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 allows a program to to terminate immediately and provide feedback
20 /// to the caller of the program. `panic!` should be used when a program reaches
21 /// an unrecoverable problem.
23 /// This macro is the perfect way to assert conditions in example code and in
24 /// tests. `panic!` is closely tied with the `unwrap` method of both [`Option`]
25 /// and [`Result`][runwrap] enums. Both implementations call `panic!` when they are set
26 /// to None or Err variants.
28 /// This macro is used to inject panic into a Rust thread, causing the thread to
29 /// panic entirely. Each thread's panic can be reaped as the `Box<Any>` type,
30 /// and the single-argument form of the `panic!` macro will be the value which
33 /// [`Result`] enum is often a better solution for recovering from errors than
34 /// using the `panic!` macro. This macro should be used to avoid proceeding using
35 /// incorrect values, such as from external sources. Detailed information about
36 /// error handling is found in the [book].
38 /// The multi-argument form of this macro panics with a string and has the
39 /// [`format!`] syntax for building a string.
41 /// [runwrap]: ../std/result/enum.Result.html#method.unwrap
42 /// [`Option`]: ../std/option/enum.Option.html#method.unwrap
43 /// [`Result`]: ../std/result/enum.Result.html
44 /// [`format!`]: ../std/macro.format.html
45 /// [book]: ../book/second-edition/ch09-01-unrecoverable-errors-with-panic.html
47 /// # Current implementation
49 /// If the main thread panics it will terminate all your threads and end your
50 /// program with code `101`.
55 /// # #![allow(unreachable_code)]
57 /// panic!("this is a terrible mistake!");
58 /// panic!(4); // panic with the value of 4 to be collected elsewhere
59 /// panic!("this is a {} {message}", "fancy", message = "message");
62 #[stable(feature = "rust1", since = "1.0.0")]
63 #[allow_internal_unstable]
66 panic!("explicit panic")
69 $crate::rt::begin_panic($msg, &(file!(), line!(), __rust_unstable_column!()))
71 ($fmt:expr, $($arg:tt)+) => ({
72 $crate::rt::begin_panic_fmt(&format_args!($fmt, $($arg)+),
73 &(file!(), line!(), __rust_unstable_column!()))
77 /// Macro for printing to the standard output.
79 /// Equivalent to the [`println!`] macro except that a newline is not printed at
80 /// the end of the message.
82 /// Note that stdout is frequently line-buffered by default so it may be
83 /// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted
86 /// Use `print!` only for the primary output of your program. Use
87 /// [`eprint!`] instead to print error and progress messages.
89 /// [`println!`]: ../std/macro.println.html
90 /// [flush]: ../std/io/trait.Write.html#tymethod.flush
91 /// [`eprint!`]: ../std/macro.eprint.html
95 /// Panics if writing to `io::stdout()` fails.
100 /// use std::io::{self, Write};
110 /// io::stdout().flush().unwrap();
112 /// print!("this string has a newline, why not choose println! instead?\n");
114 /// io::stdout().flush().unwrap();
117 #[stable(feature = "rust1", since = "1.0.0")]
118 #[allow_internal_unstable]
120 ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
123 /// Macro for printing to the standard output, with a newline.
125 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
126 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
128 /// Use the [`format!`] syntax to write data to the standard output.
129 /// See [`std::fmt`] for more information.
131 /// Use `println!` only for the primary output of your program. Use
132 /// [`eprintln!`] instead to print error and progress messages.
134 /// [`format!`]: ../std/macro.format.html
135 /// [`std::fmt`]: ../std/fmt/index.html
136 /// [`eprintln!`]: ../std/macro.eprint.html
139 /// Panics if writing to `io::stdout` fails.
144 /// println!(); // prints just a newline
145 /// println!("hello there!");
146 /// println!("format {} arguments", "some");
149 #[stable(feature = "rust1", since = "1.0.0")]
150 macro_rules! println {
151 () => (print!("\n"));
152 ($fmt:expr) => (print!(concat!($fmt, "\n")));
153 ($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*));
156 /// Macro for printing to the standard error.
158 /// Equivalent to the [`print!`] macro, except that output goes to
159 /// [`io::stderr`] instead of `io::stdout`. See [`print!`] for
162 /// Use `eprint!` only for error and progress messages. Use `print!`
163 /// instead for the primary output of your program.
165 /// [`io::stderr`]: ../std/io/struct.Stderr.html
166 /// [`print!`]: ../std/macro.print.html
170 /// Panics if writing to `io::stderr` fails.
175 /// eprint!("Error: Could not complete task");
178 #[stable(feature = "eprint", since = "1.19.0")]
179 #[allow_internal_unstable]
180 macro_rules! eprint {
181 ($($arg:tt)*) => ($crate::io::_eprint(format_args!($($arg)*)));
184 /// Macro for printing to the standard error, with a newline.
186 /// Equivalent to the [`println!`] macro, except that output goes to
187 /// [`io::stderr`] instead of `io::stdout`. See [`println!`] for
190 /// Use `eprintln!` only for error and progress messages. Use `println!`
191 /// instead for the primary output of your program.
193 /// [`io::stderr`]: ../std/io/struct.Stderr.html
194 /// [`println!`]: ../std/macro.println.html
198 /// Panics if writing to `io::stderr` fails.
203 /// eprintln!("Error: Could not complete task");
206 #[stable(feature = "eprint", since = "1.19.0")]
207 macro_rules! eprintln {
208 () => (eprint!("\n"));
209 ($fmt:expr) => (eprint!(concat!($fmt, "\n")));
210 ($fmt:expr, $($arg:tt)*) => (eprint!(concat!($fmt, "\n"), $($arg)*));
213 /// A macro to select an event from a number of receivers.
215 /// This macro is used to wait for the first event to occur on a number of
216 /// receivers. It places no restrictions on the types of receivers given to
217 /// this macro, this can be viewed as a heterogeneous select.
222 /// #![feature(mpsc_select)]
225 /// use std::sync::mpsc;
227 /// // two placeholder functions for now
228 /// fn long_running_thread() {}
229 /// fn calculate_the_answer() -> u32 { 42 }
231 /// let (tx1, rx1) = mpsc::channel();
232 /// let (tx2, rx2) = mpsc::channel();
234 /// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); });
235 /// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); });
238 /// _ = rx1.recv() => println!("the long running thread finished first"),
239 /// answer = rx2.recv() => {
240 /// println!("the answer was: {}", answer.unwrap());
243 /// # drop(rx1.recv());
244 /// # drop(rx2.recv());
247 /// For more information about select, see the `std::sync::mpsc::Select` structure.
249 #[unstable(feature = "mpsc_select", issue = "27800")]
250 macro_rules! select {
252 $($name:pat = $rx:ident.$meth:ident() => $code:expr),+
254 use $crate::sync::mpsc::Select;
255 let sel = Select::new();
256 $( let mut $rx = sel.handle(&$rx); )+
260 let ret = sel.wait();
261 $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+
267 macro_rules! assert_approx_eq {
268 ($a:expr, $b:expr) => ({
269 let (a, b) = (&$a, &$b);
270 assert!((*a - *b).abs() < 1.0e-6,
271 "{} is not approximately equal to {}", *a, *b);
275 /// Built-in macros to the compiler itself.
277 /// These macros do not have any corresponding definition with a `macro_rules!`
278 /// macro, but are documented here. Their implementations can be found hardcoded
279 /// into libsyntax itself.
283 /// Unconditionally causes compilation to fail with the given error message when encountered.
285 /// This macro should be used when a crate uses a conditional compilation strategy to provide
286 /// better error messages for errornous conditions.
290 /// Two such examples are macros and `#[cfg]` environments.
292 /// Emit better compiler error if a macro is passed invalid values.
295 /// macro_rules! give_me_foo_or_bar {
299 /// compile_error!("This macro only accepts `foo` or `bar`");
303 /// give_me_foo_or_bar!(neither);
304 /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
307 /// Emit compiler error if one of a number of features isn't available.
310 /// #[cfg(not(any(feature = "foo", feature = "bar")))]
311 /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.")
313 #[stable(feature = "compile_error_macro", since = "1.20.0")]
315 macro_rules! compile_error { ($msg:expr) => ({ /* compiler built-in */ }) }
317 /// The core macro for formatted string creation & output.
319 /// This macro functions by taking a formatting string literal containing
320 /// `{}` for each additional argument passed. `format_args!` prepares the
321 /// additional parameters to ensure the output can be interpreted as a string
322 /// and canonicalizes the arguments into a single type. Any value that implements
323 /// the [`Display`] trait can be passed to `format_args!`, as can any
324 /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
326 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
327 /// passed to the macros within [`std::fmt`] for performing useful redirection.
328 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
329 /// proxied through this one. `format_args!`, unlike its derived macros, avoids
330 /// heap allocations.
332 /// For more information, see the documentation in [`std::fmt`].
334 /// [`Display`]: ../std/fmt/trait.Display.html
335 /// [`Debug`]: ../std/fmt/trait.Debug.html
336 /// [`fmt::Arguments`]: ../std/fmt/struct.Arguments.html
337 /// [`std::fmt`]: ../std/fmt/index.html
338 /// [`format!`]: ../std/macro.format.html
339 /// [`write!`]: ../std/macro.write.html
340 /// [`println!`]: ../std/macro.println.html
347 /// let s = fmt::format(format_args!("hello {}", "world"));
348 /// assert_eq!(s, format!("hello {}", "world"));
351 #[stable(feature = "rust1", since = "1.0.0")]
353 macro_rules! format_args {
354 ($fmt:expr) => ({ /* compiler built-in */ });
355 ($fmt:expr, $($args:tt)*) => ({ /* compiler built-in */ });
358 /// Inspect an environment variable at compile time.
360 /// This macro will expand to the value of the named environment variable at
361 /// compile time, yielding an expression of type `&'static str`.
363 /// If the environment variable is not defined, then a compilation error
364 /// will be emitted. To not emit a compile error, use the [`option_env!`]
367 /// [`option_env!`]: ../std/macro.option_env.html
372 /// let path: &'static str = env!("PATH");
373 /// println!("the $PATH variable at the time of compiling was: {}", path);
375 #[stable(feature = "rust1", since = "1.0.0")]
378 ($name:expr) => ({ /* compiler built-in */ });
379 ($name:expr,) => ({ /* compiler built-in */ });
382 /// Optionally inspect an environment variable at compile time.
384 /// If the named environment variable is present at compile time, this will
385 /// expand into an expression of type `Option<&'static str>` whose value is
386 /// `Some` of the value of the environment variable. If the environment
387 /// variable is not present, then this will expand to `None`. See
388 /// [`Option<T>`][option] for more information on this type.
390 /// A compile time error is never emitted when using this macro regardless
391 /// of whether the environment variable is present or not.
393 /// [option]: ../std/option/enum.Option.html
398 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
399 /// println!("the secret key might be: {:?}", key);
401 #[stable(feature = "rust1", since = "1.0.0")]
403 macro_rules! option_env { ($name:expr) => ({ /* compiler built-in */ }) }
405 /// Concatenate identifiers into one identifier.
407 /// This macro takes any number of comma-separated identifiers, and
408 /// concatenates them all into one, yielding an expression which is a new
409 /// identifier. Note that hygiene makes it such that this macro cannot
410 /// capture local variables. Also, as a general rule, macros are only
411 /// allowed in item, statement or expression position. That means while
412 /// you may use this macro for referring to existing variables, functions or
413 /// modules etc, you cannot define a new one with it.
418 /// #![feature(concat_idents)]
421 /// fn foobar() -> u32 { 23 }
423 /// let f = concat_idents!(foo, bar);
424 /// println!("{}", f());
426 /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
429 #[unstable(feature = "concat_idents_macro", issue = "29599")]
431 macro_rules! concat_idents {
432 ($($e:ident),*) => ({ /* compiler built-in */ });
433 ($($e:ident,)*) => ({ /* compiler built-in */ });
436 /// Concatenates literals into a static string slice.
438 /// This macro takes any number of comma-separated literals, yielding an
439 /// expression of type `&'static str` which represents all of the literals
440 /// concatenated left-to-right.
442 /// Integer and floating point literals are stringified in order to be
448 /// let s = concat!("test", 10, 'b', true);
449 /// assert_eq!(s, "test10btrue");
451 #[stable(feature = "rust1", since = "1.0.0")]
453 macro_rules! concat {
454 ($($e:expr),*) => ({ /* compiler built-in */ });
455 ($($e:expr,)*) => ({ /* compiler built-in */ });
458 /// A macro which expands to the line number on which it was invoked.
460 /// With [`column!`] and [`file!`], these macros provide debugging information for
461 /// developers about the location within the source.
463 /// The expanded expression has type `u32` and is 1-based, so the first line
464 /// in each file evaluates to 1, the second to 2, etc. This is consistent
465 /// with error messages by common compilers or popular editors.
466 /// The returned column is not the invocation of the `line!` macro itself,
467 /// but rather the first macro invocation leading up to the invocation
468 /// of the `line!` macro.
470 /// [`column!`]: macro.column.html
471 /// [`file!`]: macro.file.html
476 /// let current_line = line!();
477 /// println!("defined on line: {}", current_line);
479 #[stable(feature = "rust1", since = "1.0.0")]
481 macro_rules! line { () => ({ /* compiler built-in */ }) }
483 /// A macro which expands to the column number on which it was invoked.
485 /// With [`line!`] and [`file!`], these macros provide debugging information for
486 /// developers about the location within the source.
488 /// The expanded expression has type `u32` and is 1-based, so the first column
489 /// in each line evaluates to 1, the second to 2, etc. This is consistent
490 /// with error messages by common compilers or popular editors.
491 /// The returned column is not the invocation of the `column!` macro itself,
492 /// but rather the first macro invocation leading up to the invocation
493 /// of the `column!` macro.
495 /// [`line!`]: macro.line.html
496 /// [`file!`]: macro.file.html
501 /// let current_col = column!();
502 /// println!("defined on column: {}", current_col);
504 #[stable(feature = "rust1", since = "1.0.0")]
506 macro_rules! column { () => ({ /* compiler built-in */ }) }
508 /// A macro which expands to the file name from which it was invoked.
510 /// With [`line!`] and [`column!`], these macros provide debugging information for
511 /// developers about the location within the source.
514 /// The expanded expression has type `&'static str`, and the returned file
515 /// is not the invocation of the `file!` macro itself, but rather the
516 /// first macro invocation leading up to the invocation of the `file!`
519 /// [`line!`]: macro.line.html
520 /// [`column!`]: macro.column.html
525 /// let this_file = file!();
526 /// println!("defined in file: {}", this_file);
528 #[stable(feature = "rust1", since = "1.0.0")]
530 macro_rules! file { () => ({ /* compiler built-in */ }) }
532 /// A macro which stringifies its arguments.
534 /// This macro will yield an expression of type `&'static str` which is the
535 /// stringification of all the tokens passed to the macro. No restrictions
536 /// are placed on the syntax of the macro invocation itself.
538 /// Note that the expanded results of the input tokens may change in the
539 /// future. You should be careful if you rely on the output.
544 /// let one_plus_one = stringify!(1 + 1);
545 /// assert_eq!(one_plus_one, "1 + 1");
547 #[stable(feature = "rust1", since = "1.0.0")]
549 macro_rules! stringify { ($($t:tt)*) => ({ /* compiler built-in */ }) }
551 /// Includes a utf8-encoded file as a string.
553 /// The file is located relative to the current file. (similarly to how
554 /// modules are found)
556 /// This macro will yield an expression of type `&'static str` which is the
557 /// contents of the file.
561 /// Assume there are two files in the same directory with the following
564 /// File 'spanish.in':
572 /// ```ignore (cannot-doctest-external-file-dependency)
574 /// let my_str = include_str!("spanish.in");
575 /// assert_eq!(my_str, "adiós\n");
576 /// print!("{}", my_str);
580 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
581 #[stable(feature = "rust1", since = "1.0.0")]
583 macro_rules! include_str { ($file:expr) => ({ /* compiler built-in */ }) }
585 /// Includes a file as a reference to a byte array.
587 /// The file is located relative to the current file. (similarly to how
588 /// modules are found)
590 /// This macro will yield an expression of type `&'static [u8; N]` which is
591 /// the contents of the file.
595 /// Assume there are two files in the same directory with the following
598 /// File 'spanish.in':
606 /// ```ignore (cannot-doctest-external-file-dependency)
608 /// let bytes = include_bytes!("spanish.in");
609 /// assert_eq!(bytes, b"adi\xc3\xb3s\n");
610 /// print!("{}", String::from_utf8_lossy(bytes));
614 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
615 #[stable(feature = "rust1", since = "1.0.0")]
617 macro_rules! include_bytes { ($file:expr) => ({ /* compiler built-in */ }) }
619 /// Expands to a string that represents the current module path.
621 /// The current module path can be thought of as the hierarchy of modules
622 /// leading back up to the crate root. The first component of the path
623 /// returned is the name of the crate currently being compiled.
630 /// assert!(module_path!().ends_with("test"));
636 #[stable(feature = "rust1", since = "1.0.0")]
638 macro_rules! module_path { () => ({ /* compiler built-in */ }) }
640 /// Boolean evaluation of configuration flags, at compile-time.
642 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
643 /// boolean expression evaluation of configuration flags. This frequently
644 /// leads to less duplicated code.
646 /// The syntax given to this macro is the same syntax as [the `cfg`
647 /// attribute](../book/first-edition/conditional-compilation.html).
652 /// let my_directory = if cfg!(windows) {
653 /// "windows-specific-directory"
658 #[stable(feature = "rust1", since = "1.0.0")]
660 macro_rules! cfg { ($($cfg:tt)*) => ({ /* compiler built-in */ }) }
662 /// Parse a file as an expression or an item according to the context.
664 /// The file is located relative to the current file (similarly to how
665 /// modules are found).
667 /// Using this macro is often a bad idea, because if the file is
668 /// parsed as an expression, it is going to be placed in the
669 /// surrounding code unhygienically. This could result in variables
670 /// or functions being different from what the file expected if
671 /// there are variables or functions that have the same name in
672 /// the current file.
676 /// Assume there are two files in the same directory with the following
679 /// File 'monkeys.in':
681 /// ```ignore (only-for-syntax-highlight)
686 /// .collect::<String>()
691 /// ```ignore (cannot-doctest-external-file-dependency)
693 /// let my_string = include!("monkeys.in");
694 /// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
695 /// println!("{}", my_string);
699 /// Compiling 'main.rs' and running the resulting binary will print
701 #[stable(feature = "rust1", since = "1.0.0")]
703 macro_rules! include { ($file:expr) => ({ /* compiler built-in */ }) }
706 /// A macro for defining #[cfg] if-else statements.
708 /// This is similar to the `if/elif` C preprocessor macro by allowing definition
709 /// of a cascade of `#[cfg]` cases, emitting the implementation which matches
712 /// This allows you to conveniently provide a long list #[cfg]'d blocks of code
713 /// without having to rewrite each clause multiple times.
714 macro_rules! cfg_if {
716 if #[cfg($($meta:meta),*)] { $($it:item)* }
722 $( ( ($($meta),*) ($($it)*) ), )*
728 macro_rules! __cfg_if_items {
729 (($($not:meta,)*) ; ) => {};
730 (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
731 __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* }
732 __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* }
736 macro_rules! __cfg_if_apply {
737 ($m:meta, $($it:item)*) => {