1 //! Standard library macros
3 //! This modules contains a set of macros which are exported from the standard
4 //! library. Each macro is available for use when linking against the standard
7 /// The entry point for panic of Rust threads.
9 /// This allows a program to terminate immediately and provide feedback
10 /// to the caller of the program. `panic!` should be used when a program reaches
11 /// an unrecoverable problem.
13 /// This macro is the perfect way to assert conditions in example code and in
14 /// tests. `panic!` is closely tied with the `unwrap` method of both [`Option`]
15 /// and [`Result`][runwrap] enums. Both implementations call `panic!` when they are set
16 /// to None or Err variants.
18 /// This macro is used to inject panic into a Rust thread, causing the thread to
19 /// panic entirely. Each thread's panic can be reaped as the `Box<Any>` type,
20 /// and the single-argument form of the `panic!` macro will be the value which
23 /// [`Result`] enum is often a better solution for recovering from errors than
24 /// using the `panic!` macro. This macro should be used to avoid proceeding using
25 /// incorrect values, such as from external sources. Detailed information about
26 /// error handling is found in the [book].
28 /// The multi-argument form of this macro panics with a string and has the
29 /// [`format!`] syntax for building a string.
31 /// See also the macro [`compile_error!`], for raising errors during compilation.
33 /// [runwrap]: ../std/result/enum.Result.html#method.unwrap
34 /// [`Option`]: ../std/option/enum.Option.html#method.unwrap
35 /// [`Result`]: ../std/result/enum.Result.html
36 /// [`format!`]: ../std/macro.format.html
37 /// [`compile_error!`]: ../std/macro.compile_error.html
38 /// [book]: ../book/ch09-00-error-handling.html
40 /// # Current implementation
42 /// If the main thread panics it will terminate all your threads and end your
43 /// program with code `101`.
48 /// # #![allow(unreachable_code)]
50 /// panic!("this is a terrible mistake!");
51 /// panic!(4); // panic with the value of 4 to be collected elsewhere
52 /// panic!("this is a {} {message}", "fancy", message = "message");
55 #[stable(feature = "rust1", since = "1.0.0")]
56 #[allow_internal_unstable]
59 panic!("explicit panic")
62 $crate::rt::begin_panic($msg, &(file!(), line!(), __rust_unstable_column!()))
67 ($fmt:expr, $($arg:tt)+) => ({
68 $crate::rt::begin_panic_fmt(&format_args!($fmt, $($arg)+),
69 &(file!(), line!(), __rust_unstable_column!()))
73 /// Macro for printing to the standard output.
75 /// Equivalent to the [`println!`] macro except that a newline is not printed at
76 /// the end of the message.
78 /// Note that stdout is frequently line-buffered by default so it may be
79 /// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted
82 /// Use `print!` only for the primary output of your program. Use
83 /// [`eprint!`] instead to print error and progress messages.
85 /// [`println!`]: ../std/macro.println.html
86 /// [flush]: ../std/io/trait.Write.html#tymethod.flush
87 /// [`eprint!`]: ../std/macro.eprint.html
91 /// Panics if writing to `io::stdout()` fails.
96 /// use std::io::{self, Write};
106 /// io::stdout().flush().unwrap();
108 /// print!("this string has a newline, why not choose println! instead?\n");
110 /// io::stdout().flush().unwrap();
113 #[stable(feature = "rust1", since = "1.0.0")]
114 #[allow_internal_unstable]
116 ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
119 /// Macro for printing to the standard output, with a newline.
121 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
122 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
124 /// Use the [`format!`] syntax to write data to the standard output.
125 /// See [`std::fmt`] for more information.
127 /// Use `println!` only for the primary output of your program. Use
128 /// [`eprintln!`] instead to print error and progress messages.
130 /// [`format!`]: ../std/macro.format.html
131 /// [`std::fmt`]: ../std/fmt/index.html
132 /// [`eprintln!`]: ../std/macro.eprintln.html
135 /// Panics if writing to `io::stdout` fails.
140 /// println!(); // prints just a newline
141 /// println!("hello there!");
142 /// println!("format {} arguments", "some");
145 #[stable(feature = "rust1", since = "1.0.0")]
146 #[allow_internal_unstable]
147 macro_rules! println {
148 () => (print!("\n"));
150 $crate::io::_print(format_args_nl!($($arg)*));
154 /// Macro for printing to the standard error.
156 /// Equivalent to the [`print!`] macro, except that output goes to
157 /// [`io::stderr`] instead of `io::stdout`. See [`print!`] for
160 /// Use `eprint!` only for error and progress messages. Use `print!`
161 /// instead for the primary output of your program.
163 /// [`io::stderr`]: ../std/io/struct.Stderr.html
164 /// [`print!`]: ../std/macro.print.html
168 /// Panics if writing to `io::stderr` fails.
173 /// eprint!("Error: Could not complete task");
176 #[stable(feature = "eprint", since = "1.19.0")]
177 #[allow_internal_unstable]
178 macro_rules! eprint {
179 ($($arg:tt)*) => ($crate::io::_eprint(format_args!($($arg)*)));
182 /// Macro for printing to the standard error, with a newline.
184 /// Equivalent to the [`println!`] macro, except that output goes to
185 /// [`io::stderr`] instead of `io::stdout`. See [`println!`] for
188 /// Use `eprintln!` only for error and progress messages. Use `println!`
189 /// instead for the primary output of your program.
191 /// [`io::stderr`]: ../std/io/struct.Stderr.html
192 /// [`println!`]: ../std/macro.println.html
196 /// Panics if writing to `io::stderr` fails.
201 /// eprintln!("Error: Could not complete task");
204 #[stable(feature = "eprint", since = "1.19.0")]
205 #[allow_internal_unstable]
206 macro_rules! eprintln {
207 () => (eprint!("\n"));
209 $crate::io::_eprint(format_args_nl!($($arg)*));
213 /// A macro for quick and dirty debugging with which you can inspect
214 /// the value of a given expression. An example:
218 /// let b = dbg!(a * 2) + 1;
219 /// // ^-- prints: [src/main.rs:2] a * 2 = 4
220 /// assert_eq!(b, 5);
223 /// The macro works by using the `Debug` implementation of the type of
224 /// the given expression to print the value to [stderr] along with the
225 /// source location of the macro invocation as well as the source code
226 /// of the expression.
228 /// Invoking the macro on an expression moves and takes ownership of it
229 /// before returning the evaluated expression unchanged. If the type
230 /// of the expression does not implement `Copy` and you don't want
231 /// to give up ownership, you can instead borrow with `dbg!(&expr)`
232 /// for some expression `expr`.
234 /// Note that the macro is intended as a debugging tool and therefore you
235 /// should avoid having uses of it in version control for longer periods.
236 /// Use cases involving debug output that should be added to version control
237 /// may be better served by macros such as `debug!` from the `log` crate.
241 /// The exact output printed by this macro should not be relied upon
242 /// and is subject to future changes.
246 /// Panics if writing to `io::stderr` fails.
248 /// # Further examples
250 /// With a method call:
253 /// fn foo(n: usize) {
254 /// if let Some(_) = dbg!(n.checked_sub(4)) {
262 /// This prints to [stderr]:
265 /// [src/main.rs:4] n.checked_sub(4) = None
268 /// Naive factorial implementation:
271 /// fn factorial(n: u32) -> u32 {
272 /// if dbg!(n <= 1) {
275 /// dbg!(n * factorial(n - 1))
279 /// dbg!(factorial(4));
282 /// This prints to [stderr]:
285 /// [src/main.rs:3] n <= 1 = false
286 /// [src/main.rs:3] n <= 1 = false
287 /// [src/main.rs:3] n <= 1 = false
288 /// [src/main.rs:3] n <= 1 = true
289 /// [src/main.rs:4] 1 = 1
290 /// [src/main.rs:5] n * factorial(n - 1) = 2
291 /// [src/main.rs:5] n * factorial(n - 1) = 6
292 /// [src/main.rs:5] n * factorial(n - 1) = 24
293 /// [src/main.rs:11] factorial(4) = 24
296 /// The `dbg!(..)` macro moves the input:
299 /// /// A wrapper around `usize` which importantly is not Copyable.
301 /// struct NoCopy(usize);
303 /// let a = NoCopy(42);
304 /// let _ = dbg!(a); // <-- `a` is moved here.
305 /// let _ = dbg!(a); // <-- `a` is moved again; error!
308 /// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
310 #[stable(feature = "dbg_macro", since = "1.32.0")]
313 // Use of `match` here is intentional because it affects the lifetimes
314 // of temporaries - https://stackoverflow.com/a/48732525/1063961
317 eprintln!("[{}:{}] {} = {:#?}",
318 file!(), line!(), stringify!($val), &tmp);
325 /// A macro to await on an async call.
327 #[unstable(feature = "await_macro", issue = "50547")]
328 #[allow_internal_unstable]
329 #[allow_internal_unsafe]
334 if let $crate::task::Poll::Ready(x) =
335 $crate::future::poll_with_tls_waker(unsafe {
336 $crate::pin::Pin::new_unchecked(&mut pinned)
341 // FIXME(cramertj) prior to stabilizing await, we have to ensure that this
342 // can't be used to create a generator on stable via `|| await!()`.
348 /// A macro to select an event from a number of receivers.
350 /// This macro is used to wait for the first event to occur on a number of
351 /// receivers. It places no restrictions on the types of receivers given to
352 /// this macro, this can be viewed as a heterogeneous select.
357 /// #![feature(mpsc_select)]
360 /// use std::sync::mpsc;
362 /// // two placeholder functions for now
363 /// fn long_running_thread() {}
364 /// fn calculate_the_answer() -> u32 { 42 }
366 /// let (tx1, rx1) = mpsc::channel();
367 /// let (tx2, rx2) = mpsc::channel();
369 /// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); });
370 /// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); });
373 /// _ = rx1.recv() => println!("the long running thread finished first"),
374 /// answer = rx2.recv() => {
375 /// println!("the answer was: {}", answer.unwrap());
378 /// # drop(rx1.recv());
379 /// # drop(rx2.recv());
382 /// For more information about select, see the `std::sync::mpsc::Select` structure.
384 #[unstable(feature = "mpsc_select", issue = "27800")]
385 #[rustc_deprecated(since = "1.32.0",
386 reason = "channel selection will be removed in a future release")]
387 macro_rules! select {
389 $($name:pat = $rx:ident.$meth:ident() => $code:expr),+
391 use $crate::sync::mpsc::Select;
392 let sel = Select::new();
393 $( let mut $rx = sel.handle(&$rx); )+
397 let ret = sel.wait();
398 $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+
404 macro_rules! assert_approx_eq {
405 ($a:expr, $b:expr) => ({
406 let (a, b) = (&$a, &$b);
407 assert!((*a - *b).abs() < 1.0e-6,
408 "{} is not approximately equal to {}", *a, *b);
412 /// Built-in macros to the compiler itself.
414 /// These macros do not have any corresponding definition with a `macro_rules!`
415 /// macro, but are documented here. Their implementations can be found hardcoded
416 /// into libsyntax itself.
420 /// Unconditionally causes compilation to fail with the given error message when encountered.
422 /// This macro should be used when a crate uses a conditional compilation strategy to provide
423 /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
424 /// which emits an error at *runtime*, rather than during compilation.
428 /// Two such examples are macros and `#[cfg]` environments.
430 /// Emit better compiler error if a macro is passed invalid values. Without the final branch,
431 /// the compiler would still emit an error, but the error's message would not mention the two
435 /// macro_rules! give_me_foo_or_bar {
439 /// compile_error!("This macro only accepts `foo` or `bar`");
443 /// give_me_foo_or_bar!(neither);
444 /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
447 /// Emit compiler error if one of a number of features isn't available.
450 /// #[cfg(not(any(feature = "foo", feature = "bar")))]
451 /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.")
454 /// [`panic!`]: ../std/macro.panic.html
455 #[stable(feature = "compile_error_macro", since = "1.20.0")]
456 #[rustc_doc_only_macro]
457 macro_rules! compile_error {
458 ($msg:expr) => ({ /* compiler built-in */ });
459 ($msg:expr,) => ({ /* compiler built-in */ });
462 /// The core macro for formatted string creation & output.
464 /// This macro functions by taking a formatting string literal containing
465 /// `{}` for each additional argument passed. `format_args!` prepares the
466 /// additional parameters to ensure the output can be interpreted as a string
467 /// and canonicalizes the arguments into a single type. Any value that implements
468 /// the [`Display`] trait can be passed to `format_args!`, as can any
469 /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
471 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
472 /// passed to the macros within [`std::fmt`] for performing useful redirection.
473 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
474 /// proxied through this one. `format_args!`, unlike its derived macros, avoids
475 /// heap allocations.
477 /// You can use the [`fmt::Arguments`] value that `format_args!` returns
478 /// in `Debug` and `Display` contexts as seen below. The example also shows
479 /// that `Debug` and `Display` format to the same thing: the interpolated
480 /// format string in `format_args!`.
483 /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
484 /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
485 /// assert_eq!("1 foo 2", display);
486 /// assert_eq!(display, debug);
489 /// For more information, see the documentation in [`std::fmt`].
491 /// [`Display`]: ../std/fmt/trait.Display.html
492 /// [`Debug`]: ../std/fmt/trait.Debug.html
493 /// [`fmt::Arguments`]: ../std/fmt/struct.Arguments.html
494 /// [`std::fmt`]: ../std/fmt/index.html
495 /// [`format!`]: ../std/macro.format.html
496 /// [`write!`]: ../std/macro.write.html
497 /// [`println!`]: ../std/macro.println.html
504 /// let s = fmt::format(format_args!("hello {}", "world"));
505 /// assert_eq!(s, format!("hello {}", "world"));
507 #[stable(feature = "rust1", since = "1.0.0")]
508 #[rustc_doc_only_macro]
509 macro_rules! format_args {
510 ($fmt:expr) => ({ /* compiler built-in */ });
511 ($fmt:expr, $($args:tt)*) => ({ /* compiler built-in */ });
514 /// Inspect an environment variable at compile time.
516 /// This macro will expand to the value of the named environment variable at
517 /// compile time, yielding an expression of type `&'static str`.
519 /// If the environment variable is not defined, then a compilation error
520 /// will be emitted. To not emit a compile error, use the [`option_env!`]
523 /// [`option_env!`]: ../std/macro.option_env.html
528 /// let path: &'static str = env!("PATH");
529 /// println!("the $PATH variable at the time of compiling was: {}", path);
532 /// You can customize the error message by passing a string as the second
536 /// let doc: &'static str = env!("documentation", "what's that?!");
539 /// If the `documentation` environment variable is not defined, you'll get
540 /// the following error:
543 /// error: what's that?!
545 #[stable(feature = "rust1", since = "1.0.0")]
546 #[rustc_doc_only_macro]
548 ($name:expr) => ({ /* compiler built-in */ });
549 ($name:expr,) => ({ /* compiler built-in */ });
552 /// Optionally inspect an environment variable at compile time.
554 /// If the named environment variable is present at compile time, this will
555 /// expand into an expression of type `Option<&'static str>` whose value is
556 /// `Some` of the value of the environment variable. If the environment
557 /// variable is not present, then this will expand to `None`. See
558 /// [`Option<T>`][option] for more information on this type.
560 /// A compile time error is never emitted when using this macro regardless
561 /// of whether the environment variable is present or not.
563 /// [option]: ../std/option/enum.Option.html
568 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
569 /// println!("the secret key might be: {:?}", key);
571 #[stable(feature = "rust1", since = "1.0.0")]
572 #[rustc_doc_only_macro]
573 macro_rules! option_env {
574 ($name:expr) => ({ /* compiler built-in */ });
575 ($name:expr,) => ({ /* compiler built-in */ });
578 /// Concatenate identifiers into one identifier.
580 /// This macro takes any number of comma-separated identifiers, and
581 /// concatenates them all into one, yielding an expression which is a new
582 /// identifier. Note that hygiene makes it such that this macro cannot
583 /// capture local variables. Also, as a general rule, macros are only
584 /// allowed in item, statement or expression position. That means while
585 /// you may use this macro for referring to existing variables, functions or
586 /// modules etc, you cannot define a new one with it.
591 /// #![feature(concat_idents)]
594 /// fn foobar() -> u32 { 23 }
596 /// let f = concat_idents!(foo, bar);
597 /// println!("{}", f());
599 /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
602 #[unstable(feature = "concat_idents_macro", issue = "29599")]
603 #[rustc_doc_only_macro]
604 macro_rules! concat_idents {
605 ($($e:ident),+) => ({ /* compiler built-in */ });
606 ($($e:ident,)+) => ({ /* compiler built-in */ });
609 /// Concatenates literals into a static string slice.
611 /// This macro takes any number of comma-separated literals, yielding an
612 /// expression of type `&'static str` which represents all of the literals
613 /// concatenated left-to-right.
615 /// Integer and floating point literals are stringified in order to be
621 /// let s = concat!("test", 10, 'b', true);
622 /// assert_eq!(s, "test10btrue");
624 #[stable(feature = "rust1", since = "1.0.0")]
625 #[rustc_doc_only_macro]
626 macro_rules! concat {
627 ($($e:expr),*) => ({ /* compiler built-in */ });
628 ($($e:expr,)*) => ({ /* compiler built-in */ });
631 /// A macro which expands to the line number on which it was invoked.
633 /// With [`column!`] and [`file!`], these macros provide debugging information for
634 /// developers about the location within the source.
636 /// The expanded expression has type `u32` and is 1-based, so the first line
637 /// in each file evaluates to 1, the second to 2, etc. This is consistent
638 /// with error messages by common compilers or popular editors.
639 /// The returned line is *not necessarily* the line of the `line!` invocation itself,
640 /// but rather the first macro invocation leading up to the invocation
641 /// of the `line!` macro.
643 /// [`column!`]: macro.column.html
644 /// [`file!`]: macro.file.html
649 /// let current_line = line!();
650 /// println!("defined on line: {}", current_line);
652 #[stable(feature = "rust1", since = "1.0.0")]
653 #[rustc_doc_only_macro]
654 macro_rules! line { () => ({ /* compiler built-in */ }) }
656 /// A macro which expands to the column number on which it was invoked.
658 /// With [`line!`] and [`file!`], these macros provide debugging information for
659 /// developers about the location within the source.
661 /// The expanded expression has type `u32` and is 1-based, so the first column
662 /// in each line evaluates to 1, the second to 2, etc. This is consistent
663 /// with error messages by common compilers or popular editors.
664 /// The returned column is *not necessarily* the line of the `column!` invocation itself,
665 /// but rather the first macro invocation leading up to the invocation
666 /// of the `column!` macro.
668 /// [`line!`]: macro.line.html
669 /// [`file!`]: macro.file.html
674 /// let current_col = column!();
675 /// println!("defined on column: {}", current_col);
677 #[stable(feature = "rust1", since = "1.0.0")]
678 #[rustc_doc_only_macro]
679 macro_rules! column { () => ({ /* compiler built-in */ }) }
681 /// A macro which expands to the file name from which it was invoked.
683 /// With [`line!`] and [`column!`], these macros provide debugging information for
684 /// developers about the location within the source.
687 /// The expanded expression has type `&'static str`, and the returned file
688 /// is not the invocation of the `file!` macro itself, but rather the
689 /// first macro invocation leading up to the invocation of the `file!`
692 /// [`line!`]: macro.line.html
693 /// [`column!`]: macro.column.html
698 /// let this_file = file!();
699 /// println!("defined in file: {}", this_file);
701 #[stable(feature = "rust1", since = "1.0.0")]
702 #[rustc_doc_only_macro]
703 macro_rules! file { () => ({ /* compiler built-in */ }) }
705 /// A macro which stringifies its arguments.
707 /// This macro will yield an expression of type `&'static str` which is the
708 /// stringification of all the tokens passed to the macro. No restrictions
709 /// are placed on the syntax of the macro invocation itself.
711 /// Note that the expanded results of the input tokens may change in the
712 /// future. You should be careful if you rely on the output.
717 /// let one_plus_one = stringify!(1 + 1);
718 /// assert_eq!(one_plus_one, "1 + 1");
720 #[stable(feature = "rust1", since = "1.0.0")]
721 #[rustc_doc_only_macro]
722 macro_rules! stringify { ($($t:tt)*) => ({ /* compiler built-in */ }) }
724 /// Includes a utf8-encoded file as a string.
726 /// The file is located relative to the current file. (similarly to how
727 /// modules are found)
729 /// This macro will yield an expression of type `&'static str` which is the
730 /// contents of the file.
734 /// Assume there are two files in the same directory with the following
737 /// File 'spanish.in':
745 /// ```ignore (cannot-doctest-external-file-dependency)
747 /// let my_str = include_str!("spanish.in");
748 /// assert_eq!(my_str, "adiós\n");
749 /// print!("{}", my_str);
753 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
754 #[stable(feature = "rust1", since = "1.0.0")]
755 #[rustc_doc_only_macro]
756 macro_rules! include_str {
757 ($file:expr) => ({ /* compiler built-in */ });
758 ($file:expr,) => ({ /* compiler built-in */ });
761 /// Includes a file as a reference to a byte array.
763 /// The file is located relative to the current file. (similarly to how
764 /// modules are found)
766 /// This macro will yield an expression of type `&'static [u8; N]` which is
767 /// the contents of the file.
771 /// Assume there are two files in the same directory with the following
774 /// File 'spanish.in':
782 /// ```ignore (cannot-doctest-external-file-dependency)
784 /// let bytes = include_bytes!("spanish.in");
785 /// assert_eq!(bytes, b"adi\xc3\xb3s\n");
786 /// print!("{}", String::from_utf8_lossy(bytes));
790 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
791 #[stable(feature = "rust1", since = "1.0.0")]
792 #[rustc_doc_only_macro]
793 macro_rules! include_bytes {
794 ($file:expr) => ({ /* compiler built-in */ });
795 ($file:expr,) => ({ /* compiler built-in */ });
798 /// Expands to a string that represents the current module path.
800 /// The current module path can be thought of as the hierarchy of modules
801 /// leading back up to the crate root. The first component of the path
802 /// returned is the name of the crate currently being compiled.
809 /// assert!(module_path!().ends_with("test"));
815 #[stable(feature = "rust1", since = "1.0.0")]
816 #[rustc_doc_only_macro]
817 macro_rules! module_path { () => ({ /* compiler built-in */ }) }
819 /// Boolean evaluation of configuration flags, at compile-time.
821 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
822 /// boolean expression evaluation of configuration flags. This frequently
823 /// leads to less duplicated code.
825 /// The syntax given to this macro is the same syntax as the `cfg`
831 /// let my_directory = if cfg!(windows) {
832 /// "windows-specific-directory"
837 #[stable(feature = "rust1", since = "1.0.0")]
838 #[rustc_doc_only_macro]
839 macro_rules! cfg { ($($cfg:tt)*) => ({ /* compiler built-in */ }) }
841 /// Parse a file as an expression or an item according to the context.
843 /// The file is located relative to the current file (similarly to how
844 /// modules are found).
846 /// Using this macro is often a bad idea, because if the file is
847 /// parsed as an expression, it is going to be placed in the
848 /// surrounding code unhygienically. This could result in variables
849 /// or functions being different from what the file expected if
850 /// there are variables or functions that have the same name in
851 /// the current file.
855 /// Assume there are two files in the same directory with the following
858 /// File 'monkeys.in':
860 /// ```ignore (only-for-syntax-highlight)
865 /// .collect::<String>()
870 /// ```ignore (cannot-doctest-external-file-dependency)
872 /// let my_string = include!("monkeys.in");
873 /// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
874 /// println!("{}", my_string);
878 /// Compiling 'main.rs' and running the resulting binary will print
880 #[stable(feature = "rust1", since = "1.0.0")]
881 #[rustc_doc_only_macro]
882 macro_rules! include {
883 ($file:expr) => ({ /* compiler built-in */ });
884 ($file:expr,) => ({ /* compiler built-in */ });
887 /// Ensure that a boolean expression is `true` at runtime.
889 /// This will invoke the [`panic!`] macro if the provided expression cannot be
890 /// evaluated to `true` at runtime.
894 /// Assertions are always checked in both debug and release builds, and cannot
895 /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
896 /// release builds by default.
898 /// Unsafe code relies on `assert!` to enforce run-time invariants that, if
899 /// violated could lead to unsafety.
901 /// Other use-cases of `assert!` include testing and enforcing run-time
902 /// invariants in safe code (whose violation cannot result in unsafety).
904 /// # Custom Messages
906 /// This macro has a second form, where a custom panic message can
907 /// be provided with or without arguments for formatting. See [`std::fmt`]
908 /// for syntax for this form.
910 /// [`panic!`]: macro.panic.html
911 /// [`debug_assert!`]: macro.debug_assert.html
912 /// [`std::fmt`]: ../std/fmt/index.html
917 /// // the panic message for these assertions is the stringified value of the
918 /// // expression given.
921 /// fn some_computation() -> bool { true } // a very simple function
923 /// assert!(some_computation());
925 /// // assert with a custom message
927 /// assert!(x, "x wasn't true!");
929 /// let a = 3; let b = 27;
930 /// assert!(a + b == 30, "a = {}, b = {}", a, b);
932 #[stable(feature = "rust1", since = "1.0.0")]
933 #[rustc_doc_only_macro]
934 macro_rules! assert {
935 ($cond:expr) => ({ /* compiler built-in */ });
936 ($cond:expr,) => ({ /* compiler built-in */ });
937 ($cond:expr, $($arg:tt)+) => ({ /* compiler built-in */ });
941 /// A macro for defining `#[cfg]` if-else statements.
943 /// This is similar to the `if/elif` C preprocessor macro by allowing definition
944 /// of a cascade of `#[cfg]` cases, emitting the implementation which matches
947 /// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
948 /// without having to rewrite each clause multiple times.
949 macro_rules! cfg_if {
951 if #[cfg($($meta:meta),*)] { $($it:item)* }
957 $( ( ($($meta),*) ($($it)*) ), )*
963 macro_rules! __cfg_if_items {
964 (($($not:meta,)*) ; ) => {};
965 (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
966 __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* }
967 __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* }
971 macro_rules! __cfg_if_apply {
972 ($m:meta, $($it:item)*) => {