1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Standard library macros
13 //! This modules contains a set of macros which are exported from the standard
14 //! library. Each macro is available for use when linking against the standard
17 /// The entry point for panic of Rust threads.
19 /// This macro is used to inject panic into a Rust thread, causing the thread to
20 /// unwind and panic entirely. Each thread's panic can be reaped as the
21 /// `Box<Any>` type, and the single-argument form of the `panic!` macro will be
22 /// the value which is transmitted.
24 /// The multi-argument form of this macro panics with a string and has the
25 /// `format!` syntax for building a string.
30 /// # #![allow(unreachable_code)]
32 /// panic!("this is a terrible mistake!");
33 /// panic!(4); // panic with the value of 4 to be collected elsewhere
34 /// panic!("this is a {} {message}", "fancy", message = "message");
37 #[stable(feature = "rust1", since = "1.0.0")]
38 #[allow_internal_unstable]
41 panic!("explicit panic")
44 $crate::rt::begin_unwind($msg, {
45 // static requires less code at runtime, more constant data
46 static _FILE_LINE: (&'static str, u32) = (file!(), line!());
50 ($fmt:expr, $($arg:tt)+) => ({
51 $crate::rt::begin_unwind_fmt(format_args!($fmt, $($arg)+), {
52 // The leading _'s are to avoid dead code warnings if this is
53 // used inside a dead function. Just `#[allow(dead_code)]` is
54 // insufficient, since the user may have
55 // `#[forbid(dead_code)]` and which cannot be overridden.
56 static _FILE_LINE: (&'static str, u32) = (file!(), line!());
62 /// Macro for printing to the standard output.
64 /// Equivalent to the `println!` macro except that a newline is not printed at
65 /// the end of the message.
67 /// Note that stdout is frequently line-buffered by default so it may be
68 /// necessary to use `io::stdout().flush()` to ensure the output is emitted
74 /// use std::io::{self, Write};
84 /// io::stdout().flush().unwrap();
86 /// print!("this string has a newline, why not choose println! instead?\n");
88 /// io::stdout().flush().unwrap();
91 #[stable(feature = "rust1", since = "1.0.0")]
92 #[allow_internal_unstable]
94 ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
97 /// Macro for printing to the standard output.
99 /// Use the `format!` syntax to write data to the standard output.
100 /// See `std::fmt` for more information.
105 /// println!("hello there!");
106 /// println!("format {} arguments", "some");
109 #[stable(feature = "rust1", since = "1.0.0")]
110 macro_rules! println {
111 ($fmt:expr) => (print!(concat!($fmt, "\n")));
112 ($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*));
115 /// Helper macro for unwrapping `Result` values while returning early with an
116 /// error if the value of the expression is `Err`. Can only be used in
117 /// functions that return `Result` because of the early return of `Err` that
124 /// use std::fs::File;
125 /// use std::io::prelude::*;
127 /// fn write_to_file_using_try() -> Result<(), io::Error> {
128 /// let mut file = try!(File::create("my_best_friends.txt"));
129 /// try!(file.write_all(b"This is a list of my best friends."));
130 /// println!("I wrote to the file");
133 /// // This is equivalent to:
134 /// fn write_to_file_using_match() -> Result<(), io::Error> {
135 /// let mut file = try!(File::create("my_best_friends.txt"));
136 /// match file.write_all(b"This is a list of my best friends.") {
138 /// Err(e) => return Err(e),
140 /// println!("I wrote to the file");
145 #[stable(feature = "rust1", since = "1.0.0")]
147 ($expr:expr) => (match $expr {
148 $crate::result::Result::Ok(val) => val,
149 $crate::result::Result::Err(err) => {
150 return $crate::result::Result::Err($crate::convert::From::from(err))
155 /// A macro to select an event from a number of receivers.
157 /// This macro is used to wait for the first event to occur on a number of
158 /// receivers. It places no restrictions on the types of receivers given to
159 /// this macro, this can be viewed as a heterogeneous select.
164 /// #![feature(mpsc_select)]
167 /// use std::sync::mpsc;
169 /// // two placeholder functions for now
170 /// fn long_running_thread() {}
171 /// fn calculate_the_answer() -> u32 { 42 }
173 /// let (tx1, rx1) = mpsc::channel();
174 /// let (tx2, rx2) = mpsc::channel();
176 /// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); });
177 /// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); });
180 /// _ = rx1.recv() => println!("the long running thread finished first"),
181 /// answer = rx2.recv() => {
182 /// println!("the answer was: {}", answer.unwrap());
185 /// # drop(rx1.recv());
186 /// # drop(rx2.recv());
189 /// For more information about select, see the `std::sync::mpsc::Select` structure.
191 #[unstable(feature = "mpsc_select", issue = "27800")]
192 macro_rules! select {
194 $($name:pat = $rx:ident.$meth:ident() => $code:expr),+
196 use $crate::sync::mpsc::Select;
197 let sel = Select::new();
198 $( let mut $rx = sel.handle(&$rx); )+
202 let ret = sel.wait();
203 $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+
208 // When testing the standard library, we link to the liblog crate to get the
209 // logging macros. In doing so, the liblog crate was linked against the real
210 // version of libstd, and uses a different std::fmt module than the test crate
211 // uses. To get around this difference, we redefine the log!() macro here to be
212 // just a dumb version of what it should be.
215 ($lvl:expr, $($args:tt)*) => (
216 if log_enabled!($lvl) { println!($($args)*) }
221 macro_rules! assert_approx_eq {
222 ($a:expr, $b:expr) => ({
223 let (a, b) = (&$a, &$b);
224 assert!((*a - *b).abs() < 1.0e-6,
225 "{} is not approximately equal to {}", *a, *b);
229 /// Built-in macros to the compiler itself.
231 /// These macros do not have any corresponding definition with a `macro_rules!`
232 /// macro, but are documented here. Their implementations can be found hardcoded
233 /// into libsyntax itself.
236 /// The core macro for formatted string creation & output.
238 /// This macro produces a value of type `fmt::Arguments`. This value can be
239 /// passed to the functions in `std::fmt` for performing useful functions.
240 /// All other formatting macros (`format!`, `write!`, `println!`, etc) are
241 /// proxied through this one.
243 /// For more information, see the documentation in `std::fmt`.
250 /// let s = fmt::format(format_args!("hello {}", "world"));
251 /// assert_eq!(s, format!("hello {}", "world"));
255 macro_rules! format_args { ($fmt:expr, $($args:tt)*) => ({
256 /* compiler built-in */
259 /// Inspect an environment variable at compile time.
261 /// This macro will expand to the value of the named environment variable at
262 /// compile time, yielding an expression of type `&'static str`.
264 /// If the environment variable is not defined, then a compilation error
265 /// will be emitted. To not emit a compile error, use the `option_env!`
271 /// let path: &'static str = env!("PATH");
272 /// println!("the $PATH variable at the time of compiling was: {}", path);
275 macro_rules! env { ($name:expr) => ({ /* compiler built-in */ }) }
277 /// Optionally inspect an environment variable at compile time.
279 /// If the named environment variable is present at compile time, this will
280 /// expand into an expression of type `Option<&'static str>` whose value is
281 /// `Some` of the value of the environment variable. If the environment
282 /// variable is not present, then this will expand to `None`.
284 /// A compile time error is never emitted when using this macro regardless
285 /// of whether the environment variable is present or not.
290 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
291 /// println!("the secret key might be: {:?}", key);
294 macro_rules! option_env { ($name:expr) => ({ /* compiler built-in */ }) }
296 /// Concatenate identifiers into one identifier.
298 /// This macro takes any number of comma-separated identifiers, and
299 /// concatenates them all into one, yielding an expression which is a new
300 /// identifier. Note that hygiene makes it such that this macro cannot
301 /// capture local variables, and macros are only allowed in item,
302 /// statement or expression position, meaning this macro may be difficult to
303 /// use in some situations.
308 /// #![feature(concat_idents)]
311 /// fn foobar() -> u32 { 23 }
313 /// let f = concat_idents!(foo, bar);
314 /// println!("{}", f());
318 macro_rules! concat_idents {
319 ($($e:ident),*) => ({ /* compiler built-in */ })
322 /// Concatenates literals into a static string slice.
324 /// This macro takes any number of comma-separated literals, yielding an
325 /// expression of type `&'static str` which represents all of the literals
326 /// concatenated left-to-right.
328 /// Integer and floating point literals are stringified in order to be
334 /// let s = concat!("test", 10, 'b', true);
335 /// assert_eq!(s, "test10btrue");
338 macro_rules! concat { ($($e:expr),*) => ({ /* compiler built-in */ }) }
340 /// A macro which expands to the line number on which it was invoked.
342 /// The expanded expression has type `u32`, and the returned line is not
343 /// the invocation of the `line!()` macro itself, but rather the first macro
344 /// invocation leading up to the invocation of the `line!()` macro.
349 /// let current_line = line!();
350 /// println!("defined on line: {}", current_line);
353 macro_rules! line { () => ({ /* compiler built-in */ }) }
355 /// A macro which expands to the column number on which it was invoked.
357 /// The expanded expression has type `u32`, and the returned column is not
358 /// the invocation of the `column!()` macro itself, but rather the first macro
359 /// invocation leading up to the invocation of the `column!()` macro.
364 /// let current_col = column!();
365 /// println!("defined on column: {}", current_col);
368 macro_rules! column { () => ({ /* compiler built-in */ }) }
370 /// A macro which expands to the file name from which it was invoked.
372 /// The expanded expression has type `&'static str`, and the returned file
373 /// is not the invocation of the `file!()` macro itself, but rather the
374 /// first macro invocation leading up to the invocation of the `file!()`
380 /// let this_file = file!();
381 /// println!("defined in file: {}", this_file);
384 macro_rules! file { () => ({ /* compiler built-in */ }) }
386 /// A macro which stringifies its argument.
388 /// This macro will yield an expression of type `&'static str` which is the
389 /// stringification of all the tokens passed to the macro. No restrictions
390 /// are placed on the syntax of the macro invocation itself.
395 /// let one_plus_one = stringify!(1 + 1);
396 /// assert_eq!(one_plus_one, "1 + 1");
399 macro_rules! stringify { ($t:tt) => ({ /* compiler built-in */ }) }
401 /// Includes a utf8-encoded file as a string.
403 /// This macro will yield an expression of type `&'static str` which is the
404 /// contents of the filename specified. The file is located relative to the
405 /// current file (similarly to how modules are found),
410 /// let secret_key = include_str!("secret-key.ascii");
413 macro_rules! include_str { ($file:expr) => ({ /* compiler built-in */ }) }
415 /// Includes a file as a reference to a byte array.
417 /// This macro will yield an expression of type `&'static [u8; N]` which is
418 /// the contents of the filename specified. The file is located relative to
419 /// the current file (similarly to how modules are found),
424 /// let secret_key = include_bytes!("secret-key.bin");
427 macro_rules! include_bytes { ($file:expr) => ({ /* compiler built-in */ }) }
429 /// Expands to a string that represents the current module path.
431 /// The current module path can be thought of as the hierarchy of modules
432 /// leading back up to the crate root. The first component of the path
433 /// returned is the name of the crate currently being compiled.
440 /// assert!(module_path!().ends_with("test"));
447 macro_rules! module_path { () => ({ /* compiler built-in */ }) }
449 /// Boolean evaluation of configuration flags.
451 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
452 /// boolean expression evaluation of configuration flags. This frequently
453 /// leads to less duplicated code.
455 /// The syntax given to this macro is the same syntax as the `cfg`
461 /// let my_directory = if cfg!(windows) {
462 /// "windows-specific-directory"
468 macro_rules! cfg { ($cfg:tt) => ({ /* compiler built-in */ }) }
470 /// Parse the current given file as an expression.
472 /// This is generally a bad idea, because it's going to behave unhygienically.
478 /// include!("/path/to/a/file")
482 macro_rules! include { ($cfg:tt) => ({ /* compiler built-in */ }) }