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

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    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Standard library macros
    //!
    //! This modules contains a set of macros which are exported from the standard
    //! library. Each macro is available for use when linking against the standard
    //! library.

    /// The entry point for panic of Rust threads.
    ///
    /// This macro is used to inject panic into a Rust thread, causing the thread to
    /// panic entirely. Each thread's panic can be reaped as the `Box<Any>` type,
    /// and the single-argument form of the `panic!` macro will be the value which
    /// is transmitted.
    ///
    /// The multi-argument form of this macro panics with a string and has the
    /// `format!` syntax for building a string.
    ///
    /// # Examples
    ///
    /// ```should_panic
    /// # #![allow(unreachable_code)]
    /// panic!();
    /// panic!("this is a terrible mistake!");
    /// panic!(4); // panic with the value of 4 to be collected elsewhere
    /// panic!("this is a {} {message}", "fancy", message = "message");
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[allow_internal_unstable]
    macro_rules! panic {
        () => ({
            panic!("explicit panic")
        });
        ($msg:expr) => ({
            $crate::rt::begin_panic($msg, {
                // static requires less code at runtime, more constant data
                static _FILE_LINE: (&'static str, u32) = (file!(), line!());
                &_FILE_LINE
            })
        });
        ($fmt:expr, $($arg:tt)+) => ({
            $crate::rt::begin_panic_fmt(&format_args!($fmt, $($arg)+), {
                // The leading _'s are to avoid dead code warnings if this is
                // used inside a dead function. Just `#[allow(dead_code)]` is
                // insufficient, since the user may have
                // `#[forbid(dead_code)]` and which cannot be overridden.
                static _FILE_LINE: (&'static str, u32) = (file!(), line!());
                &_FILE_LINE
            })
        });
    }

    /// Macro for printing to the standard output.
    ///
    /// Equivalent to the `println!` macro except that a newline is not printed at
    /// the end of the message.
    ///
    /// Note that stdout is frequently line-buffered by default so it may be
    /// necessary to use `io::stdout().flush()` to ensure the output is emitted
    /// immediately.
    ///
    /// # Panics
    ///
    /// Panics if writing to `io::stdout()` fails.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::{self, Write};
    ///
    /// print!("this ");
    /// print!("will ");
    /// print!("be ");
    /// print!("on ");
    /// print!("the ");
    /// print!("same ");
    /// print!("line ");
    ///
    /// io::stdout().flush().unwrap();
    ///
    /// print!("this string has a newline, why not choose println! instead?\n");
    ///
    /// io::stdout().flush().unwrap();
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[allow_internal_unstable]
    macro_rules! print {
        ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
    }

    /// Macro for printing to the standard output, with a newline. On all
    /// platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
    /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
    ///
    /// Use the `format!` syntax to write data to the standard output.
    /// See `std::fmt` for more information.
    ///
    /// # Panics
    ///
    /// Panics if writing to `io::stdout()` fails.
    ///
    /// # Examples
    ///
    /// ```
    /// println!();
    /// println!("hello there!");
    /// println!("format {} arguments", "some");
    /// ```
    #[macro_export]
    #[stable(feature = "rust1", since = "1.0.0")]
    macro_rules! println {
        () => (print!("\n"));
        ($fmt:expr) => (print!(concat!($fmt, "\n")));
        ($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*));
    }

    /// A macro to select an event from a number of receivers.
    ///
    /// This macro is used to wait for the first event to occur on a number of
    /// receivers. It places no restrictions on the types of receivers given to
    /// this macro, this can be viewed as a heterogeneous select.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(mpsc_select)]
    ///
    /// use std::thread;
    /// use std::sync::mpsc;
    ///
    /// // two placeholder functions for now
    /// fn long_running_thread() {}
    /// fn calculate_the_answer() -> u32 { 42 }
    ///
    /// let (tx1, rx1) = mpsc::channel();
    /// let (tx2, rx2) = mpsc::channel();
    ///
    /// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); });
    /// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); });
    ///
    /// select! {
    ///     _ = rx1.recv() => println!("the long running thread finished first"),
    ///     answer = rx2.recv() => {
    ///         println!("the answer was: {}", answer.unwrap());
    ///     }
    /// }
    /// # drop(rx1.recv());
    /// # drop(rx2.recv());
    /// ```
    ///
    /// For more information about select, see the `std::sync::mpsc::Select` structure.
    #[macro_export]
    #[unstable(feature = "mpsc_select", issue = "27800")]
    macro_rules! select {
        (
            $($name:pat = $rx:ident.$meth:ident() => $code:expr),+
        ) => ({
            use $crate::sync::mpsc::Select;
            let sel = Select::new();
            $( let mut $rx = sel.handle(&$rx); )+
            unsafe {
                $( $rx.add(); )+
            }
            let ret = sel.wait();
            $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+
            { unreachable!() }
        })
    }

    #[cfg(test)]
    macro_rules! assert_approx_eq {
        ($a:expr, $b:expr) => ({
            let (a, b) = (&$a, &$b);
            assert!((*a - *b).abs() < 1.0e-6,
                    "{} is not approximately equal to {}", *a, *b);
        })
    }

    /// Built-in macros to the compiler itself.
    ///
    /// These macros do not have any corresponding definition with a `macro_rules!`
    /// macro, but are documented here. Their implementations can be found hardcoded
    /// into libsyntax itself.
    #[cfg(dox)]
    pub mod builtin {
        /// The core macro for formatted string creation & output.
        ///
        /// This macro produces a value of type [`fmt::Arguments`]. This value can be
        /// passed to the functions in [`std::fmt`] for performing useful functions.
        /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
        /// proxied through this one.
        ///
        /// For more information, see the documentation in [`std::fmt`].
        ///
        /// [`fmt::Arguments`]: ../std/fmt/struct.Arguments.html
        /// [`std::fmt`]: ../std/fmt/index.html
        /// [`format!`]: ../std/macro.format.html
        /// [`write!`]: ../std/macro.write.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")]
        #[macro_export]
        macro_rules! format_args { ($fmt:expr, $($args:tt)*) => ({
            /* compiler built-in */
        }) }

        /// Inspect 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 $PATH variable at the time of compiling was: {}", path);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[macro_export]
        macro_rules! env { ($name:expr) => ({ /* compiler built-in */ }) }

        /// Optionally inspect 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`.
        ///
        /// 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")]
        #[macro_export]
        macro_rules! option_env { ($name:expr) => ({ /* compiler built-in */ }) }

        /// Concatenate 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_macro", issue = "29599")]
        #[macro_export]
        macro_rules! concat_idents {
            ($($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")]
        #[macro_export]
        macro_rules! concat { ($($e:expr),*) => ({ /* compiler built-in */ }) }

        /// A macro which expands to the line number on which it was invoked.
        ///
        /// The expanded expression has type `u32`, and the returned line is not
        /// the invocation of the `line!()` macro 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")]
        #[macro_export]
        macro_rules! line { () => ({ /* compiler built-in */ }) }

        /// A macro which expands to the column number on which it was invoked.
        ///
        /// The expanded expression has type `u32`, and the returned column is not
        /// the invocation of the `column!()` macro 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")]
        #[macro_export]
        macro_rules! column { () => ({ /* compiler built-in */ }) }

        /// A macro which expands to the file name from which it was invoked.
        ///
        /// 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")]
        #[macro_export]
        macro_rules! file { () => ({ /* compiler built-in */ }) }

        /// A macro which stringifies its argument.
        ///
        /// 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")]
        #[macro_export]
        macro_rules! stringify { ($t:tt) => ({ /* compiler built-in */ }) }

        /// Includes a utf8-encoded file as a string.
        ///
        /// The file is located relative to the current file. (similarly to how
        /// modules are found)
        ///
        /// This macro will yield an expression of type `&'static str` which is the
        /// contents of the file.
        ///
        /// # Examples
        ///
        /// ```rust,ignore
        /// let secret_key = include_str!("secret-key.ascii");
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[macro_export]
        macro_rules! include_str { ($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)
        ///
        /// This macro will yield an expression of type `&'static [u8; N]` which is
        /// the contents of the file.
        ///
        /// # Examples
        ///
        /// ```rust,ignore
        /// let secret_key = include_bytes!("secret-key.bin");
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[macro_export]
        macro_rules! include_bytes { ($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")]
        #[macro_export]
        macro_rules! module_path { () => ({ /* compiler built-in */ }) }

        /// Boolean evaluation of configuration flags.
        ///
        /// 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](../reference.html#conditional-compilation).
        ///
        /// # Examples
        ///
        /// ```
        /// let my_directory = if cfg!(windows) {
        ///     "windows-specific-directory"
        /// } else {
        ///     "unix-directory"
        /// };
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[macro_export]
        macro_rules! cfg { ($($cfg:tt)*) => ({ /* compiler built-in */ }) }

        /// Parse 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)
        ///
        /// 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 unhygenically. 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
        ///
        /// ```ignore
        /// fn foo() {
        ///     include!("/path/to/a/file")
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[macro_export]
        macro_rules! include { ($file:expr) => ({ /* compiler built-in */ }) }
    }

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