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 /// panic entirely. Each thread's panic can be reaped as the `Box<Any>` type,
21 /// and the single-argument form of the `panic!` macro will be the value which
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 $crate::rt::begin_panic($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 $crate::rt::begin_panic_fmt(&format_args!($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
71 /// Use `print!` only for the primary output of your program. Use
72 /// `eprint!` instead to print error and progress messages.
76 /// Panics if writing to `io::stdout()` fails.
81 /// use std::io::{self, Write};
91 /// io::stdout().flush().unwrap();
93 /// print!("this string has a newline, why not choose println! instead?\n");
95 /// io::stdout().flush().unwrap();
98 #[stable(feature = "rust1", since = "1.0.0")]
99 #[allow_internal_unstable]
101 ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
104 /// Macro for printing to the standard output, with a newline. On all
105 /// platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
106 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
108 /// Use the `format!` syntax to write data to the standard output.
109 /// See `std::fmt` for more information.
111 /// Use `println!` only for the primary output of your program. Use
112 /// `eprintln!` instead to print error and progress messages.
116 /// Panics if writing to `io::stdout` fails.
121 /// println!(); // prints just a newline
122 /// println!("hello there!");
123 /// println!("format {} arguments", "some");
126 #[stable(feature = "rust1", since = "1.0.0")]
127 macro_rules! println {
128 () => (print!("\n"));
129 ($fmt:expr) => (print!(concat!($fmt, "\n")));
130 ($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*));
133 /// Macro for printing to the standard error.
135 /// Equivalent to the `print!` macro, except that output goes to
136 /// `io::stderr` instead of `io::stdout`. See `print!` for
139 /// Use `eprint!` only for error and progress messages. Use `print!`
140 /// instead for the primary output of your program.
144 /// Panics if writing to `io::stderr` fails.
146 #[stable(feature = "eprint", since = "1.19.0")]
147 #[allow_internal_unstable]
148 macro_rules! eprint {
149 ($($arg:tt)*) => ($crate::io::_eprint(format_args!($($arg)*)));
152 /// Macro for printing to the standard error, with a newline.
154 /// Equivalent to the `println!` macro, except that output goes to
155 /// `io::stderr` instead of `io::stdout`. See `println!` for
158 /// Use `eprintln!` only for error and progress messages. Use `println!`
159 /// instead for the primary output of your program.
163 /// Panics if writing to `io::stderr` fails.
165 #[stable(feature = "eprint", since = "1.19.0")]
166 macro_rules! eprintln {
167 () => (eprint!("\n"));
168 ($fmt:expr) => (eprint!(concat!($fmt, "\n")));
169 ($fmt:expr, $($arg:tt)*) => (eprint!(concat!($fmt, "\n"), $($arg)*));
172 /// A macro to select an event from a number of receivers.
174 /// This macro is used to wait for the first event to occur on a number of
175 /// receivers. It places no restrictions on the types of receivers given to
176 /// this macro, this can be viewed as a heterogeneous select.
181 /// #![feature(mpsc_select)]
184 /// use std::sync::mpsc;
186 /// // two placeholder functions for now
187 /// fn long_running_thread() {}
188 /// fn calculate_the_answer() -> u32 { 42 }
190 /// let (tx1, rx1) = mpsc::channel();
191 /// let (tx2, rx2) = mpsc::channel();
193 /// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); });
194 /// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); });
197 /// _ = rx1.recv() => println!("the long running thread finished first"),
198 /// answer = rx2.recv() => {
199 /// println!("the answer was: {}", answer.unwrap());
202 /// # drop(rx1.recv());
203 /// # drop(rx2.recv());
206 /// For more information about select, see the `std::sync::mpsc::Select` structure.
208 #[unstable(feature = "mpsc_select", issue = "27800")]
209 macro_rules! select {
211 $($name:pat = $rx:ident.$meth:ident() => $code:expr),+
213 use $crate::sync::mpsc::Select;
214 let sel = Select::new();
215 $( let mut $rx = sel.handle(&$rx); )+
219 let ret = sel.wait();
220 $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+
226 macro_rules! assert_approx_eq {
227 ($a:expr, $b:expr) => ({
228 let (a, b) = (&$a, &$b);
229 assert!((*a - *b).abs() < 1.0e-6,
230 "{} is not approximately equal to {}", *a, *b);
234 /// Built-in macros to the compiler itself.
236 /// These macros do not have any corresponding definition with a `macro_rules!`
237 /// macro, but are documented here. Their implementations can be found hardcoded
238 /// into libsyntax itself.
242 /// Unconditionally causes compilation to fail with the given error message when encountered.
244 /// For more information, see the [RFC].
246 /// [RFC]: https://github.com/rust-lang/rfcs/blob/master/text/1695-add-error-macro.md
247 #[unstable(feature = "compile_error_macro", issue = "40872")]
249 macro_rules! compile_error { ($msg:expr) => ({ /* compiler built-in */ }) }
251 /// The core macro for formatted string creation & output.
253 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
254 /// passed to the functions in [`std::fmt`] for performing useful functions.
255 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
256 /// proxied through this one.
258 /// For more information, see the documentation in [`std::fmt`].
260 /// [`fmt::Arguments`]: ../std/fmt/struct.Arguments.html
261 /// [`std::fmt`]: ../std/fmt/index.html
262 /// [`format!`]: ../std/macro.format.html
263 /// [`write!`]: ../std/macro.write.html
264 /// [`println!`]: ../std/macro.println.html
271 /// let s = fmt::format(format_args!("hello {}", "world"));
272 /// assert_eq!(s, format!("hello {}", "world"));
275 #[stable(feature = "rust1", since = "1.0.0")]
277 macro_rules! format_args { ($fmt:expr, $($args:tt)*) => ({
278 /* compiler built-in */
281 /// Inspect an environment variable at compile time.
283 /// This macro will expand to the value of the named environment variable at
284 /// compile time, yielding an expression of type `&'static str`.
286 /// If the environment variable is not defined, then a compilation error
287 /// will be emitted. To not emit a compile error, use the `option_env!`
293 /// let path: &'static str = env!("PATH");
294 /// println!("the $PATH variable at the time of compiling was: {}", path);
296 #[stable(feature = "rust1", since = "1.0.0")]
298 macro_rules! env { ($name:expr) => ({ /* compiler built-in */ }) }
300 /// Optionally inspect an environment variable at compile time.
302 /// If the named environment variable is present at compile time, this will
303 /// expand into an expression of type `Option<&'static str>` whose value is
304 /// `Some` of the value of the environment variable. If the environment
305 /// variable is not present, then this will expand to `None`.
307 /// A compile time error is never emitted when using this macro regardless
308 /// of whether the environment variable is present or not.
313 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
314 /// println!("the secret key might be: {:?}", key);
316 #[stable(feature = "rust1", since = "1.0.0")]
318 macro_rules! option_env { ($name:expr) => ({ /* compiler built-in */ }) }
320 /// Concatenate identifiers into one identifier.
322 /// This macro takes any number of comma-separated identifiers, and
323 /// concatenates them all into one, yielding an expression which is a new
324 /// identifier. Note that hygiene makes it such that this macro cannot
325 /// capture local variables. Also, as a general rule, macros are only
326 /// allowed in item, statement or expression position. That means while
327 /// you may use this macro for referring to existing variables, functions or
328 /// modules etc, you cannot define a new one with it.
333 /// #![feature(concat_idents)]
336 /// fn foobar() -> u32 { 23 }
338 /// let f = concat_idents!(foo, bar);
339 /// println!("{}", f());
341 /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
344 #[unstable(feature = "concat_idents_macro", issue = "29599")]
346 macro_rules! concat_idents {
347 ($($e:ident),*) => ({ /* compiler built-in */ })
350 /// Concatenates literals into a static string slice.
352 /// This macro takes any number of comma-separated literals, yielding an
353 /// expression of type `&'static str` which represents all of the literals
354 /// concatenated left-to-right.
356 /// Integer and floating point literals are stringified in order to be
362 /// let s = concat!("test", 10, 'b', true);
363 /// assert_eq!(s, "test10btrue");
365 #[stable(feature = "rust1", since = "1.0.0")]
367 macro_rules! concat { ($($e:expr),*) => ({ /* compiler built-in */ }) }
369 /// A macro which expands to the line number on which it was invoked.
371 /// The expanded expression has type `u32`, and the returned line is not
372 /// the invocation of the `line!()` macro itself, but rather the first macro
373 /// invocation leading up to the invocation of the `line!()` macro.
378 /// let current_line = line!();
379 /// println!("defined on line: {}", current_line);
381 #[stable(feature = "rust1", since = "1.0.0")]
383 macro_rules! line { () => ({ /* compiler built-in */ }) }
385 /// A macro which expands to the column number on which it was invoked.
387 /// The expanded expression has type `u32`, and the returned column is not
388 /// the invocation of the `column!()` macro itself, but rather the first macro
389 /// invocation leading up to the invocation of the `column!()` macro.
394 /// let current_col = column!();
395 /// println!("defined on column: {}", current_col);
397 #[stable(feature = "rust1", since = "1.0.0")]
399 macro_rules! column { () => ({ /* compiler built-in */ }) }
401 /// A macro which expands to the file name from which it was invoked.
403 /// The expanded expression has type `&'static str`, and the returned file
404 /// is not the invocation of the `file!()` macro itself, but rather the
405 /// first macro invocation leading up to the invocation of the `file!()`
411 /// let this_file = file!();
412 /// println!("defined in file: {}", this_file);
414 #[stable(feature = "rust1", since = "1.0.0")]
416 macro_rules! file { () => ({ /* compiler built-in */ }) }
418 /// A macro which stringifies its argument.
420 /// This macro will yield an expression of type `&'static str` which is the
421 /// stringification of all the tokens passed to the macro. No restrictions
422 /// are placed on the syntax of the macro invocation itself.
424 /// Note that the expanded results of the input tokens may change in the
425 /// future. You should be careful if you rely on the output.
430 /// let one_plus_one = stringify!(1 + 1);
431 /// assert_eq!(one_plus_one, "1 + 1");
433 #[stable(feature = "rust1", since = "1.0.0")]
435 macro_rules! stringify { ($t:tt) => ({ /* compiler built-in */ }) }
437 /// Includes a utf8-encoded file as a string.
439 /// The file is located relative to the current file. (similarly to how
440 /// modules are found)
442 /// This macro will yield an expression of type `&'static str` which is the
443 /// contents of the file.
447 /// ```ignore (cannot-doctest-external-file-dependency)
448 /// let secret_key = include_str!("secret-key.ascii");
450 #[stable(feature = "rust1", since = "1.0.0")]
452 macro_rules! include_str { ($file:expr) => ({ /* compiler built-in */ }) }
454 /// Includes a file as a reference to a byte array.
456 /// The file is located relative to the current file. (similarly to how
457 /// modules are found)
459 /// This macro will yield an expression of type `&'static [u8; N]` which is
460 /// the contents of the file.
464 /// ```ignore (cannot-doctest-external-file-dependency)
465 /// let secret_key = include_bytes!("secret-key.bin");
467 #[stable(feature = "rust1", since = "1.0.0")]
469 macro_rules! include_bytes { ($file:expr) => ({ /* compiler built-in */ }) }
471 /// Expands to a string that represents the current module path.
473 /// The current module path can be thought of as the hierarchy of modules
474 /// leading back up to the crate root. The first component of the path
475 /// returned is the name of the crate currently being compiled.
482 /// assert!(module_path!().ends_with("test"));
488 #[stable(feature = "rust1", since = "1.0.0")]
490 macro_rules! module_path { () => ({ /* compiler built-in */ }) }
492 /// Boolean evaluation of configuration flags.
494 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
495 /// boolean expression evaluation of configuration flags. This frequently
496 /// leads to less duplicated code.
498 /// The syntax given to this macro is the same syntax as [the `cfg`
499 /// attribute](../book/first-edition/conditional-compilation.html).
504 /// let my_directory = if cfg!(windows) {
505 /// "windows-specific-directory"
510 #[stable(feature = "rust1", since = "1.0.0")]
512 macro_rules! cfg { ($($cfg:tt)*) => ({ /* compiler built-in */ }) }
514 /// Parse a file as an expression or an item according to the context.
516 /// The file is located relative to the current file (similarly to how
517 /// modules are found).
519 /// Using this macro is often a bad idea, because if the file is
520 /// parsed as an expression, it is going to be placed in the
521 /// surrounding code unhygienically. This could result in variables
522 /// or functions being different from what the file expected if
523 /// there are variables or functions that have the same name in
524 /// the current file.
528 /// Assume there are two files in the same directory with the following
531 /// File 'my_str.in':
533 /// ```ignore (only-for-syntax-highlight)
539 /// ```ignore (cannot-doctest-external-file-dependency)
541 /// let my_str = include!("my_str.in");
542 /// println!("{}", my_str);
546 /// Compiling 'main.rs' and running the resulting binary will print "Hello
548 #[stable(feature = "rust1", since = "1.0.0")]
550 macro_rules! include { ($file:expr) => ({ /* compiler built-in */ }) }