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 /// See also the macro [`compile_error!`], for raising errors during compilation.
43 /// [runwrap]: ../std/result/enum.Result.html#method.unwrap
44 /// [`Option`]: ../std/option/enum.Option.html#method.unwrap
45 /// [`Result`]: ../std/result/enum.Result.html
46 /// [`format!`]: ../std/macro.format.html
47 /// [`compile_error!`]: ../std/macro.compile_error.html
48 /// [book]: ../book/second-edition/ch09-01-unrecoverable-errors-with-panic.html
50 /// # Current implementation
52 /// If the main thread panics it will terminate all your threads and end your
53 /// program with code `101`.
58 /// # #![allow(unreachable_code)]
60 /// panic!("this is a terrible mistake!");
61 /// panic!(4); // panic with the value of 4 to be collected elsewhere
62 /// panic!("this is a {} {message}", "fancy", message = "message");
65 #[stable(feature = "rust1", since = "1.0.0")]
66 #[allow_internal_unstable]
69 panic!("explicit panic")
72 $crate::rt::begin_panic($msg, &(file!(), line!(), __rust_unstable_column!()))
77 ($fmt:expr, $($arg:tt)+) => ({
78 $crate::rt::begin_panic_fmt(&format_args!($fmt, $($arg)+),
79 &(file!(), line!(), __rust_unstable_column!()))
83 /// Macro for printing to the standard output.
85 /// Equivalent to the [`println!`] macro except that a newline is not printed at
86 /// the end of the message.
88 /// Note that stdout is frequently line-buffered by default so it may be
89 /// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted
92 /// Use `print!` only for the primary output of your program. Use
93 /// [`eprint!`] instead to print error and progress messages.
95 /// [`println!`]: ../std/macro.println.html
96 /// [flush]: ../std/io/trait.Write.html#tymethod.flush
97 /// [`eprint!`]: ../std/macro.eprint.html
101 /// Panics if writing to `io::stdout()` fails.
106 /// use std::io::{self, Write};
116 /// io::stdout().flush().unwrap();
118 /// print!("this string has a newline, why not choose println! instead?\n");
120 /// io::stdout().flush().unwrap();
123 #[stable(feature = "rust1", since = "1.0.0")]
124 #[allow_internal_unstable]
126 ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
129 /// Macro for printing to the standard output, with a newline.
131 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
132 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
134 /// Use the [`format!`] syntax to write data to the standard output.
135 /// See [`std::fmt`] for more information.
137 /// Use `println!` only for the primary output of your program. Use
138 /// [`eprintln!`] instead to print error and progress messages.
140 /// [`format!`]: ../std/macro.format.html
141 /// [`std::fmt`]: ../std/fmt/index.html
142 /// [`eprintln!`]: ../std/macro.eprint.html
145 /// Panics if writing to `io::stdout` fails.
150 /// println!(); // prints just a newline
151 /// println!("hello there!");
152 /// println!("format {} arguments", "some");
155 #[stable(feature = "rust1", since = "1.0.0")]
156 #[allow_internal_unstable]
157 macro_rules! println {
158 () => (print!("\n"));
160 $crate::io::_print(format_args_nl!($($arg)*));
164 /// Macro for printing to the standard error.
166 /// Equivalent to the [`print!`] macro, except that output goes to
167 /// [`io::stderr`] instead of `io::stdout`. See [`print!`] for
170 /// Use `eprint!` only for error and progress messages. Use `print!`
171 /// instead for the primary output of your program.
173 /// [`io::stderr`]: ../std/io/struct.Stderr.html
174 /// [`print!`]: ../std/macro.print.html
178 /// Panics if writing to `io::stderr` fails.
183 /// eprint!("Error: Could not complete task");
186 #[stable(feature = "eprint", since = "1.19.0")]
187 #[allow_internal_unstable]
188 macro_rules! eprint {
189 ($($arg:tt)*) => ($crate::io::_eprint(format_args!($($arg)*)));
192 /// Macro for printing to the standard error, with a newline.
194 /// Equivalent to the [`println!`] macro, except that output goes to
195 /// [`io::stderr`] instead of `io::stdout`. See [`println!`] for
198 /// Use `eprintln!` only for error and progress messages. Use `println!`
199 /// instead for the primary output of your program.
201 /// [`io::stderr`]: ../std/io/struct.Stderr.html
202 /// [`println!`]: ../std/macro.println.html
206 /// Panics if writing to `io::stderr` fails.
211 /// eprintln!("Error: Could not complete task");
214 #[stable(feature = "eprint", since = "1.19.0")]
215 #[allow_internal_unstable]
216 macro_rules! eprintln {
217 () => (eprint!("\n"));
219 $crate::io::_eprint(format_args_nl!($($arg)*));
224 #[unstable(feature = "await_macro", issue = "50547")]
225 #[allow_internal_unstable]
226 #[allow_internal_unsafe]
231 if let $crate::task::Poll::Ready(x) =
232 $crate::future::poll_in_task_cx(unsafe {
233 $crate::pin::PinMut::new_unchecked(&mut pinned)
238 // FIXME(cramertj) prior to stabilizing await, we have to ensure that this
239 // can't be used to create a generator on stable via `|| await!()`.
245 /// A macro to select an event from a number of receivers.
247 /// This macro is used to wait for the first event to occur on a number of
248 /// receivers. It places no restrictions on the types of receivers given to
249 /// this macro, this can be viewed as a heterogeneous select.
254 /// #![feature(mpsc_select)]
257 /// use std::sync::mpsc;
259 /// // two placeholder functions for now
260 /// fn long_running_thread() {}
261 /// fn calculate_the_answer() -> u32 { 42 }
263 /// let (tx1, rx1) = mpsc::channel();
264 /// let (tx2, rx2) = mpsc::channel();
266 /// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); });
267 /// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); });
270 /// _ = rx1.recv() => println!("the long running thread finished first"),
271 /// answer = rx2.recv() => {
272 /// println!("the answer was: {}", answer.unwrap());
275 /// # drop(rx1.recv());
276 /// # drop(rx2.recv());
279 /// For more information about select, see the `std::sync::mpsc::Select` structure.
281 #[unstable(feature = "mpsc_select", issue = "27800")]
282 macro_rules! select {
284 $($name:pat = $rx:ident.$meth:ident() => $code:expr),+
286 use $crate::sync::mpsc::Select;
287 let sel = Select::new();
288 $( let mut $rx = sel.handle(&$rx); )+
292 let ret = sel.wait();
293 $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+
299 macro_rules! assert_approx_eq {
300 ($a:expr, $b:expr) => ({
301 let (a, b) = (&$a, &$b);
302 assert!((*a - *b).abs() < 1.0e-6,
303 "{} is not approximately equal to {}", *a, *b);
307 /// Built-in macros to the compiler itself.
309 /// These macros do not have any corresponding definition with a `macro_rules!`
310 /// macro, but are documented here. Their implementations can be found hardcoded
311 /// into libsyntax itself.
315 /// Unconditionally causes compilation to fail with the given error message when encountered.
317 /// This macro should be used when a crate uses a conditional compilation strategy to provide
318 /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
319 /// which emits an error at *runtime*, rather than during compilation.
323 /// Two such examples are macros and `#[cfg]` environments.
325 /// Emit better compiler error if a macro is passed invalid values. Without the final branch,
326 /// the compiler would still emit an error, but the error's message would not mention the two
330 /// macro_rules! give_me_foo_or_bar {
334 /// compile_error!("This macro only accepts `foo` or `bar`");
338 /// give_me_foo_or_bar!(neither);
339 /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
342 /// Emit compiler error if one of a number of features isn't available.
345 /// #[cfg(not(any(feature = "foo", feature = "bar")))]
346 /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.")
349 /// [`panic!`]: ../std/macro.panic.html
350 #[stable(feature = "compile_error_macro", since = "1.20.0")]
351 #[rustc_doc_only_macro]
352 macro_rules! compile_error {
353 ($msg:expr) => ({ /* compiler built-in */ });
354 ($msg:expr,) => ({ /* compiler built-in */ });
357 /// The core macro for formatted string creation & output.
359 /// This macro functions by taking a formatting string literal containing
360 /// `{}` for each additional argument passed. `format_args!` prepares the
361 /// additional parameters to ensure the output can be interpreted as a string
362 /// and canonicalizes the arguments into a single type. Any value that implements
363 /// the [`Display`] trait can be passed to `format_args!`, as can any
364 /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
366 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
367 /// passed to the macros within [`std::fmt`] for performing useful redirection.
368 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
369 /// proxied through this one. `format_args!`, unlike its derived macros, avoids
370 /// heap allocations.
372 /// You can use the [`fmt::Arguments`] value that `format_args!` returns
373 /// in `Debug` and `Display` contexts as seen below. The example also shows
374 /// that `Debug` and `Display` format to the same thing: the interpolated
375 /// format string in `format_args!`.
378 /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
379 /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
380 /// assert_eq!("1 foo 2", display);
381 /// assert_eq!(display, debug);
384 /// For more information, see the documentation in [`std::fmt`].
386 /// [`Display`]: ../std/fmt/trait.Display.html
387 /// [`Debug`]: ../std/fmt/trait.Debug.html
388 /// [`fmt::Arguments`]: ../std/fmt/struct.Arguments.html
389 /// [`std::fmt`]: ../std/fmt/index.html
390 /// [`format!`]: ../std/macro.format.html
391 /// [`write!`]: ../std/macro.write.html
392 /// [`println!`]: ../std/macro.println.html
399 /// let s = fmt::format(format_args!("hello {}", "world"));
400 /// assert_eq!(s, format!("hello {}", "world"));
402 #[stable(feature = "rust1", since = "1.0.0")]
403 #[rustc_doc_only_macro]
404 macro_rules! format_args {
405 ($fmt:expr) => ({ /* compiler built-in */ });
406 ($fmt:expr, $($args:tt)*) => ({ /* compiler built-in */ });
409 /// Inspect an environment variable at compile time.
411 /// This macro will expand to the value of the named environment variable at
412 /// compile time, yielding an expression of type `&'static str`.
414 /// If the environment variable is not defined, then a compilation error
415 /// will be emitted. To not emit a compile error, use the [`option_env!`]
418 /// [`option_env!`]: ../std/macro.option_env.html
423 /// let path: &'static str = env!("PATH");
424 /// println!("the $PATH variable at the time of compiling was: {}", path);
427 /// You can customize the error message by passing a string as the second
431 /// let doc: &'static str = env!("documentation", "what's that?!");
434 /// If the `documentation` environment variable is not defined, you'll get
435 /// the following error:
438 /// error: what's that?!
440 #[stable(feature = "rust1", since = "1.0.0")]
441 #[rustc_doc_only_macro]
443 ($name:expr) => ({ /* compiler built-in */ });
444 ($name:expr,) => ({ /* compiler built-in */ });
447 /// Optionally inspect an environment variable at compile time.
449 /// If the named environment variable is present at compile time, this will
450 /// expand into an expression of type `Option<&'static str>` whose value is
451 /// `Some` of the value of the environment variable. If the environment
452 /// variable is not present, then this will expand to `None`. See
453 /// [`Option<T>`][option] for more information on this type.
455 /// A compile time error is never emitted when using this macro regardless
456 /// of whether the environment variable is present or not.
458 /// [option]: ../std/option/enum.Option.html
463 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
464 /// println!("the secret key might be: {:?}", key);
466 #[stable(feature = "rust1", since = "1.0.0")]
467 #[rustc_doc_only_macro]
468 macro_rules! option_env {
469 ($name:expr) => ({ /* compiler built-in */ });
470 ($name:expr,) => ({ /* compiler built-in */ });
473 /// Concatenate identifiers into one identifier.
475 /// This macro takes any number of comma-separated identifiers, and
476 /// concatenates them all into one, yielding an expression which is a new
477 /// identifier. Note that hygiene makes it such that this macro cannot
478 /// capture local variables. Also, as a general rule, macros are only
479 /// allowed in item, statement or expression position. That means while
480 /// you may use this macro for referring to existing variables, functions or
481 /// modules etc, you cannot define a new one with it.
486 /// #![feature(concat_idents)]
489 /// fn foobar() -> u32 { 23 }
491 /// let f = concat_idents!(foo, bar);
492 /// println!("{}", f());
494 /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
497 #[unstable(feature = "concat_idents_macro", issue = "29599")]
498 #[rustc_doc_only_macro]
499 macro_rules! concat_idents {
500 ($($e:ident),+) => ({ /* compiler built-in */ });
501 ($($e:ident,)+) => ({ /* compiler built-in */ });
504 /// Concatenates literals into a static string slice.
506 /// This macro takes any number of comma-separated literals, yielding an
507 /// expression of type `&'static str` which represents all of the literals
508 /// concatenated left-to-right.
510 /// Integer and floating point literals are stringified in order to be
516 /// let s = concat!("test", 10, 'b', true);
517 /// assert_eq!(s, "test10btrue");
519 #[stable(feature = "rust1", since = "1.0.0")]
520 #[rustc_doc_only_macro]
521 macro_rules! concat {
522 ($($e:expr),*) => ({ /* compiler built-in */ });
523 ($($e:expr,)*) => ({ /* compiler built-in */ });
526 /// A macro which expands to the line number on which it was invoked.
528 /// With [`column!`] and [`file!`], these macros provide debugging information for
529 /// developers about the location within the source.
531 /// The expanded expression has type `u32` and is 1-based, so the first line
532 /// in each file evaluates to 1, the second to 2, etc. This is consistent
533 /// with error messages by common compilers or popular editors.
534 /// The returned line is *not necessarily* the line of the `line!` invocation itself,
535 /// but rather the first macro invocation leading up to the invocation
536 /// of the `line!` macro.
538 /// [`column!`]: macro.column.html
539 /// [`file!`]: macro.file.html
544 /// let current_line = line!();
545 /// println!("defined on line: {}", current_line);
547 #[stable(feature = "rust1", since = "1.0.0")]
548 #[rustc_doc_only_macro]
549 macro_rules! line { () => ({ /* compiler built-in */ }) }
551 /// A macro which expands to the column number on which it was invoked.
553 /// With [`line!`] and [`file!`], these macros provide debugging information for
554 /// developers about the location within the source.
556 /// The expanded expression has type `u32` and is 1-based, so the first column
557 /// in each line evaluates to 1, the second to 2, etc. This is consistent
558 /// with error messages by common compilers or popular editors.
559 /// The returned column is *not necessarily* the line of the `column!` invocation itself,
560 /// but rather the first macro invocation leading up to the invocation
561 /// of the `column!` macro.
563 /// [`line!`]: macro.line.html
564 /// [`file!`]: macro.file.html
569 /// let current_col = column!();
570 /// println!("defined on column: {}", current_col);
572 #[stable(feature = "rust1", since = "1.0.0")]
573 #[rustc_doc_only_macro]
574 macro_rules! column { () => ({ /* compiler built-in */ }) }
576 /// A macro which expands to the file name from which it was invoked.
578 /// With [`line!`] and [`column!`], these macros provide debugging information for
579 /// developers about the location within the source.
582 /// The expanded expression has type `&'static str`, and the returned file
583 /// is not the invocation of the `file!` macro itself, but rather the
584 /// first macro invocation leading up to the invocation of the `file!`
587 /// [`line!`]: macro.line.html
588 /// [`column!`]: macro.column.html
593 /// let this_file = file!();
594 /// println!("defined in file: {}", this_file);
596 #[stable(feature = "rust1", since = "1.0.0")]
597 #[rustc_doc_only_macro]
598 macro_rules! file { () => ({ /* compiler built-in */ }) }
600 /// A macro which stringifies its arguments.
602 /// This macro will yield an expression of type `&'static str` which is the
603 /// stringification of all the tokens passed to the macro. No restrictions
604 /// are placed on the syntax of the macro invocation itself.
606 /// Note that the expanded results of the input tokens may change in the
607 /// future. You should be careful if you rely on the output.
612 /// let one_plus_one = stringify!(1 + 1);
613 /// assert_eq!(one_plus_one, "1 + 1");
615 #[stable(feature = "rust1", since = "1.0.0")]
616 #[rustc_doc_only_macro]
617 macro_rules! stringify { ($($t:tt)*) => ({ /* compiler built-in */ }) }
619 /// Includes a utf8-encoded file as a string.
621 /// The file is located relative to the current file. (similarly to how
622 /// modules are found)
624 /// This macro will yield an expression of type `&'static str` which is the
625 /// contents of the file.
629 /// Assume there are two files in the same directory with the following
632 /// File 'spanish.in':
640 /// ```ignore (cannot-doctest-external-file-dependency)
642 /// let my_str = include_str!("spanish.in");
643 /// assert_eq!(my_str, "adiós\n");
644 /// print!("{}", my_str);
648 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
649 #[stable(feature = "rust1", since = "1.0.0")]
650 #[rustc_doc_only_macro]
651 macro_rules! include_str {
652 ($file:expr) => ({ /* compiler built-in */ });
653 ($file:expr,) => ({ /* compiler built-in */ });
656 /// Includes a file as a reference to a byte array.
658 /// The file is located relative to the current file. (similarly to how
659 /// modules are found)
661 /// This macro will yield an expression of type `&'static [u8; N]` which is
662 /// the contents of the file.
666 /// Assume there are two files in the same directory with the following
669 /// File 'spanish.in':
677 /// ```ignore (cannot-doctest-external-file-dependency)
679 /// let bytes = include_bytes!("spanish.in");
680 /// assert_eq!(bytes, b"adi\xc3\xb3s\n");
681 /// print!("{}", String::from_utf8_lossy(bytes));
685 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
686 #[stable(feature = "rust1", since = "1.0.0")]
687 #[rustc_doc_only_macro]
688 macro_rules! include_bytes {
689 ($file:expr) => ({ /* compiler built-in */ });
690 ($file:expr,) => ({ /* compiler built-in */ });
693 /// Expands to a string that represents the current module path.
695 /// The current module path can be thought of as the hierarchy of modules
696 /// leading back up to the crate root. The first component of the path
697 /// returned is the name of the crate currently being compiled.
704 /// assert!(module_path!().ends_with("test"));
710 #[stable(feature = "rust1", since = "1.0.0")]
711 #[rustc_doc_only_macro]
712 macro_rules! module_path { () => ({ /* compiler built-in */ }) }
714 /// Boolean evaluation of configuration flags, at compile-time.
716 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
717 /// boolean expression evaluation of configuration flags. This frequently
718 /// leads to less duplicated code.
720 /// The syntax given to this macro is the same syntax as [the `cfg`
721 /// attribute](../book/first-edition/conditional-compilation.html).
726 /// let my_directory = if cfg!(windows) {
727 /// "windows-specific-directory"
732 #[stable(feature = "rust1", since = "1.0.0")]
733 #[rustc_doc_only_macro]
734 macro_rules! cfg { ($($cfg:tt)*) => ({ /* compiler built-in */ }) }
736 /// Parse a file as an expression or an item according to the context.
738 /// The file is located relative to the current file (similarly to how
739 /// modules are found).
741 /// Using this macro is often a bad idea, because if the file is
742 /// parsed as an expression, it is going to be placed in the
743 /// surrounding code unhygienically. This could result in variables
744 /// or functions being different from what the file expected if
745 /// there are variables or functions that have the same name in
746 /// the current file.
750 /// Assume there are two files in the same directory with the following
753 /// File 'monkeys.in':
755 /// ```ignore (only-for-syntax-highlight)
760 /// .collect::<String>()
765 /// ```ignore (cannot-doctest-external-file-dependency)
767 /// let my_string = include!("monkeys.in");
768 /// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
769 /// println!("{}", my_string);
773 /// Compiling 'main.rs' and running the resulting binary will print
775 #[stable(feature = "rust1", since = "1.0.0")]
776 #[rustc_doc_only_macro]
777 macro_rules! include {
778 ($file:expr) => ({ /* compiler built-in */ });
779 ($file:expr,) => ({ /* compiler built-in */ });
782 /// Ensure that a boolean expression is `true` at runtime.
784 /// This will invoke the [`panic!`] macro if the provided expression cannot be
785 /// evaluated to `true` at runtime.
789 /// Assertions are always checked in both debug and release builds, and cannot
790 /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
791 /// release builds by default.
793 /// Unsafe code relies on `assert!` to enforce run-time invariants that, if
794 /// violated could lead to unsafety.
796 /// Other use-cases of `assert!` include [testing] and enforcing run-time
797 /// invariants in safe code (whose violation cannot result in unsafety).
799 /// # Custom Messages
801 /// This macro has a second form, where a custom panic message can
802 /// be provided with or without arguments for formatting. See [`std::fmt`]
803 /// for syntax for this form.
805 /// [`panic!`]: macro.panic.html
806 /// [`debug_assert!`]: macro.debug_assert.html
807 /// [testing]: ../book/second-edition/ch11-01-writing-tests.html#checking-results-with-the-assert-macro
808 /// [`std::fmt`]: ../std/fmt/index.html
813 /// // the panic message for these assertions is the stringified value of the
814 /// // expression given.
817 /// fn some_computation() -> bool { true } // a very simple function
819 /// assert!(some_computation());
821 /// // assert with a custom message
823 /// assert!(x, "x wasn't true!");
825 /// let a = 3; let b = 27;
826 /// assert!(a + b == 30, "a = {}, b = {}", a, b);
828 #[stable(feature = "rust1", since = "1.0.0")]
829 #[rustc_doc_only_macro]
830 macro_rules! assert {
831 ($cond:expr) => ({ /* compiler built-in */ });
832 ($cond:expr,) => ({ /* compiler built-in */ });
833 ($cond:expr, $($arg:tt)+) => ({ /* compiler built-in */ });
837 /// A macro for defining `#[cfg]` if-else statements.
839 /// This is similar to the `if/elif` C preprocessor macro by allowing definition
840 /// of a cascade of `#[cfg]` cases, emitting the implementation which matches
843 /// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
844 /// without having to rewrite each clause multiple times.
845 macro_rules! cfg_if {
847 if #[cfg($($meta:meta),*)] { $($it:item)* }
853 $( ( ($($meta),*) ($($it)*) ), )*
859 macro_rules! __cfg_if_items {
860 (($($not:meta,)*) ; ) => {};
861 (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
862 __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* }
863 __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* }
867 macro_rules! __cfg_if_apply {
868 ($m:meta, $($it:item)*) => {