<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source to the Rust file `libstd/macros.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>macros.rs.html -- source</title><link rel="stylesheet" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script src="../../storage.js"></script><link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"></head><body class="rustdoc source"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../std/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><pre class="line-numbers"><span id="1"> 1</span>
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</pre><pre class="rust ">
<span class="comment">// Copyright 2014 The Rust Project Developers. See the COPYRIGHT</span>
<span class="comment">// file at the top-level directory of this distribution and at</span>
<span class="comment">// http://rust-lang.org/COPYRIGHT.</span>
<span class="comment">//</span>
<span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span>
<span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span>
<span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span>
<span class="comment">// option. This file may not be copied, modified, or distributed</span>
<span class="comment">// except according to those terms.</span>
<span class="doccomment">//! Standard library macros</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! This modules contains a set of macros which are exported from the standard</span>
<span class="doccomment">//! library. Each macro is available for use when linking against the standard</span>
<span class="doccomment">//! library.</span>
<span class="doccomment">/// The entry point for panic of Rust threads.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This allows a program to to terminate immediately and provide feedback</span>
<span class="doccomment">/// to the caller of the program. `panic!` should be used when a program reaches</span>
<span class="doccomment">/// an unrecoverable problem.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro is the perfect way to assert conditions in example code and in</span>
<span class="doccomment">/// tests. `panic!` is closely tied with the `unwrap` method of both [`Option`]</span>
<span class="doccomment">/// and [`Result`][runwrap] enums. Both implementations call `panic!` when they are set</span>
<span class="doccomment">/// to None or Err variants.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro is used to inject panic into a Rust thread, causing the thread to</span>
<span class="doccomment">/// panic entirely. Each thread's panic can be reaped as the `Box<Any>` type,</span>
<span class="doccomment">/// and the single-argument form of the `panic!` macro will be the value which</span>
<span class="doccomment">/// is transmitted.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Result`] enum is often a better solution for recovering from errors than</span>
<span class="doccomment">/// using the `panic!` macro. This macro should be used to avoid proceeding using</span>
<span class="doccomment">/// incorrect values, such as from external sources. Detailed information about</span>
<span class="doccomment">/// error handling is found in the [book].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The multi-argument form of this macro panics with a string and has the</span>
<span class="doccomment">/// [`format!`] syntax for building a string.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [runwrap]: ../std/result/enum.Result.html#method.unwrap</span>
<span class="doccomment">/// [`Option`]: ../std/option/enum.Option.html#method.unwrap</span>
<span class="doccomment">/// [`Result`]: ../std/result/enum.Result.html</span>
<span class="doccomment">/// [`format!`]: ../std/macro.format.html</span>
<span class="doccomment">/// [book]: ../book/second-edition/ch09-01-unrecoverable-errors-with-panic.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Current implementation</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the main thread panics it will terminate all your threads and end your</span>
<span class="doccomment">/// program with code `101`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```should_panic</span>
<span class="doccomment">/// # #![allow(unreachable_code)]</span>
<span class="doccomment">/// panic!();</span>
<span class="doccomment">/// panic!("this is a terrible mistake!");</span>
<span class="doccomment">/// panic!(4); // panic with the value of 4 to be collected elsewhere</span>
<span class="doccomment">/// panic!("this is a {} {message}", "fancy", message = "message");</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">allow_internal_unstable</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">panic</span> {
() <span class="op">=></span> ({
<span class="macro">panic</span><span class="macro">!</span>(<span class="string">"explicit panic"</span>)
});
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">msg</span>:<span class="ident">expr</span>) <span class="op">=></span> ({
<span class="macro-nonterminal">$</span><span class="kw">crate</span>::<span class="macro-nonterminal">rt</span>::<span class="ident">begin_panic</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">msg</span>, <span class="kw-2">&</span>(<span class="macro">file</span><span class="macro">!</span>(), <span class="macro">line</span><span class="macro">!</span>(), <span class="macro">__rust_unstable_column</span><span class="macro">!</span>()))
});
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">msg</span>:<span class="ident">expr</span>,) <span class="op">=></span> ({
<span class="macro">panic</span><span class="macro">!</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">msg</span>)
});
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>:<span class="ident">tt</span>)<span class="op">+</span>) <span class="op">=></span> ({
<span class="macro-nonterminal">$</span><span class="kw">crate</span>::<span class="macro-nonterminal">rt</span>::<span class="ident">begin_panic_fmt</span>(<span class="kw-2">&</span><span class="macro">format_args</span><span class="macro">!</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>)<span class="op">+</span>),
<span class="kw-2">&</span>(<span class="macro">file</span><span class="macro">!</span>(), <span class="macro">line</span><span class="macro">!</span>(), <span class="macro">__rust_unstable_column</span><span class="macro">!</span>()))
});
}
<span class="doccomment">/// Macro for printing to the standard output.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Equivalent to the [`println!`] macro except that a newline is not printed at</span>
<span class="doccomment">/// the end of the message.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Note that stdout is frequently line-buffered by default so it may be</span>
<span class="doccomment">/// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted</span>
<span class="doccomment">/// immediately.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Use `print!` only for the primary output of your program. Use</span>
<span class="doccomment">/// [`eprint!`] instead to print error and progress messages.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`println!`]: ../std/macro.println.html</span>
<span class="doccomment">/// [flush]: ../std/io/trait.Write.html#tymethod.flush</span>
<span class="doccomment">/// [`eprint!`]: ../std/macro.eprint.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Panics</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Panics if writing to `io::stdout()` fails.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::io::{self, Write};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// print!("this ");</span>
<span class="doccomment">/// print!("will ");</span>
<span class="doccomment">/// print!("be ");</span>
<span class="doccomment">/// print!("on ");</span>
<span class="doccomment">/// print!("the ");</span>
<span class="doccomment">/// print!("same ");</span>
<span class="doccomment">/// print!("line ");</span>
<span class="doccomment">///</span>
<span class="doccomment">/// io::stdout().flush().unwrap();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// print!("this string has a newline, why not choose println! instead?\n");</span>
<span class="doccomment">///</span>
<span class="doccomment">/// io::stdout().flush().unwrap();</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">allow_internal_unstable</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">print</span> {
($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> (<span class="macro-nonterminal">$</span><span class="kw">crate</span>::<span class="macro-nonterminal">io</span>::<span class="ident">_print</span>(<span class="macro">format_args</span><span class="macro">!</span>($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>)<span class="kw-2">*</span>)));
}
<span class="doccomment">/// Macro for printing to the standard output, with a newline.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone</span>
<span class="doccomment">/// (no additional CARRIAGE RETURN (`\r`/`U+000D`).</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Use the [`format!`] syntax to write data to the standard output.</span>
<span class="doccomment">/// See [`std::fmt`] for more information.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Use `println!` only for the primary output of your program. Use</span>
<span class="doccomment">/// [`eprintln!`] instead to print error and progress messages.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`format!`]: ../std/macro.format.html</span>
<span class="doccomment">/// [`std::fmt`]: ../std/fmt/index.html</span>
<span class="doccomment">/// [`eprintln!`]: ../std/macro.eprint.html</span>
<span class="doccomment">/// # Panics</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Panics if writing to `io::stdout` fails.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// println!(); // prints just a newline</span>
<span class="doccomment">/// println!("hello there!");</span>
<span class="doccomment">/// println!("format {} arguments", "some");</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">println</span> {
() <span class="op">=></span> (<span class="macro">print</span><span class="macro">!</span>(<span class="string">"\n"</span>));
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>) <span class="op">=></span> (<span class="macro">print</span><span class="macro">!</span>(<span class="macro">concat</span><span class="macro">!</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>, <span class="string">"\n"</span>)));
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> (<span class="macro">print</span><span class="macro">!</span>(<span class="macro">concat</span><span class="macro">!</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>, <span class="string">"\n"</span>), $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>)<span class="kw-2">*</span>));
}
<span class="doccomment">/// Macro for printing to the standard error.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Equivalent to the [`print!`] macro, except that output goes to</span>
<span class="doccomment">/// [`io::stderr`] instead of `io::stdout`. See [`print!`] for</span>
<span class="doccomment">/// example usage.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Use `eprint!` only for error and progress messages. Use `print!`</span>
<span class="doccomment">/// instead for the primary output of your program.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`io::stderr`]: ../std/io/struct.Stderr.html</span>
<span class="doccomment">/// [`print!`]: ../std/macro.print.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Panics</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Panics if writing to `io::stderr` fails.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// eprint!("Error: Could not complete task");</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"eprint"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.19.0"</span>)]</span>
<span class="attribute">#[<span class="ident">allow_internal_unstable</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">eprint</span> {
($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> (<span class="macro-nonterminal">$</span><span class="kw">crate</span>::<span class="macro-nonterminal">io</span>::<span class="ident">_eprint</span>(<span class="macro">format_args</span><span class="macro">!</span>($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>)<span class="kw-2">*</span>)));
}
<span class="doccomment">/// Macro for printing to the standard error, with a newline.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Equivalent to the [`println!`] macro, except that output goes to</span>
<span class="doccomment">/// [`io::stderr`] instead of `io::stdout`. See [`println!`] for</span>
<span class="doccomment">/// example usage.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Use `eprintln!` only for error and progress messages. Use `println!`</span>
<span class="doccomment">/// instead for the primary output of your program.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`io::stderr`]: ../std/io/struct.Stderr.html</span>
<span class="doccomment">/// [`println!`]: ../std/macro.println.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Panics</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Panics if writing to `io::stderr` fails.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// eprintln!("Error: Could not complete task");</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"eprint"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.19.0"</span>)]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">eprintln</span> {
() <span class="op">=></span> (<span class="macro">eprint</span><span class="macro">!</span>(<span class="string">"\n"</span>));
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>) <span class="op">=></span> (<span class="macro">eprint</span><span class="macro">!</span>(<span class="macro">concat</span><span class="macro">!</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>, <span class="string">"\n"</span>)));
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> (<span class="macro">eprint</span><span class="macro">!</span>(<span class="macro">concat</span><span class="macro">!</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>, <span class="string">"\n"</span>), $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>)<span class="kw-2">*</span>));
}
<span class="doccomment">/// A macro to select an event from a number of receivers.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro is used to wait for the first event to occur on a number of</span>
<span class="doccomment">/// receivers. It places no restrictions on the types of receivers given to</span>
<span class="doccomment">/// this macro, this can be viewed as a heterogeneous select.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// #![feature(mpsc_select)]</span>
<span class="doccomment">///</span>
<span class="doccomment">/// use std::thread;</span>
<span class="doccomment">/// use std::sync::mpsc;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // two placeholder functions for now</span>
<span class="doccomment">/// fn long_running_thread() {}</span>
<span class="doccomment">/// fn calculate_the_answer() -> u32 { 42 }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let (tx1, rx1) = mpsc::channel();</span>
<span class="doccomment">/// let (tx2, rx2) = mpsc::channel();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); });</span>
<span class="doccomment">/// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); });</span>
<span class="doccomment">///</span>
<span class="doccomment">/// select! {</span>
<span class="doccomment">/// _ = rx1.recv() => println!("the long running thread finished first"),</span>
<span class="doccomment">/// answer = rx2.recv() => {</span>
<span class="doccomment">/// println!("the answer was: {}", answer.unwrap());</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// # drop(rx1.recv());</span>
<span class="doccomment">/// # drop(rx2.recv());</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// For more information about select, see the `std::sync::mpsc::Select` structure.</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"mpsc_select"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27800"</span>)]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">select</span> {
(
$(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">pat</span> <span class="op">=</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">rx</span>:<span class="ident">ident</span>.<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meth</span>:<span class="ident">ident</span>() <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">code</span>:<span class="ident">expr</span>),<span class="op">+</span>
) <span class="op">=></span> ({
<span class="kw">use</span> <span class="macro-nonterminal">$</span><span class="kw">crate</span>::<span class="macro-nonterminal">sync</span>::<span class="ident">mpsc</span>::<span class="ident">Select</span>;
<span class="kw">let</span> <span class="ident">sel</span> <span class="op">=</span> <span class="ident">Select</span>::<span class="ident">new</span>();
$( <span class="kw">let</span> <span class="kw-2">mut</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">rx</span> <span class="op">=</span> <span class="ident">sel</span>.<span class="ident">handle</span>(<span class="kw-2">&</span><span class="macro-nonterminal">$</span><span class="macro-nonterminal">rx</span>); )<span class="op">+</span>
<span class="kw">unsafe</span> {
$( <span class="macro-nonterminal">$</span><span class="macro-nonterminal">rx</span>.<span class="ident">add</span>(); )<span class="op">+</span>
}
<span class="kw">let</span> <span class="ident">ret</span> <span class="op">=</span> <span class="ident">sel</span>.<span class="ident">wait</span>();
$( <span class="kw">if</span> <span class="ident">ret</span> <span class="op">==</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">rx</span>.<span class="ident">id</span>() { <span class="kw">let</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span> <span class="op">=</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">rx</span>.<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meth</span>(); <span class="macro-nonterminal">$</span><span class="macro-nonterminal">code</span> } <span class="kw">else</span> )<span class="op">+</span>
{ <span class="macro">unreachable</span><span class="macro">!</span>() }
})
}
<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">test</span>)]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">assert_approx_eq</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">a</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">b</span>:<span class="ident">expr</span>) <span class="op">=></span> ({
<span class="kw">let</span> (<span class="ident">a</span>, <span class="ident">b</span>) <span class="op">=</span> (<span class="kw-2">&</span><span class="macro-nonterminal">$</span><span class="macro-nonterminal">a</span>, <span class="kw-2">&</span><span class="macro-nonterminal">$</span><span class="macro-nonterminal">b</span>);
<span class="macro">assert</span><span class="macro">!</span>((<span class="kw-2">*</span><span class="ident">a</span> <span class="op">-</span> <span class="kw-2">*</span><span class="ident">b</span>).<span class="ident">abs</span>() <span class="op"><</span> <span class="number">1.0e-6</span>,
<span class="string">"{} is not approximately equal to {}"</span>, <span class="kw-2">*</span><span class="ident">a</span>, <span class="kw-2">*</span><span class="ident">b</span>);
})
}
<span class="doccomment">/// Built-in macros to the compiler itself.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// These macros do not have any corresponding definition with a `macro_rules!`</span>
<span class="doccomment">/// macro, but are documented here. Their implementations can be found hardcoded</span>
<span class="doccomment">/// into libsyntax itself.</span>
<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">dox</span>)]</span>
<span class="kw">pub</span> <span class="kw">mod</span> <span class="ident">builtin</span> {
<span class="doccomment">/// Unconditionally causes compilation to fail with the given error message when encountered.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro should be used when a crate uses a conditional compilation strategy to provide</span>
<span class="doccomment">/// better error messages for erroneous conditions.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Two such examples are macros and `#[cfg]` environments.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Emit better compiler error if a macro is passed invalid values.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```compile_fail</span>
<span class="doccomment">/// macro_rules! give_me_foo_or_bar {</span>
<span class="doccomment">/// (foo) => {};</span>
<span class="doccomment">/// (bar) => {};</span>
<span class="doccomment">/// ($x:ident) => {</span>
<span class="doccomment">/// compile_error!("This macro only accepts `foo` or `bar`");</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// give_me_foo_or_bar!(neither);</span>
<span class="doccomment">/// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Emit compiler error if one of a number of features isn't available.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```compile_fail</span>
<span class="doccomment">/// #[cfg(not(any(feature = "foo", feature = "bar")))]</span>
<span class="doccomment">/// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.")</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"compile_error_macro"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.20.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">compile_error</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">msg</span>:<span class="ident">expr</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">msg</span>:<span class="ident">expr</span>,) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
}
<span class="doccomment">/// The core macro for formatted string creation & output.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro functions by taking a formatting string literal containing</span>
<span class="doccomment">/// `{}` for each additional argument passed. `format_args!` prepares the</span>
<span class="doccomment">/// additional parameters to ensure the output can be interpreted as a string</span>
<span class="doccomment">/// and canonicalizes the arguments into a single type. Any value that implements</span>
<span class="doccomment">/// the [`Display`] trait can be passed to `format_args!`, as can any</span>
<span class="doccomment">/// [`Debug`] implementation be passed to a `{:?}` within the formatting string.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro produces a value of type [`fmt::Arguments`]. This value can be</span>
<span class="doccomment">/// passed to the macros within [`std::fmt`] for performing useful redirection.</span>
<span class="doccomment">/// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are</span>
<span class="doccomment">/// proxied through this one. `format_args!`, unlike its derived macros, avoids</span>
<span class="doccomment">/// heap allocations.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// You can use the [`fmt::Arguments`] value that `format_args!` returns</span>
<span class="doccomment">/// in `Debug` and `Display` contexts as seen below. The example also shows</span>
<span class="doccomment">/// that `Debug` and `Display` format to the same thing: the interpolated</span>
<span class="doccomment">/// format string in `format_args!`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```rust</span>
<span class="doccomment">/// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));</span>
<span class="doccomment">/// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));</span>
<span class="doccomment">/// assert_eq!("1 foo 2", display);</span>
<span class="doccomment">/// assert_eq!(display, debug);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// For more information, see the documentation in [`std::fmt`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Display`]: ../std/fmt/trait.Display.html</span>
<span class="doccomment">/// [`Debug`]: ../std/fmt/trait.Debug.html</span>
<span class="doccomment">/// [`fmt::Arguments`]: ../std/fmt/struct.Arguments.html</span>
<span class="doccomment">/// [`std::fmt`]: ../std/fmt/index.html</span>
<span class="doccomment">/// [`format!`]: ../std/macro.format.html</span>
<span class="doccomment">/// [`write!`]: ../std/macro.write.html</span>
<span class="doccomment">/// [`println!`]: ../std/macro.println.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::fmt;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let s = fmt::format(format_args!("hello {}", "world"));</span>
<span class="doccomment">/// assert_eq!(s, format!("hello {}", "world"));</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">format_args</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">args</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
}
<span class="doccomment">/// Inspect an environment variable at compile time.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro will expand to the value of the named environment variable at</span>
<span class="doccomment">/// compile time, yielding an expression of type `&'static str`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the environment variable is not defined, then a compilation error</span>
<span class="doccomment">/// will be emitted. To not emit a compile error, use the [`option_env!`]</span>
<span class="doccomment">/// macro instead.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`option_env!`]: ../std/macro.option_env.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let path: &'static str = env!("PATH");</span>
<span class="doccomment">/// println!("the $PATH variable at the time of compiling was: {}", path);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// You can customize the error message by passing a string as the second</span>
<span class="doccomment">/// parameter:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```compile_fail</span>
<span class="doccomment">/// let doc: &'static str = env!("documentation", "what's that?!");</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the `documentation` environment variable is not defined, you'll get</span>
<span class="doccomment">/// the following error:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```text</span>
<span class="doccomment">/// error: what's that?!</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">env</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">expr</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">expr</span>,) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
}
<span class="doccomment">/// Optionally inspect an environment variable at compile time.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the named environment variable is present at compile time, this will</span>
<span class="doccomment">/// expand into an expression of type `Option<&'static str>` whose value is</span>
<span class="doccomment">/// `Some` of the value of the environment variable. If the environment</span>
<span class="doccomment">/// variable is not present, then this will expand to `None`. See</span>
<span class="doccomment">/// [`Option<T>`][option] for more information on this type.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// A compile time error is never emitted when using this macro regardless</span>
<span class="doccomment">/// of whether the environment variable is present or not.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [option]: ../std/option/enum.Option.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let key: Option<&'static str> = option_env!("SECRET_KEY");</span>
<span class="doccomment">/// println!("the secret key might be: {:?}", key);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">option_env</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">expr</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">expr</span>,) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
}
<span class="doccomment">/// Concatenate identifiers into one identifier.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro takes any number of comma-separated identifiers, and</span>
<span class="doccomment">/// concatenates them all into one, yielding an expression which is a new</span>
<span class="doccomment">/// identifier. Note that hygiene makes it such that this macro cannot</span>
<span class="doccomment">/// capture local variables. Also, as a general rule, macros are only</span>
<span class="doccomment">/// allowed in item, statement or expression position. That means while</span>
<span class="doccomment">/// you may use this macro for referring to existing variables, functions or</span>
<span class="doccomment">/// modules etc, you cannot define a new one with it.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// #![feature(concat_idents)]</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # fn main() {</span>
<span class="doccomment">/// fn foobar() -> u32 { 23 }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let f = concat_idents!(foo, bar);</span>
<span class="doccomment">/// println!("{}", f());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // fn concat_idents!(new, fun, name) { } // not usable in this way!</span>
<span class="doccomment">/// # }</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"concat_idents_macro"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"29599"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">concat_idents</span> {
($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e</span>:<span class="ident">ident</span>),<span class="op">+</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e</span>:<span class="ident">ident</span>,)<span class="op">+</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
}
<span class="doccomment">/// Concatenates literals into a static string slice.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro takes any number of comma-separated literals, yielding an</span>
<span class="doccomment">/// expression of type `&'static str` which represents all of the literals</span>
<span class="doccomment">/// concatenated left-to-right.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Integer and floating point literals are stringified in order to be</span>
<span class="doccomment">/// concatenated.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let s = concat!("test", 10, 'b', true);</span>
<span class="doccomment">/// assert_eq!(s, "test10btrue");</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">concat</span> {
($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e</span>:<span class="ident">expr</span>),<span class="kw-2">*</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e</span>:<span class="ident">expr</span>,)<span class="kw-2">*</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
}
<span class="doccomment">/// A macro which expands to the line number on which it was invoked.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// With [`column!`] and [`file!`], these macros provide debugging information for</span>
<span class="doccomment">/// developers about the location within the source.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The expanded expression has type `u32` and is 1-based, so the first line</span>
<span class="doccomment">/// in each file evaluates to 1, the second to 2, etc. This is consistent</span>
<span class="doccomment">/// with error messages by common compilers or popular editors.</span>
<span class="doccomment">/// The returned line is *not necessarily* the line of the `line!` invocation itself,</span>
<span class="doccomment">/// but rather the first macro invocation leading up to the invocation</span>
<span class="doccomment">/// of the `line!` macro.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`column!`]: macro.column.html</span>
<span class="doccomment">/// [`file!`]: macro.file.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let current_line = line!();</span>
<span class="doccomment">/// println!("defined on line: {}", current_line);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">line</span> { () <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> }) }
<span class="doccomment">/// A macro which expands to the column number on which it was invoked.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// With [`line!`] and [`file!`], these macros provide debugging information for</span>
<span class="doccomment">/// developers about the location within the source.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The expanded expression has type `u32` and is 1-based, so the first column</span>
<span class="doccomment">/// in each line evaluates to 1, the second to 2, etc. This is consistent</span>
<span class="doccomment">/// with error messages by common compilers or popular editors.</span>
<span class="doccomment">/// The returned column is *not necessarily* the line of the `column!` invocation itself,</span>
<span class="doccomment">/// but rather the first macro invocation leading up to the invocation</span>
<span class="doccomment">/// of the `column!` macro.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`line!`]: macro.line.html</span>
<span class="doccomment">/// [`file!`]: macro.file.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let current_col = column!();</span>
<span class="doccomment">/// println!("defined on column: {}", current_col);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">column</span> { () <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> }) }
<span class="doccomment">/// A macro which expands to the file name from which it was invoked.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// With [`line!`] and [`column!`], these macros provide debugging information for</span>
<span class="doccomment">/// developers about the location within the source.</span>
<span class="doccomment">///</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The expanded expression has type `&'static str`, and the returned file</span>
<span class="doccomment">/// is not the invocation of the `file!` macro itself, but rather the</span>
<span class="doccomment">/// first macro invocation leading up to the invocation of the `file!`</span>
<span class="doccomment">/// macro.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`line!`]: macro.line.html</span>
<span class="doccomment">/// [`column!`]: macro.column.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let this_file = file!();</span>
<span class="doccomment">/// println!("defined in file: {}", this_file);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">file</span> { () <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> }) }
<span class="doccomment">/// A macro which stringifies its arguments.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro will yield an expression of type `&'static str` which is the</span>
<span class="doccomment">/// stringification of all the tokens passed to the macro. No restrictions</span>
<span class="doccomment">/// are placed on the syntax of the macro invocation itself.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Note that the expanded results of the input tokens may change in the</span>
<span class="doccomment">/// future. You should be careful if you rely on the output.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let one_plus_one = stringify!(1 + 1);</span>
<span class="doccomment">/// assert_eq!(one_plus_one, "1 + 1");</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">stringify</span> { ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> }) }
<span class="doccomment">/// Includes a utf8-encoded file as a string.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The file is located relative to the current file. (similarly to how</span>
<span class="doccomment">/// modules are found)</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro will yield an expression of type `&'static str` which is the</span>
<span class="doccomment">/// contents of the file.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Assume there are two files in the same directory with the following</span>
<span class="doccomment">/// contents:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// File 'spanish.in':</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```text</span>
<span class="doccomment">/// adiós</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// File 'main.rs':</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```ignore (cannot-doctest-external-file-dependency)</span>
<span class="doccomment">/// fn main() {</span>
<span class="doccomment">/// let my_str = include_str!("spanish.in");</span>
<span class="doccomment">/// assert_eq!(my_str, "adiós\n");</span>
<span class="doccomment">/// print!("{}", my_str);</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Compiling 'main.rs' and running the resulting binary will print "adiós".</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">include_str</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">file</span>:<span class="ident">expr</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">file</span>:<span class="ident">expr</span>,) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
}
<span class="doccomment">/// Includes a file as a reference to a byte array.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The file is located relative to the current file. (similarly to how</span>
<span class="doccomment">/// modules are found)</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro will yield an expression of type `&'static [u8; N]` which is</span>
<span class="doccomment">/// the contents of the file.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Assume there are two files in the same directory with the following</span>
<span class="doccomment">/// contents:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// File 'spanish.in':</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```text</span>
<span class="doccomment">/// adiós</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// File 'main.rs':</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```ignore (cannot-doctest-external-file-dependency)</span>
<span class="doccomment">/// fn main() {</span>
<span class="doccomment">/// let bytes = include_bytes!("spanish.in");</span>
<span class="doccomment">/// assert_eq!(bytes, b"adi\xc3\xb3s\n");</span>
<span class="doccomment">/// print!("{}", String::from_utf8_lossy(bytes));</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Compiling 'main.rs' and running the resulting binary will print "adiós".</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">include_bytes</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">file</span>:<span class="ident">expr</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">file</span>:<span class="ident">expr</span>,) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
}
<span class="doccomment">/// Expands to a string that represents the current module path.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The current module path can be thought of as the hierarchy of modules</span>
<span class="doccomment">/// leading back up to the crate root. The first component of the path</span>
<span class="doccomment">/// returned is the name of the crate currently being compiled.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// mod test {</span>
<span class="doccomment">/// pub fn foo() {</span>
<span class="doccomment">/// assert!(module_path!().ends_with("test"));</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// test::foo();</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">module_path</span> { () <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> }) }
<span class="doccomment">/// Boolean evaluation of configuration flags, at compile-time.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// In addition to the `#[cfg]` attribute, this macro is provided to allow</span>
<span class="doccomment">/// boolean expression evaluation of configuration flags. This frequently</span>
<span class="doccomment">/// leads to less duplicated code.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The syntax given to this macro is the same syntax as [the `cfg`</span>
<span class="doccomment">/// attribute](../book/first-edition/conditional-compilation.html).</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let my_directory = if cfg!(windows) {</span>
<span class="doccomment">/// "windows-specific-directory"</span>
<span class="doccomment">/// } else {</span>
<span class="doccomment">/// "unix-directory"</span>
<span class="doccomment">/// };</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">cfg</span> { ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">cfg</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> }) }
<span class="doccomment">/// Parse a file as an expression or an item according to the context.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The file is located relative to the current file (similarly to how</span>
<span class="doccomment">/// modules are found).</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Using this macro is often a bad idea, because if the file is</span>
<span class="doccomment">/// parsed as an expression, it is going to be placed in the</span>
<span class="doccomment">/// surrounding code unhygienically. This could result in variables</span>
<span class="doccomment">/// or functions being different from what the file expected if</span>
<span class="doccomment">/// there are variables or functions that have the same name in</span>
<span class="doccomment">/// the current file.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Assume there are two files in the same directory with the following</span>
<span class="doccomment">/// contents:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// File 'monkeys.in':</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```ignore (only-for-syntax-highlight)</span>
<span class="doccomment">/// ['🙈', '🙊', '🙉']</span>
<span class="doccomment">/// .iter()</span>
<span class="doccomment">/// .cycle()</span>
<span class="doccomment">/// .take(6)</span>
<span class="doccomment">/// .collect::<String>()</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// File 'main.rs':</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```ignore (cannot-doctest-external-file-dependency)</span>
<span class="doccomment">/// fn main() {</span>
<span class="doccomment">/// let my_string = include!("monkeys.in");</span>
<span class="doccomment">/// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);</span>
<span class="doccomment">/// println!("{}", my_string);</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Compiling 'main.rs' and running the resulting binary will print</span>
<span class="doccomment">/// "🙈🙊🙉🙈🙊🙉".</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">include</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">file</span>:<span class="ident">expr</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">file</span>:<span class="ident">expr</span>,) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
}
<span class="doccomment">/// Ensure that a boolean expression is `true` at runtime.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This will invoke the [`panic!`] macro if the provided expression cannot be</span>
<span class="doccomment">/// evaluated to `true` at runtime.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Uses</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Assertions are always checked in both debug and release builds, and cannot</span>
<span class="doccomment">/// be disabled. See [`debug_assert!`] for assertions that are not enabled in</span>
<span class="doccomment">/// release builds by default.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Unsafe code relies on `assert!` to enforce run-time invariants that, if</span>
<span class="doccomment">/// violated could lead to unsafety.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Other use-cases of `assert!` include [testing] and enforcing run-time</span>
<span class="doccomment">/// invariants in safe code (whose violation cannot result in unsafety).</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Custom Messages</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This macro has a second form, where a custom panic message can</span>
<span class="doccomment">/// be provided with or without arguments for formatting. See [`std::fmt`]</span>
<span class="doccomment">/// for syntax for this form.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`panic!`]: macro.panic.html</span>
<span class="doccomment">/// [`debug_assert!`]: macro.debug_assert.html</span>
<span class="doccomment">/// [testing]: ../book/second-edition/ch11-01-writing-tests.html#checking-results-with-the-assert-macro</span>
<span class="doccomment">/// [`std::fmt`]: ../std/fmt/index.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// // the panic message for these assertions is the stringified value of the</span>
<span class="doccomment">/// // expression given.</span>
<span class="doccomment">/// assert!(true);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// fn some_computation() -> bool { true } // a very simple function</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert!(some_computation());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // assert with a custom message</span>
<span class="doccomment">/// let x = true;</span>
<span class="doccomment">/// assert!(x, "x wasn't true!");</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let a = 3; let b = 27;</span>
<span class="doccomment">/// assert!(a + b == 30, "a = {}, b = {}", a, b);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">assert</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">cond</span>:<span class="ident">expr</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">cond</span>:<span class="ident">expr</span>,) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">cond</span>:<span class="ident">expr</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>:<span class="ident">tt</span>)<span class="op">+</span>) <span class="op">=></span> ({ <span class="comment">/* compiler built-in */</span> });
}
}
<span class="doccomment">/// A macro for defining `#[cfg]` if-else statements.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This is similar to the `if/elif` C preprocessor macro by allowing definition</span>
<span class="doccomment">/// of a cascade of `#[cfg]` cases, emitting the implementation which matches</span>
<span class="doccomment">/// first.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code</span>
<span class="doccomment">/// without having to rewrite each clause multiple times.</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">cfg_if</span> {
($(
<span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meta</span>:<span class="ident">meta</span>),<span class="kw-2">*</span>)]</span> { $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">it</span>:<span class="ident">item</span>)<span class="op">*</span> }
) <span class="kw">else</span> <span class="op">*</span> <span class="kw">else</span> {
$(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">it2</span>:<span class="ident">item</span>)<span class="op">*</span>
}) <span class="op">=></span> {
<span class="macro">__cfg_if_items</span><span class="macro">!</span> {
() ;
$( ( ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meta</span>),<span class="kw-2">*</span>) ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">it</span>)<span class="kw-2">*</span>) ), )<span class="op">*</span>
( () ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">it2</span>)<span class="kw-2">*</span>) ),
}
}
}
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">__cfg_if_items</span> {
(($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">not</span>:<span class="ident">meta</span>,)<span class="kw-2">*</span>) ; ) <span class="op">=></span> {};
(($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">not</span>:<span class="ident">meta</span>,)<span class="kw-2">*</span>) ; ( ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">m</span>:<span class="ident">meta</span>),<span class="kw-2">*</span>) ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">it</span>:<span class="ident">item</span>)<span class="kw-2">*</span>) ), $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">rest</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> {
<span class="macro">__cfg_if_apply</span><span class="macro">!</span> { <span class="ident">cfg</span>(<span class="ident">all</span>(<span class="ident">not</span>(<span class="ident">any</span>($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">not</span>),<span class="kw-2">*</span>)), $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">m</span>,)<span class="kw-2">*</span>)), $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">it</span>)<span class="op">*</span> }
<span class="macro">__cfg_if_items</span><span class="macro">!</span> { ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">not</span>,)<span class="op">*</span> $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">m</span>,)<span class="kw-2">*</span>) ; $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">rest</span>)<span class="op">*</span> }
}
}
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">__cfg_if_apply</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">m</span>:<span class="ident">meta</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">it</span>:<span class="ident">item</span>)<span class="kw-2">*</span>) <span class="op">=></span> {
$(<span class="attribute">#[<span class="macro-nonterminal">$</span><span class="macro-nonterminal">m</span>]</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">it</span>)<span class="op">*</span>
}
}
</pre>
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