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 /// Panics the current thread.
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 state.
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(__rust_unstable_column, libstd_sys_internals)]
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 /// Prints 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(print_internals)]
116 ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
119 /// Prints 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(print_internals, format_args_nl)]
147 macro_rules! println {
148 () => (print!("\n"));
150 $crate::io::_print(format_args_nl!($($arg)*));
154 /// Prints to the standard error.
156 /// Equivalent to the [`print!`] macro, except that output goes to
157 /// [`io::stderr`] instead of `io::stdout`. See [`print!`] for
160 /// Use `eprint!` only for error and progress messages. Use `print!`
161 /// instead for the primary output of your program.
163 /// [`io::stderr`]: ../std/io/struct.Stderr.html
164 /// [`print!`]: ../std/macro.print.html
168 /// Panics if writing to `io::stderr` fails.
173 /// eprint!("Error: Could not complete task");
176 #[stable(feature = "eprint", since = "1.19.0")]
177 #[allow_internal_unstable(print_internals)]
178 macro_rules! eprint {
179 ($($arg:tt)*) => ($crate::io::_eprint(format_args!($($arg)*)));
182 /// Prints 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(print_internals, format_args_nl)]
206 macro_rules! eprintln {
207 () => (eprint!("\n"));
209 $crate::io::_eprint(format_args_nl!($($arg)*));
213 /// Prints and returns the value of a given expression for quick and dirty
220 /// let b = dbg!(a * 2) + 1;
221 /// // ^-- prints: [src/main.rs:2] a * 2 = 4
222 /// assert_eq!(b, 5);
225 /// The macro works by using the `Debug` implementation of the type of
226 /// the given expression to print the value to [stderr] along with the
227 /// source location of the macro invocation as well as the source code
228 /// of the expression.
230 /// Invoking the macro on an expression moves and takes ownership of it
231 /// before returning the evaluated expression unchanged. If the type
232 /// of the expression does not implement `Copy` and you don't want
233 /// to give up ownership, you can instead borrow with `dbg!(&expr)`
234 /// for some expression `expr`.
236 /// The `dbg!` macro works exactly the same in release builds.
237 /// This is useful when debugging issues that only occur in release
238 /// builds or when debugging in release mode is significantly faster.
240 /// Note that the macro is intended as a debugging tool and therefore you
241 /// should avoid having uses of it in version control for longer periods.
242 /// Use cases involving debug output that should be added to version control
243 /// are better served by macros such as [`debug!`] from the [`log`] crate.
247 /// The exact output printed by this macro should not be relied upon
248 /// and is subject to future changes.
252 /// Panics if writing to `io::stderr` fails.
254 /// # Further examples
256 /// With a method call:
259 /// fn foo(n: usize) {
260 /// if let Some(_) = dbg!(n.checked_sub(4)) {
268 /// This prints to [stderr]:
271 /// [src/main.rs:4] n.checked_sub(4) = None
274 /// Naive factorial implementation:
277 /// fn factorial(n: u32) -> u32 {
278 /// if dbg!(n <= 1) {
281 /// dbg!(n * factorial(n - 1))
285 /// dbg!(factorial(4));
288 /// This prints to [stderr]:
291 /// [src/main.rs:3] n <= 1 = false
292 /// [src/main.rs:3] n <= 1 = false
293 /// [src/main.rs:3] n <= 1 = false
294 /// [src/main.rs:3] n <= 1 = true
295 /// [src/main.rs:4] 1 = 1
296 /// [src/main.rs:5] n * factorial(n - 1) = 2
297 /// [src/main.rs:5] n * factorial(n - 1) = 6
298 /// [src/main.rs:5] n * factorial(n - 1) = 24
299 /// [src/main.rs:11] factorial(4) = 24
302 /// The `dbg!(..)` macro moves the input:
305 /// /// A wrapper around `usize` which importantly is not Copyable.
307 /// struct NoCopy(usize);
309 /// let a = NoCopy(42);
310 /// let _ = dbg!(a); // <-- `a` is moved here.
311 /// let _ = dbg!(a); // <-- `a` is moved again; error!
314 /// You can also use `dbg!()` without a value to just print the
315 /// file and line whenever it's reached.
317 /// Finally, if you want to `dbg!(..)` multiple values, it will treat them as
318 /// a tuple (and return it, too):
321 /// assert_eq!(dbg!(1usize, 2u32), (1, 2));
324 /// However, a single argument with a trailing comma will still not be treated
325 /// as a tuple, following the convention of ignoring trailing commas in macro
326 /// invocations. You can use a 1-tuple directly if you need one:
329 /// assert_eq!(1, dbg!(1u32,)); // trailing comma ignored
330 /// assert_eq!((1,), dbg!((1u32,))); // 1-tuple
333 /// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
334 /// [`debug!`]: https://docs.rs/log/*/log/macro.debug.html
335 /// [`log`]: https://crates.io/crates/log
337 #[stable(feature = "dbg_macro", since = "1.32.0")]
340 eprintln!("[{}:{}]", file!(), line!());
343 // Use of `match` here is intentional because it affects the lifetimes
344 // of temporaries - https://stackoverflow.com/a/48732525/1063961
347 eprintln!("[{}:{}] {} = {:#?}",
348 file!(), line!(), stringify!($val), &tmp);
353 // Trailing comma with single argument is ignored
354 ($val:expr,) => { dbg!($val) };
355 ($($val:expr),+ $(,)?) => {
360 /// Awaits the completion of an async call.
362 #[unstable(feature = "await_macro", issue = "50547")]
363 #[allow_internal_unstable(gen_future, generators)]
364 #[allow_internal_unsafe]
365 macro_rules! r#await {
369 if let $crate::task::Poll::Ready(x) =
370 $crate::future::poll_with_tls_context(unsafe {
371 $crate::pin::Pin::new_unchecked(&mut pinned)
376 // FIXME(cramertj) prior to stabilizing await, we have to ensure that this
377 // can't be used to create a generator on stable via `|| await!()`.
383 /// Selects the first successful receive event from a number of receivers.
385 /// This macro is used to wait for the first event to occur on a number of
386 /// receivers. It places no restrictions on the types of receivers given to
387 /// this macro, this can be viewed as a heterogeneous select.
392 /// #![feature(mpsc_select)]
395 /// use std::sync::mpsc;
397 /// // two placeholder functions for now
398 /// fn long_running_thread() {}
399 /// fn calculate_the_answer() -> u32 { 42 }
401 /// let (tx1, rx1) = mpsc::channel();
402 /// let (tx2, rx2) = mpsc::channel();
404 /// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); });
405 /// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); });
408 /// _ = rx1.recv() => println!("the long running thread finished first"),
409 /// answer = rx2.recv() => {
410 /// println!("the answer was: {}", answer.unwrap());
413 /// # drop(rx1.recv());
414 /// # drop(rx2.recv());
417 /// For more information about select, see the `std::sync::mpsc::Select` structure.
419 #[unstable(feature = "mpsc_select", issue = "27800")]
420 #[rustc_deprecated(since = "1.32.0",
421 reason = "channel selection will be removed in a future release")]
422 macro_rules! select {
424 $($name:pat = $rx:ident.$meth:ident() => $code:expr),+
426 use $crate::sync::mpsc::Select;
427 let sel = Select::new();
428 $( let mut $rx = sel.handle(&$rx); )+
432 let ret = sel.wait();
433 $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+
439 macro_rules! assert_approx_eq {
440 ($a:expr, $b:expr) => ({
441 let (a, b) = (&$a, &$b);
442 assert!((*a - *b).abs() < 1.0e-6,
443 "{} is not approximately equal to {}", *a, *b);
447 /// Built-in macros to the compiler itself.
449 /// These macros do not have any corresponding definition with a `macro_rules!`
450 /// macro, but are documented here. Their implementations can be found hardcoded
451 /// into libsyntax itself.
455 /// Causes compilation to fail with the given error message when encountered.
457 /// This macro should be used when a crate uses a conditional compilation strategy to provide
458 /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
459 /// which emits an error at *runtime*, rather than during compilation.
463 /// Two such examples are macros and `#[cfg]` environments.
465 /// Emit better compiler error if a macro is passed invalid values. Without the final branch,
466 /// the compiler would still emit an error, but the error's message would not mention the two
470 /// macro_rules! give_me_foo_or_bar {
474 /// compile_error!("This macro only accepts `foo` or `bar`");
478 /// give_me_foo_or_bar!(neither);
479 /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
482 /// Emit compiler error if one of a number of features isn't available.
485 /// #[cfg(not(any(feature = "foo", feature = "bar")))]
486 /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.")
489 /// [`panic!`]: ../std/macro.panic.html
490 #[stable(feature = "compile_error_macro", since = "1.20.0")]
491 #[rustc_doc_only_macro]
492 macro_rules! compile_error {
493 ($msg:expr) => ({ /* compiler built-in */ });
494 ($msg:expr,) => ({ /* compiler built-in */ });
497 /// Constructs parameters for the other string-formatting macros.
499 /// This macro functions by taking a formatting string literal containing
500 /// `{}` for each additional argument passed. `format_args!` prepares the
501 /// additional parameters to ensure the output can be interpreted as a string
502 /// and canonicalizes the arguments into a single type. Any value that implements
503 /// the [`Display`] trait can be passed to `format_args!`, as can any
504 /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
506 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
507 /// passed to the macros within [`std::fmt`] for performing useful redirection.
508 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
509 /// proxied through this one. `format_args!`, unlike its derived macros, avoids
510 /// heap allocations.
512 /// You can use the [`fmt::Arguments`] value that `format_args!` returns
513 /// in `Debug` and `Display` contexts as seen below. The example also shows
514 /// that `Debug` and `Display` format to the same thing: the interpolated
515 /// format string in `format_args!`.
518 /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
519 /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
520 /// assert_eq!("1 foo 2", display);
521 /// assert_eq!(display, debug);
524 /// For more information, see the documentation in [`std::fmt`].
526 /// [`Display`]: ../std/fmt/trait.Display.html
527 /// [`Debug`]: ../std/fmt/trait.Debug.html
528 /// [`fmt::Arguments`]: ../std/fmt/struct.Arguments.html
529 /// [`std::fmt`]: ../std/fmt/index.html
530 /// [`format!`]: ../std/macro.format.html
531 /// [`write!`]: ../std/macro.write.html
532 /// [`println!`]: ../std/macro.println.html
539 /// let s = fmt::format(format_args!("hello {}", "world"));
540 /// assert_eq!(s, format!("hello {}", "world"));
542 #[stable(feature = "rust1", since = "1.0.0")]
543 #[rustc_doc_only_macro]
544 macro_rules! format_args {
545 ($fmt:expr) => ({ /* compiler built-in */ });
546 ($fmt:expr, $($args:tt)*) => ({ /* compiler built-in */ });
549 /// Inspects an environment variable at compile time.
551 /// This macro will expand to the value of the named environment variable at
552 /// compile time, yielding an expression of type `&'static str`.
554 /// If the environment variable is not defined, then a compilation error
555 /// will be emitted. To not emit a compile error, use the [`option_env!`]
558 /// [`option_env!`]: ../std/macro.option_env.html
563 /// let path: &'static str = env!("PATH");
564 /// println!("the $PATH variable at the time of compiling was: {}", path);
567 /// You can customize the error message by passing a string as the second
571 /// let doc: &'static str = env!("documentation", "what's that?!");
574 /// If the `documentation` environment variable is not defined, you'll get
575 /// the following error:
578 /// error: what's that?!
580 #[stable(feature = "rust1", since = "1.0.0")]
581 #[rustc_doc_only_macro]
583 ($name:expr) => ({ /* compiler built-in */ });
584 ($name:expr,) => ({ /* compiler built-in */ });
587 /// Optionally inspects an environment variable at compile time.
589 /// If the named environment variable is present at compile time, this will
590 /// expand into an expression of type `Option<&'static str>` whose value is
591 /// `Some` of the value of the environment variable. If the environment
592 /// variable is not present, then this will expand to `None`. See
593 /// [`Option<T>`][option] for more information on this type.
595 /// A compile time error is never emitted when using this macro regardless
596 /// of whether the environment variable is present or not.
598 /// [option]: ../std/option/enum.Option.html
603 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
604 /// println!("the secret key might be: {:?}", key);
606 #[stable(feature = "rust1", since = "1.0.0")]
607 #[rustc_doc_only_macro]
608 macro_rules! option_env {
609 ($name:expr) => ({ /* compiler built-in */ });
610 ($name:expr,) => ({ /* compiler built-in */ });
613 /// Concatenates identifiers into one identifier.
615 /// This macro takes any number of comma-separated identifiers, and
616 /// concatenates them all into one, yielding an expression which is a new
617 /// identifier. Note that hygiene makes it such that this macro cannot
618 /// capture local variables. Also, as a general rule, macros are only
619 /// allowed in item, statement or expression position. That means while
620 /// you may use this macro for referring to existing variables, functions or
621 /// modules etc, you cannot define a new one with it.
626 /// #![feature(concat_idents)]
629 /// fn foobar() -> u32 { 23 }
631 /// let f = concat_idents!(foo, bar);
632 /// println!("{}", f());
634 /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
637 #[unstable(feature = "concat_idents_macro", issue = "29599")]
638 #[rustc_doc_only_macro]
639 macro_rules! concat_idents {
640 ($($e:ident),+) => ({ /* compiler built-in */ });
641 ($($e:ident,)+) => ({ /* compiler built-in */ });
644 /// Concatenates literals into a static string slice.
646 /// This macro takes any number of comma-separated literals, yielding an
647 /// expression of type `&'static str` which represents all of the literals
648 /// concatenated left-to-right.
650 /// Integer and floating point literals are stringified in order to be
656 /// let s = concat!("test", 10, 'b', true);
657 /// assert_eq!(s, "test10btrue");
659 #[stable(feature = "rust1", since = "1.0.0")]
660 #[rustc_doc_only_macro]
661 macro_rules! concat {
662 ($($e:expr),*) => ({ /* compiler built-in */ });
663 ($($e:expr,)*) => ({ /* compiler built-in */ });
666 /// Expands to the line number on which it was invoked.
668 /// With [`column!`] and [`file!`], these macros provide debugging information for
669 /// developers about the location within the source.
671 /// The expanded expression has type `u32` and is 1-based, so the first line
672 /// in each file evaluates to 1, the second to 2, etc. This is consistent
673 /// with error messages by common compilers or popular editors.
674 /// The returned line is *not necessarily* the line of the `line!` invocation itself,
675 /// but rather the first macro invocation leading up to the invocation
676 /// of the `line!` macro.
678 /// [`column!`]: macro.column.html
679 /// [`file!`]: macro.file.html
684 /// let current_line = line!();
685 /// println!("defined on line: {}", current_line);
687 #[stable(feature = "rust1", since = "1.0.0")]
688 #[rustc_doc_only_macro]
689 macro_rules! line { () => ({ /* compiler built-in */ }) }
691 /// Expands to the column number at which it was invoked.
693 /// With [`line!`] and [`file!`], these macros provide debugging information for
694 /// developers about the location within the source.
696 /// The expanded expression has type `u32` and is 1-based, so the first column
697 /// in each line evaluates to 1, the second to 2, etc. This is consistent
698 /// with error messages by common compilers or popular editors.
699 /// The returned column is *not necessarily* the line of the `column!` invocation itself,
700 /// but rather the first macro invocation leading up to the invocation
701 /// of the `column!` macro.
703 /// [`line!`]: macro.line.html
704 /// [`file!`]: macro.file.html
709 /// let current_col = column!();
710 /// println!("defined on column: {}", current_col);
712 #[stable(feature = "rust1", since = "1.0.0")]
713 #[rustc_doc_only_macro]
714 macro_rules! column { () => ({ /* compiler built-in */ }) }
716 /// Expands to the file name in which it was invoked.
718 /// With [`line!`] and [`column!`], these macros provide debugging information for
719 /// developers about the location within the source.
722 /// The expanded expression has type `&'static str`, and the returned file
723 /// is not the invocation of the `file!` macro itself, but rather the
724 /// first macro invocation leading up to the invocation of the `file!`
727 /// [`line!`]: macro.line.html
728 /// [`column!`]: macro.column.html
733 /// let this_file = file!();
734 /// println!("defined in file: {}", this_file);
736 #[stable(feature = "rust1", since = "1.0.0")]
737 #[rustc_doc_only_macro]
738 macro_rules! file { () => ({ /* compiler built-in */ }) }
740 /// Stringifies its arguments.
742 /// This macro will yield an expression of type `&'static str` which is the
743 /// stringification of all the tokens passed to the macro. No restrictions
744 /// are placed on the syntax of the macro invocation itself.
746 /// Note that the expanded results of the input tokens may change in the
747 /// future. You should be careful if you rely on the output.
752 /// let one_plus_one = stringify!(1 + 1);
753 /// assert_eq!(one_plus_one, "1 + 1");
755 #[stable(feature = "rust1", since = "1.0.0")]
756 #[rustc_doc_only_macro]
757 macro_rules! stringify { ($($t:tt)*) => ({ /* compiler built-in */ }) }
759 /// Includes a utf8-encoded file as a string.
761 /// The file is located relative to the current file. (similarly to how
762 /// modules are found)
764 /// This macro will yield an expression of type `&'static str` which is the
765 /// contents of the file.
769 /// Assume there are two files in the same directory with the following
772 /// File 'spanish.in':
780 /// ```ignore (cannot-doctest-external-file-dependency)
782 /// let my_str = include_str!("spanish.in");
783 /// assert_eq!(my_str, "adiรณs\n");
784 /// print!("{}", my_str);
788 /// Compiling 'main.rs' and running the resulting binary will print "adiรณs".
789 #[stable(feature = "rust1", since = "1.0.0")]
790 #[rustc_doc_only_macro]
791 macro_rules! include_str {
792 ($file:expr) => ({ /* compiler built-in */ });
793 ($file:expr,) => ({ /* compiler built-in */ });
796 /// Includes a file as a reference to a byte array.
798 /// The file is located relative to the current file. (similarly to how
799 /// modules are found)
801 /// This macro will yield an expression of type `&'static [u8; N]` which is
802 /// the contents of the file.
806 /// Assume there are two files in the same directory with the following
809 /// File 'spanish.in':
817 /// ```ignore (cannot-doctest-external-file-dependency)
819 /// let bytes = include_bytes!("spanish.in");
820 /// assert_eq!(bytes, b"adi\xc3\xb3s\n");
821 /// print!("{}", String::from_utf8_lossy(bytes));
825 /// Compiling 'main.rs' and running the resulting binary will print "adiรณs".
826 #[stable(feature = "rust1", since = "1.0.0")]
827 #[rustc_doc_only_macro]
828 macro_rules! include_bytes {
829 ($file:expr) => ({ /* compiler built-in */ });
830 ($file:expr,) => ({ /* compiler built-in */ });
833 /// Expands to a string that represents the current module path.
835 /// The current module path can be thought of as the hierarchy of modules
836 /// leading back up to the crate root. The first component of the path
837 /// returned is the name of the crate currently being compiled.
844 /// assert!(module_path!().ends_with("test"));
850 #[stable(feature = "rust1", since = "1.0.0")]
851 #[rustc_doc_only_macro]
852 macro_rules! module_path { () => ({ /* compiler built-in */ }) }
854 /// Evaluates boolean combinations of configuration flags at compile-time.
856 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
857 /// boolean expression evaluation of configuration flags. This frequently
858 /// leads to less duplicated code.
860 /// The syntax given to this macro is the same syntax as the [`cfg`]
863 /// [`cfg`]: ../reference/conditional-compilation.html#the-cfg-attribute
868 /// let my_directory = if cfg!(windows) {
869 /// "windows-specific-directory"
874 #[stable(feature = "rust1", since = "1.0.0")]
875 #[rustc_doc_only_macro]
876 macro_rules! cfg { ($($cfg:tt)*) => ({ /* compiler built-in */ }) }
878 /// Parses a file as an expression or an item according to the context.
880 /// The file is located relative to the current file (similarly to how
881 /// modules are found).
883 /// Using this macro is often a bad idea, because if the file is
884 /// parsed as an expression, it is going to be placed in the
885 /// surrounding code unhygienically. This could result in variables
886 /// or functions being different from what the file expected if
887 /// there are variables or functions that have the same name in
888 /// the current file.
892 /// Assume there are two files in the same directory with the following
895 /// File 'monkeys.in':
897 /// ```ignore (only-for-syntax-highlight)
898 /// ['๐', '๐', '๐']
902 /// .collect::<String>()
907 /// ```ignore (cannot-doctest-external-file-dependency)
909 /// let my_string = include!("monkeys.in");
910 /// assert_eq!("๐๐๐๐๐๐", my_string);
911 /// println!("{}", my_string);
915 /// Compiling 'main.rs' and running the resulting binary will print
916 /// "๐๐๐๐๐๐".
917 #[stable(feature = "rust1", since = "1.0.0")]
918 #[rustc_doc_only_macro]
919 macro_rules! include {
920 ($file:expr) => ({ /* compiler built-in */ });
921 ($file:expr,) => ({ /* compiler built-in */ });
924 /// Asserts that a boolean expression is `true` at runtime.
926 /// This will invoke the [`panic!`] macro if the provided expression cannot be
927 /// evaluated to `true` at runtime.
931 /// Assertions are always checked in both debug and release builds, and cannot
932 /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
933 /// release builds by default.
935 /// Unsafe code relies on `assert!` to enforce run-time invariants that, if
936 /// violated could lead to unsafety.
938 /// Other use-cases of `assert!` include testing and enforcing run-time
939 /// invariants in safe code (whose violation cannot result in unsafety).
941 /// # Custom Messages
943 /// This macro has a second form, where a custom panic message can
944 /// be provided with or without arguments for formatting. See [`std::fmt`]
945 /// for syntax for this form.
947 /// [`panic!`]: macro.panic.html
948 /// [`debug_assert!`]: macro.debug_assert.html
949 /// [`std::fmt`]: ../std/fmt/index.html
954 /// // the panic message for these assertions is the stringified value of the
955 /// // expression given.
958 /// fn some_computation() -> bool { true } // a very simple function
960 /// assert!(some_computation());
962 /// // assert with a custom message
964 /// assert!(x, "x wasn't true!");
966 /// let a = 3; let b = 27;
967 /// assert!(a + b == 30, "a = {}, b = {}", a, b);
969 #[stable(feature = "rust1", since = "1.0.0")]
970 #[rustc_doc_only_macro]
971 macro_rules! assert {
972 ($cond:expr) => ({ /* compiler built-in */ });
973 ($cond:expr,) => ({ /* compiler built-in */ });
974 ($cond:expr, $($arg:tt)+) => ({ /* compiler built-in */ });
978 /// Defines `#[cfg]` if-else statements.
980 /// This is similar to the `if/elif` C preprocessor macro by allowing definition
981 /// of a cascade of `#[cfg]` cases, emitting the implementation which matches
984 /// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
985 /// without having to rewrite each clause multiple times.
986 macro_rules! cfg_if {
988 if #[cfg($($meta:meta),*)] { $($it:item)* }
994 $( ( ($($meta),*) ($($it)*) ), )*
1000 macro_rules! __cfg_if_items {
1001 (($($not:meta,)*) ; ) => {};
1002 (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
1003 __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* }
1004 __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* }
1008 macro_rules! __cfg_if_apply {
1009 ($m:meta, $($it:item)*) => {