!(some_computation());
        ///
        /// // assert with a custom message
        /// let x = true;
        /// assert!(x, "x wasn't true!");
        ///
        /// let a = 3; let b = 27;
        /// assert!(a + b == 30, "a = {}, b = {}", a, b);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! assert {
            ($cond:expr) => {{ /* compiler built-in */ }};
            ($cond:expr,) => {{ /* compiler built-in */ }};
            ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};
        }

        /// Inline assembly.
        ///
        /// Read the [unstable book] for the usage.
        ///
        /// [unstable book]: ../unstable-book/library-features/asm.html
        #[unstable(
            feature = "asm",
            issue = "72016",
            reason = "inline assembly is not stable enough for use and is subject to change"
        )]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! asm {
            ("assembly template",
                $(operands,)*
                $(options($(option),*))?
            ) => {
                /* compiler built-in */
            };
        }

        /// LLVM-style inline assembly.
        ///
        /// Read the [unstable book] for the usage.
        ///
        /// [unstable book]: ../unstable-book/library-features/llvm-asm.html
        #[unstable(
            feature = "llvm_asm",
            issue = "70173",
            reason = "prefer using the new asm! syntax instead"
        )]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! llvm_asm {
            ("assembly template"
                            : $("output"(operand),)*
                            : $("input"(operand),)*
                            : $("clobbers",)*
                            : $("options",)*) => {
                /* compiler built-in */
            };
        }

        /// Module-level inline assembly.
        #[unstable(
            feature = "global_asm",
            issue = "35119",
            reason = "`global_asm!` is not stable enough for use and is subject to change"
        )]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! global_asm {
            ("assembly") => {
                /* compiler built-in */
            };
        }

        /// Prints passed tokens into the standard output.
        #[unstable(
            feature = "log_syntax",
            issue = "29598",
            reason = "`log_syntax!` is not stable enough for use and is subject to change"
        )]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! log_syntax {
            ($($arg:tt)*) => {
                /* compiler built-in */
            };
        }

        /// Enables or disables tracing functionality used for debugging other macros.
        #[unstable(
            feature = "trace_macros",
            issue = "29598",
            reason = "`trace_macros` is not stable enough for use and is subject to change"
        )]
        #[rustc_builtin_macro]
        #[macro_export]
        macro_rules! trace_macros {
            (true) => {{ /* compiler built-in */ }};
            (false) => {{ /* compiler built-in */ }};
        }

        /// Attribute macro applied to a function to turn it into a unit test.
        #[stable(feature = "rust1", since = "1.0.0")]
        #[allow_internal_unstable(test, rustc_attrs)]
        #[rustc_builtin_macro]
        pub macro test($item:item) {
            /* compiler built-in */
        }

        /// Attribute macro applied to a function to turn it into a benchmark test.
        #[unstable(
            feature = "test",
            issue = "50297",
            soft,
            reason = "`bench` is a part of custom test frameworks which are unstable"
        )]
        #[allow_internal_unstable(test, rustc_attrs)]
        #[rustc_builtin_macro]
        pub macro bench($item:item) {
            /* compiler built-in */
        }

        /// An implementation detail of the `#[test]` and `#[bench]` macros.
        #[unstable(
            feature = "custom_test_frameworks",
            issue = "50297",
            reason = "custom test frameworks are an unstable feature"
        )]
        #[allow_internal_unstable(test, rustc_attrs)]
        #[rustc_builtin_macro]
        pub macro test_case($item:item) {
            /* compiler built-in */
        }

        /// Attribute macro applied to a static to register it as a global allocator.
        #[stable(feature = "global_allocator", since = "1.28.0")]
        #[allow_internal_unstable(rustc_attrs)]
        #[rustc_builtin_macro]
        pub macro global_allocator($item:item) {
            /* compiler built-in */
        }

        /// Keeps the item it's applied to if the passed path is accessible, and removes it otherwise.
        #[unstable(
            feature = "cfg_accessible",
            issue = "64797",
            reason = "`cfg_accessible` is not fully implemented"
        )]
        #[rustc_builtin_macro]
        pub macro cfg_accessible($item:item) {
            /* compiler built-in */
        }

        /// Unstable implementation detail of the `rustc` compiler, do not use.
        #[rustc_builtin_macro]
        #[stable(feature = "rust1", since = "1.0.0")]
        #[allow_internal_unstable(core_intrinsics, libstd_sys_internals)]
        pub macro RustcDecodable($item:item) {
            /* compiler built-in */
        }

        /// Unstable implementation detail of the `rustc` compiler, do not use.
        #[rustc_builtin_macro]
        #[stable(feature = "rust1", since = "1.0.0")]
        #[allow_internal_unstable(core_intrinsics)]
        pub macro RustcEncodable($item:item) {
            /* compiler built-in */
        }
    }