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 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 #[cfg_attr(stage0, allow_internal_unstable)]
57 #[cfg_attr(not(stage0), allow_internal_unstable(__rust_unstable_column, libstd_sys_internals))]
60 panic!("explicit panic")
63 $crate::rt::begin_panic($msg, &(file!(), line!(), __rust_unstable_column!()))
68 ($fmt:expr, $($arg:tt)+) => ({
69 $crate::rt::begin_panic_fmt(&format_args!($fmt, $($arg)+),
70 &(file!(), line!(), __rust_unstable_column!()))
74 /// Macro for printing to the standard output.
76 /// Equivalent to the [`println!`] macro except that a newline is not printed at
77 /// the end of the message.
79 /// Note that stdout is frequently line-buffered by default so it may be
80 /// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted
83 /// Use `print!` only for the primary output of your program. Use
84 /// [`eprint!`] instead to print error and progress messages.
86 /// [`println!`]: ../std/macro.println.html
87 /// [flush]: ../std/io/trait.Write.html#tymethod.flush
88 /// [`eprint!`]: ../std/macro.eprint.html
92 /// Panics if writing to `io::stdout()` fails.
97 /// use std::io::{self, Write};
107 /// io::stdout().flush().unwrap();
109 /// print!("this string has a newline, why not choose println! instead?\n");
111 /// io::stdout().flush().unwrap();
114 #[stable(feature = "rust1", since = "1.0.0")]
115 #[cfg_attr(stage0, allow_internal_unstable)]
116 #[cfg_attr(not(stage0), allow_internal_unstable(print_internals))]
118 ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
121 /// Macro for printing to the standard output, with a newline.
123 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
124 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
126 /// Use the [`format!`] syntax to write data to the standard output.
127 /// See [`std::fmt`] for more information.
129 /// Use `println!` only for the primary output of your program. Use
130 /// [`eprintln!`] instead to print error and progress messages.
132 /// [`format!`]: ../std/macro.format.html
133 /// [`std::fmt`]: ../std/fmt/index.html
134 /// [`eprintln!`]: ../std/macro.eprintln.html
137 /// Panics if writing to `io::stdout` fails.
142 /// println!(); // prints just a newline
143 /// println!("hello there!");
144 /// println!("format {} arguments", "some");
147 #[stable(feature = "rust1", since = "1.0.0")]
148 #[cfg_attr(stage0, allow_internal_unstable)]
149 #[cfg_attr(not(stage0), allow_internal_unstable(print_internals, format_args_nl))]
150 macro_rules! println {
151 () => (print!("\n"));
153 $crate::io::_print(format_args_nl!($($arg)*));
157 /// Macro for printing to the standard error.
159 /// Equivalent to the [`print!`] macro, except that output goes to
160 /// [`io::stderr`] instead of `io::stdout`. See [`print!`] for
163 /// Use `eprint!` only for error and progress messages. Use `print!`
164 /// instead for the primary output of your program.
166 /// [`io::stderr`]: ../std/io/struct.Stderr.html
167 /// [`print!`]: ../std/macro.print.html
171 /// Panics if writing to `io::stderr` fails.
176 /// eprint!("Error: Could not complete task");
179 #[stable(feature = "eprint", since = "1.19.0")]
180 #[cfg_attr(stage0, allow_internal_unstable)]
181 #[cfg_attr(not(stage0), allow_internal_unstable(print_internals))]
182 macro_rules! eprint {
183 ($($arg:tt)*) => ($crate::io::_eprint(format_args!($($arg)*)));
186 /// Macro for printing to the standard error, with a newline.
188 /// Equivalent to the [`println!`] macro, except that output goes to
189 /// [`io::stderr`] instead of `io::stdout`. See [`println!`] for
192 /// Use `eprintln!` only for error and progress messages. Use `println!`
193 /// instead for the primary output of your program.
195 /// [`io::stderr`]: ../std/io/struct.Stderr.html
196 /// [`println!`]: ../std/macro.println.html
200 /// Panics if writing to `io::stderr` fails.
205 /// eprintln!("Error: Could not complete task");
208 #[stable(feature = "eprint", since = "1.19.0")]
209 #[cfg_attr(stage0, allow_internal_unstable)]
210 #[cfg_attr(not(stage0), allow_internal_unstable(print_internals, format_args_nl))]
211 macro_rules! eprintln {
212 () => (eprint!("\n"));
214 $crate::io::_eprint(format_args_nl!($($arg)*));
218 /// A macro for quick and dirty debugging with which you can inspect
219 /// the value of a given expression. An example:
223 /// let b = dbg!(a * 2) + 1;
224 /// // ^-- prints: [src/main.rs:2] a * 2 = 4
225 /// assert_eq!(b, 5);
228 /// The macro works by using the `Debug` implementation of the type of
229 /// the given expression to print the value to [stderr] along with the
230 /// source location of the macro invocation as well as the source code
231 /// of the expression.
233 /// Invoking the macro on an expression moves and takes ownership of it
234 /// before returning the evaluated expression unchanged. If the type
235 /// of the expression does not implement `Copy` and you don't want
236 /// to give up ownership, you can instead borrow with `dbg!(&expr)`
237 /// for some expression `expr`.
239 /// Note that the macro is intended as a debugging tool and therefore you
240 /// should avoid having uses of it in version control for longer periods.
241 /// Use cases involving debug output that should be added to version control
242 /// may be better served by macros such as `debug!` from the `log` crate.
246 /// The exact output printed by this macro should not be relied upon
247 /// and is subject to future changes.
251 /// Panics if writing to `io::stderr` fails.
253 /// # Further examples
255 /// With a method call:
258 /// fn foo(n: usize) {
259 /// if let Some(_) = dbg!(n.checked_sub(4)) {
267 /// This prints to [stderr]:
270 /// [src/main.rs:4] n.checked_sub(4) = None
273 /// Naive factorial implementation:
276 /// fn factorial(n: u32) -> u32 {
277 /// if dbg!(n <= 1) {
280 /// dbg!(n * factorial(n - 1))
284 /// dbg!(factorial(4));
287 /// This prints to [stderr]:
290 /// [src/main.rs:3] n <= 1 = false
291 /// [src/main.rs:3] n <= 1 = false
292 /// [src/main.rs:3] n <= 1 = false
293 /// [src/main.rs:3] n <= 1 = true
294 /// [src/main.rs:4] 1 = 1
295 /// [src/main.rs:5] n * factorial(n - 1) = 2
296 /// [src/main.rs:5] n * factorial(n - 1) = 6
297 /// [src/main.rs:5] n * factorial(n - 1) = 24
298 /// [src/main.rs:11] factorial(4) = 24
301 /// The `dbg!(..)` macro moves the input:
304 /// /// A wrapper around `usize` which importantly is not Copyable.
306 /// struct NoCopy(usize);
308 /// let a = NoCopy(42);
309 /// let _ = dbg!(a); // <-- `a` is moved here.
310 /// let _ = dbg!(a); // <-- `a` is moved again; error!
313 /// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
315 #[stable(feature = "dbg_macro", since = "1.32.0")]
318 // Use of `match` here is intentional because it affects the lifetimes
319 // of temporaries - https://stackoverflow.com/a/48732525/1063961
322 eprintln!("[{}:{}] {} = {:#?}",
323 file!(), line!(), stringify!($val), &tmp);
330 /// A macro to await on an async call.
332 #[unstable(feature = "await_macro", issue = "50547")]
333 #[cfg_attr(stage0, allow_internal_unstable)]
334 #[cfg_attr(not(stage0), allow_internal_unstable(gen_future, generators))]
335 #[allow_internal_unsafe]
336 macro_rules! r#await {
340 if let $crate::task::Poll::Ready(x) =
341 $crate::future::poll_with_tls_waker(unsafe {
342 $crate::pin::Pin::new_unchecked(&mut pinned)
347 // FIXME(cramertj) prior to stabilizing await, we have to ensure that this
348 // can't be used to create a generator on stable via `|| await!()`.
354 /// A macro to select an event from a number of receivers.
356 /// This macro is used to wait for the first event to occur on a number of
357 /// receivers. It places no restrictions on the types of receivers given to
358 /// this macro, this can be viewed as a heterogeneous select.
363 /// #![feature(mpsc_select)]
366 /// use std::sync::mpsc;
368 /// // two placeholder functions for now
369 /// fn long_running_thread() {}
370 /// fn calculate_the_answer() -> u32 { 42 }
372 /// let (tx1, rx1) = mpsc::channel();
373 /// let (tx2, rx2) = mpsc::channel();
375 /// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); });
376 /// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); });
379 /// _ = rx1.recv() => println!("the long running thread finished first"),
380 /// answer = rx2.recv() => {
381 /// println!("the answer was: {}", answer.unwrap());
384 /// # drop(rx1.recv());
385 /// # drop(rx2.recv());
388 /// For more information about select, see the `std::sync::mpsc::Select` structure.
390 #[unstable(feature = "mpsc_select", issue = "27800")]
391 #[rustc_deprecated(since = "1.32.0",
392 reason = "channel selection will be removed in a future release")]
393 macro_rules! select {
395 $($name:pat = $rx:ident.$meth:ident() => $code:expr),+
397 use $crate::sync::mpsc::Select;
398 let sel = Select::new();
399 $( let mut $rx = sel.handle(&$rx); )+
403 let ret = sel.wait();
404 $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+
410 macro_rules! assert_approx_eq {
411 ($a:expr, $b:expr) => ({
412 let (a, b) = (&$a, &$b);
413 assert!((*a - *b).abs() < 1.0e-6,
414 "{} is not approximately equal to {}", *a, *b);
418 /// Built-in macros to the compiler itself.
420 /// These macros do not have any corresponding definition with a `macro_rules!`
421 /// macro, but are documented here. Their implementations can be found hardcoded
422 /// into libsyntax itself.
426 /// Unconditionally causes compilation to fail with the given error message when encountered.
428 /// This macro should be used when a crate uses a conditional compilation strategy to provide
429 /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
430 /// which emits an error at *runtime*, rather than during compilation.
434 /// Two such examples are macros and `#[cfg]` environments.
436 /// Emit better compiler error if a macro is passed invalid values. Without the final branch,
437 /// the compiler would still emit an error, but the error's message would not mention the two
441 /// macro_rules! give_me_foo_or_bar {
445 /// compile_error!("This macro only accepts `foo` or `bar`");
449 /// give_me_foo_or_bar!(neither);
450 /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
453 /// Emit compiler error if one of a number of features isn't available.
456 /// #[cfg(not(any(feature = "foo", feature = "bar")))]
457 /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.")
460 /// [`panic!`]: ../std/macro.panic.html
461 #[stable(feature = "compile_error_macro", since = "1.20.0")]
462 #[rustc_doc_only_macro]
463 macro_rules! compile_error {
464 ($msg:expr) => ({ /* compiler built-in */ });
465 ($msg:expr,) => ({ /* compiler built-in */ });
468 /// The core macro for formatted string creation & output.
470 /// This macro functions by taking a formatting string literal containing
471 /// `{}` for each additional argument passed. `format_args!` prepares the
472 /// additional parameters to ensure the output can be interpreted as a string
473 /// and canonicalizes the arguments into a single type. Any value that implements
474 /// the [`Display`] trait can be passed to `format_args!`, as can any
475 /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
477 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
478 /// passed to the macros within [`std::fmt`] for performing useful redirection.
479 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
480 /// proxied through this one. `format_args!`, unlike its derived macros, avoids
481 /// heap allocations.
483 /// You can use the [`fmt::Arguments`] value that `format_args!` returns
484 /// in `Debug` and `Display` contexts as seen below. The example also shows
485 /// that `Debug` and `Display` format to the same thing: the interpolated
486 /// format string in `format_args!`.
489 /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
490 /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
491 /// assert_eq!("1 foo 2", display);
492 /// assert_eq!(display, debug);
495 /// For more information, see the documentation in [`std::fmt`].
497 /// [`Display`]: ../std/fmt/trait.Display.html
498 /// [`Debug`]: ../std/fmt/trait.Debug.html
499 /// [`fmt::Arguments`]: ../std/fmt/struct.Arguments.html
500 /// [`std::fmt`]: ../std/fmt/index.html
501 /// [`format!`]: ../std/macro.format.html
502 /// [`write!`]: ../std/macro.write.html
503 /// [`println!`]: ../std/macro.println.html
510 /// let s = fmt::format(format_args!("hello {}", "world"));
511 /// assert_eq!(s, format!("hello {}", "world"));
513 #[stable(feature = "rust1", since = "1.0.0")]
514 #[rustc_doc_only_macro]
515 macro_rules! format_args {
516 ($fmt:expr) => ({ /* compiler built-in */ });
517 ($fmt:expr, $($args:tt)*) => ({ /* compiler built-in */ });
520 /// Inspect an environment variable at compile time.
522 /// This macro will expand to the value of the named environment variable at
523 /// compile time, yielding an expression of type `&'static str`.
525 /// If the environment variable is not defined, then a compilation error
526 /// will be emitted. To not emit a compile error, use the [`option_env!`]
529 /// [`option_env!`]: ../std/macro.option_env.html
534 /// let path: &'static str = env!("PATH");
535 /// println!("the $PATH variable at the time of compiling was: {}", path);
538 /// You can customize the error message by passing a string as the second
542 /// let doc: &'static str = env!("documentation", "what's that?!");
545 /// If the `documentation` environment variable is not defined, you'll get
546 /// the following error:
549 /// error: what's that?!
551 #[stable(feature = "rust1", since = "1.0.0")]
552 #[rustc_doc_only_macro]
554 ($name:expr) => ({ /* compiler built-in */ });
555 ($name:expr,) => ({ /* compiler built-in */ });
558 /// Optionally inspect an environment variable at compile time.
560 /// If the named environment variable is present at compile time, this will
561 /// expand into an expression of type `Option<&'static str>` whose value is
562 /// `Some` of the value of the environment variable. If the environment
563 /// variable is not present, then this will expand to `None`. See
564 /// [`Option<T>`][option] for more information on this type.
566 /// A compile time error is never emitted when using this macro regardless
567 /// of whether the environment variable is present or not.
569 /// [option]: ../std/option/enum.Option.html
574 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
575 /// println!("the secret key might be: {:?}", key);
577 #[stable(feature = "rust1", since = "1.0.0")]
578 #[rustc_doc_only_macro]
579 macro_rules! option_env {
580 ($name:expr) => ({ /* compiler built-in */ });
581 ($name:expr,) => ({ /* compiler built-in */ });
584 /// Concatenate identifiers into one identifier.
586 /// This macro takes any number of comma-separated identifiers, and
587 /// concatenates them all into one, yielding an expression which is a new
588 /// identifier. Note that hygiene makes it such that this macro cannot
589 /// capture local variables. Also, as a general rule, macros are only
590 /// allowed in item, statement or expression position. That means while
591 /// you may use this macro for referring to existing variables, functions or
592 /// modules etc, you cannot define a new one with it.
597 /// #![feature(concat_idents)]
600 /// fn foobar() -> u32 { 23 }
602 /// let f = concat_idents!(foo, bar);
603 /// println!("{}", f());
605 /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
608 #[unstable(feature = "concat_idents_macro", issue = "29599")]
609 #[rustc_doc_only_macro]
610 macro_rules! concat_idents {
611 ($($e:ident),+) => ({ /* compiler built-in */ });
612 ($($e:ident,)+) => ({ /* compiler built-in */ });
615 /// Concatenates literals into a static string slice.
617 /// This macro takes any number of comma-separated literals, yielding an
618 /// expression of type `&'static str` which represents all of the literals
619 /// concatenated left-to-right.
621 /// Integer and floating point literals are stringified in order to be
627 /// let s = concat!("test", 10, 'b', true);
628 /// assert_eq!(s, "test10btrue");
630 #[stable(feature = "rust1", since = "1.0.0")]
631 #[rustc_doc_only_macro]
632 macro_rules! concat {
633 ($($e:expr),*) => ({ /* compiler built-in */ });
634 ($($e:expr,)*) => ({ /* compiler built-in */ });
637 /// A macro which expands to the line number on which it was invoked.
639 /// With [`column!`] and [`file!`], these macros provide debugging information for
640 /// developers about the location within the source.
642 /// The expanded expression has type `u32` and is 1-based, so the first line
643 /// in each file evaluates to 1, the second to 2, etc. This is consistent
644 /// with error messages by common compilers or popular editors.
645 /// The returned line is *not necessarily* the line of the `line!` invocation itself,
646 /// but rather the first macro invocation leading up to the invocation
647 /// of the `line!` macro.
649 /// [`column!`]: macro.column.html
650 /// [`file!`]: macro.file.html
655 /// let current_line = line!();
656 /// println!("defined on line: {}", current_line);
658 #[stable(feature = "rust1", since = "1.0.0")]
659 #[rustc_doc_only_macro]
660 macro_rules! line { () => ({ /* compiler built-in */ }) }
662 /// A macro which expands to the column number on which it was invoked.
664 /// With [`line!`] and [`file!`], these macros provide debugging information for
665 /// developers about the location within the source.
667 /// The expanded expression has type `u32` and is 1-based, so the first column
668 /// in each line evaluates to 1, the second to 2, etc. This is consistent
669 /// with error messages by common compilers or popular editors.
670 /// The returned column is *not necessarily* the line of the `column!` invocation itself,
671 /// but rather the first macro invocation leading up to the invocation
672 /// of the `column!` macro.
674 /// [`line!`]: macro.line.html
675 /// [`file!`]: macro.file.html
680 /// let current_col = column!();
681 /// println!("defined on column: {}", current_col);
683 #[stable(feature = "rust1", since = "1.0.0")]
684 #[rustc_doc_only_macro]
685 macro_rules! column { () => ({ /* compiler built-in */ }) }
687 /// A macro which expands to the file name from which it was invoked.
689 /// With [`line!`] and [`column!`], these macros provide debugging information for
690 /// developers about the location within the source.
693 /// The expanded expression has type `&'static str`, and the returned file
694 /// is not the invocation of the `file!` macro itself, but rather the
695 /// first macro invocation leading up to the invocation of the `file!`
698 /// [`line!`]: macro.line.html
699 /// [`column!`]: macro.column.html
704 /// let this_file = file!();
705 /// println!("defined in file: {}", this_file);
707 #[stable(feature = "rust1", since = "1.0.0")]
708 #[rustc_doc_only_macro]
709 macro_rules! file { () => ({ /* compiler built-in */ }) }
711 /// A macro which stringifies its arguments.
713 /// This macro will yield an expression of type `&'static str` which is the
714 /// stringification of all the tokens passed to the macro. No restrictions
715 /// are placed on the syntax of the macro invocation itself.
717 /// Note that the expanded results of the input tokens may change in the
718 /// future. You should be careful if you rely on the output.
723 /// let one_plus_one = stringify!(1 + 1);
724 /// assert_eq!(one_plus_one, "1 + 1");
726 #[stable(feature = "rust1", since = "1.0.0")]
727 #[rustc_doc_only_macro]
728 macro_rules! stringify { ($($t:tt)*) => ({ /* compiler built-in */ }) }
730 /// Includes a utf8-encoded file as a string.
732 /// The file is located relative to the current file. (similarly to how
733 /// modules are found)
735 /// This macro will yield an expression of type `&'static str` which is the
736 /// contents of the file.
740 /// Assume there are two files in the same directory with the following
743 /// File 'spanish.in':
751 /// ```ignore (cannot-doctest-external-file-dependency)
753 /// let my_str = include_str!("spanish.in");
754 /// assert_eq!(my_str, "adiós\n");
755 /// print!("{}", my_str);
759 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
760 #[stable(feature = "rust1", since = "1.0.0")]
761 #[rustc_doc_only_macro]
762 macro_rules! include_str {
763 ($file:expr) => ({ /* compiler built-in */ });
764 ($file:expr,) => ({ /* compiler built-in */ });
767 /// Includes a file as a reference to a byte array.
769 /// The file is located relative to the current file. (similarly to how
770 /// modules are found)
772 /// This macro will yield an expression of type `&'static [u8; N]` which is
773 /// the contents of the file.
777 /// Assume there are two files in the same directory with the following
780 /// File 'spanish.in':
788 /// ```ignore (cannot-doctest-external-file-dependency)
790 /// let bytes = include_bytes!("spanish.in");
791 /// assert_eq!(bytes, b"adi\xc3\xb3s\n");
792 /// print!("{}", String::from_utf8_lossy(bytes));
796 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
797 #[stable(feature = "rust1", since = "1.0.0")]
798 #[rustc_doc_only_macro]
799 macro_rules! include_bytes {
800 ($file:expr) => ({ /* compiler built-in */ });
801 ($file:expr,) => ({ /* compiler built-in */ });
804 /// Expands to a string that represents the current module path.
806 /// The current module path can be thought of as the hierarchy of modules
807 /// leading back up to the crate root. The first component of the path
808 /// returned is the name of the crate currently being compiled.
815 /// assert!(module_path!().ends_with("test"));
821 #[stable(feature = "rust1", since = "1.0.0")]
822 #[rustc_doc_only_macro]
823 macro_rules! module_path { () => ({ /* compiler built-in */ }) }
825 /// Boolean evaluation of configuration flags, at compile-time.
827 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
828 /// boolean expression evaluation of configuration flags. This frequently
829 /// leads to less duplicated code.
831 /// The syntax given to this macro is the same syntax as the `cfg`
837 /// let my_directory = if cfg!(windows) {
838 /// "windows-specific-directory"
843 #[stable(feature = "rust1", since = "1.0.0")]
844 #[rustc_doc_only_macro]
845 macro_rules! cfg { ($($cfg:tt)*) => ({ /* compiler built-in */ }) }
847 /// Parse a file as an expression or an item according to the context.
849 /// The file is located relative to the current file (similarly to how
850 /// modules are found).
852 /// Using this macro is often a bad idea, because if the file is
853 /// parsed as an expression, it is going to be placed in the
854 /// surrounding code unhygienically. This could result in variables
855 /// or functions being different from what the file expected if
856 /// there are variables or functions that have the same name in
857 /// the current file.
861 /// Assume there are two files in the same directory with the following
864 /// File 'monkeys.in':
866 /// ```ignore (only-for-syntax-highlight)
871 /// .collect::<String>()
876 /// ```ignore (cannot-doctest-external-file-dependency)
878 /// let my_string = include!("monkeys.in");
879 /// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
880 /// println!("{}", my_string);
884 /// Compiling 'main.rs' and running the resulting binary will print
886 #[stable(feature = "rust1", since = "1.0.0")]
887 #[rustc_doc_only_macro]
888 macro_rules! include {
889 ($file:expr) => ({ /* compiler built-in */ });
890 ($file:expr,) => ({ /* compiler built-in */ });
893 /// Ensure that a boolean expression is `true` at runtime.
895 /// This will invoke the [`panic!`] macro if the provided expression cannot be
896 /// evaluated to `true` at runtime.
900 /// Assertions are always checked in both debug and release builds, and cannot
901 /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
902 /// release builds by default.
904 /// Unsafe code relies on `assert!` to enforce run-time invariants that, if
905 /// violated could lead to unsafety.
907 /// Other use-cases of `assert!` include testing and enforcing run-time
908 /// invariants in safe code (whose violation cannot result in unsafety).
910 /// # Custom Messages
912 /// This macro has a second form, where a custom panic message can
913 /// be provided with or without arguments for formatting. See [`std::fmt`]
914 /// for syntax for this form.
916 /// [`panic!`]: macro.panic.html
917 /// [`debug_assert!`]: macro.debug_assert.html
918 /// [`std::fmt`]: ../std/fmt/index.html
923 /// // the panic message for these assertions is the stringified value of the
924 /// // expression given.
927 /// fn some_computation() -> bool { true } // a very simple function
929 /// assert!(some_computation());
931 /// // assert with a custom message
933 /// assert!(x, "x wasn't true!");
935 /// let a = 3; let b = 27;
936 /// assert!(a + b == 30, "a = {}, b = {}", a, b);
938 #[stable(feature = "rust1", since = "1.0.0")]
939 #[rustc_doc_only_macro]
940 macro_rules! assert {
941 ($cond:expr) => ({ /* compiler built-in */ });
942 ($cond:expr,) => ({ /* compiler built-in */ });
943 ($cond:expr, $($arg:tt)+) => ({ /* compiler built-in */ });
947 /// A macro for defining `#[cfg]` if-else statements.
949 /// This is similar to the `if/elif` C preprocessor macro by allowing definition
950 /// of a cascade of `#[cfg]` cases, emitting the implementation which matches
953 /// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
954 /// without having to rewrite each clause multiple times.
955 macro_rules! cfg_if {
957 if #[cfg($($meta:meta),*)] { $($it:item)* }
963 $( ( ($($meta),*) ($($it)*) ), )*
969 macro_rules! __cfg_if_items {
970 (($($not:meta,)*) ; ) => {};
971 (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
972 __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* }
973 __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* }
977 macro_rules! __cfg_if_apply {
978 ($m:meta, $($it:item)*) => {