core/macros/mod.rs
1#[doc = include_str!("panic.md")]
2#[macro_export]
3#[rustc_builtin_macro(core_panic)]
4#[allow_internal_unstable(edition_panic)]
5#[stable(feature = "core", since = "1.6.0")]
6#[rustc_diagnostic_item = "core_panic_macro"]
7macro_rules! panic {
8 // Expands to either `$crate::panic::panic_2015` or `$crate::panic::panic_2021`
9 // depending on the edition of the caller.
10 ($($arg:tt)*) => {
11 /* compiler built-in */
12 };
13}
14
15/// Asserts that two expressions are equal to each other (using [`PartialEq`]).
16///
17/// Assertions are always checked in both debug and release builds, and cannot
18/// be disabled. See [`debug_assert_eq!`] for assertions that are disabled in
19/// release builds by default.
20///
21/// [`debug_assert_eq!`]: crate::debug_assert_eq
22///
23/// On panic, this macro will print the values of the expressions with their
24/// debug representations.
25///
26/// Like [`assert!`], this macro has a second form, where a custom
27/// panic message can be provided.
28///
29/// # Examples
30///
31/// ```
32/// let a = 3;
33/// let b = 1 + 2;
34/// assert_eq!(a, b);
35///
36/// assert_eq!(a, b, "we are testing addition with {} and {}", a, b);
37/// ```
38#[macro_export]
39#[stable(feature = "rust1", since = "1.0.0")]
40#[cfg_attr(not(test), rustc_diagnostic_item = "assert_eq_macro")]
41#[allow_internal_unstable(panic_internals)]
42macro_rules! assert_eq {
43 ($left:expr, $right:expr $(,)?) => {
44 match (&$left, &$right) {
45 (left_val, right_val) => {
46 if !(*left_val == *right_val) {
47 let kind = $crate::panicking::AssertKind::Eq;
48 // The reborrows below are intentional. Without them, the stack slot for the
49 // borrow is initialized even before the values are compared, leading to a
50 // noticeable slow down.
51 $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None);
52 }
53 }
54 }
55 };
56 ($left:expr, $right:expr, $($arg:tt)+) => {
57 match (&$left, &$right) {
58 (left_val, right_val) => {
59 if !(*left_val == *right_val) {
60 let kind = $crate::panicking::AssertKind::Eq;
61 // The reborrows below are intentional. Without them, the stack slot for the
62 // borrow is initialized even before the values are compared, leading to a
63 // noticeable slow down.
64 $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)));
65 }
66 }
67 }
68 };
69}
70
71/// Asserts that two expressions are not equal to each other (using [`PartialEq`]).
72///
73/// Assertions are always checked in both debug and release builds, and cannot
74/// be disabled. See [`debug_assert_ne!`] for assertions that are disabled in
75/// release builds by default.
76///
77/// [`debug_assert_ne!`]: crate::debug_assert_ne
78///
79/// On panic, this macro will print the values of the expressions with their
80/// debug representations.
81///
82/// Like [`assert!`], this macro has a second form, where a custom
83/// panic message can be provided.
84///
85/// # Examples
86///
87/// ```
88/// let a = 3;
89/// let b = 2;
90/// assert_ne!(a, b);
91///
92/// assert_ne!(a, b, "we are testing that the values are not equal");
93/// ```
94#[macro_export]
95#[stable(feature = "assert_ne", since = "1.13.0")]
96#[cfg_attr(not(test), rustc_diagnostic_item = "assert_ne_macro")]
97#[allow_internal_unstable(panic_internals)]
98macro_rules! assert_ne {
99 ($left:expr, $right:expr $(,)?) => {
100 match (&$left, &$right) {
101 (left_val, right_val) => {
102 if *left_val == *right_val {
103 let kind = $crate::panicking::AssertKind::Ne;
104 // The reborrows below are intentional. Without them, the stack slot for the
105 // borrow is initialized even before the values are compared, leading to a
106 // noticeable slow down.
107 $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None);
108 }
109 }
110 }
111 };
112 ($left:expr, $right:expr, $($arg:tt)+) => {
113 match (&($left), &($right)) {
114 (left_val, right_val) => {
115 if *left_val == *right_val {
116 let kind = $crate::panicking::AssertKind::Ne;
117 // The reborrows below are intentional. Without them, the stack slot for the
118 // borrow is initialized even before the values are compared, leading to a
119 // noticeable slow down.
120 $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)));
121 }
122 }
123 }
124 };
125}
126
127/// Asserts that an expression matches the provided pattern.
128///
129/// This macro is generally preferable to `assert!(matches!(value, pattern))`, because it can print
130/// the debug representation of the actual value shape that did not meet expectations. In contrast,
131/// using [`assert!`] will only print that expectations were not met, but not why.
132///
133/// The pattern syntax is exactly the same as found in a match arm and the `matches!` macro. The
134/// optional if guard can be used to add additional checks that must be true for the matched value,
135/// otherwise this macro will panic.
136///
137/// Assertions are always checked in both debug and release builds, and cannot
138/// be disabled. See [`debug_assert_matches!`] for assertions that are disabled in
139/// release builds by default.
140///
141/// [`debug_assert_matches!`]: crate::assert_matches::debug_assert_matches
142///
143/// On panic, this macro will print the value of the expression with its debug representation.
144///
145/// Like [`assert!`], this macro has a second form, where a custom panic message can be provided.
146///
147/// # Examples
148///
149/// ```
150/// #![feature(assert_matches)]
151///
152/// use std::assert_matches::assert_matches;
153///
154/// let a = Some(345);
155/// let b = Some(56);
156/// assert_matches!(a, Some(_));
157/// assert_matches!(b, Some(_));
158///
159/// assert_matches!(a, Some(345));
160/// assert_matches!(a, Some(345) | None);
161///
162/// // assert_matches!(a, None); // panics
163/// // assert_matches!(b, Some(345)); // panics
164/// // assert_matches!(b, Some(345) | None); // panics
165///
166/// assert_matches!(a, Some(x) if x > 100);
167/// // assert_matches!(a, Some(x) if x < 100); // panics
168/// ```
169#[unstable(feature = "assert_matches", issue = "82775")]
170#[allow_internal_unstable(panic_internals)]
171#[rustc_macro_transparency = "semitransparent"]
172pub macro assert_matches {
173 ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
174 match $left {
175 $( $pattern )|+ $( if $guard )? => {}
176 ref left_val => {
177 $crate::panicking::assert_matches_failed(
178 left_val,
179 $crate::stringify!($($pattern)|+ $(if $guard)?),
180 $crate::option::Option::None
181 );
182 }
183 }
184 },
185 ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )?, $($arg:tt)+) => {
186 match $left {
187 $( $pattern )|+ $( if $guard )? => {}
188 ref left_val => {
189 $crate::panicking::assert_matches_failed(
190 left_val,
191 $crate::stringify!($($pattern)|+ $(if $guard)?),
192 $crate::option::Option::Some($crate::format_args!($($arg)+))
193 );
194 }
195 }
196 },
197}
198
199/// A macro for defining `#[cfg]` match-like statements.
200///
201/// It is similar to the `if/elif` C preprocessor macro by allowing definition of a cascade of
202/// `#[cfg]` cases, emitting the implementation which matches first.
203///
204/// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
205/// without having to rewrite each clause multiple times.
206///
207/// Trailing `_` wildcard match arms are **optional** and they indicate a fallback branch when
208/// all previous declarations do not evaluate to true.
209///
210/// # Example
211///
212/// ```
213/// #![feature(cfg_match)]
214///
215/// cfg_match! {
216/// cfg(unix) => {
217/// fn foo() { /* unix specific functionality */ }
218/// }
219/// cfg(target_pointer_width = "32") => {
220/// fn foo() { /* non-unix, 32-bit functionality */ }
221/// }
222/// _ => {
223/// fn foo() { /* fallback implementation */ }
224/// }
225/// }
226/// ```
227#[cfg(bootstrap)]
228#[unstable(feature = "cfg_match", issue = "115585")]
229#[rustc_diagnostic_item = "cfg_match"]
230pub macro cfg_match {
231 // with a final wildcard
232 (
233 $(cfg($initial_meta:meta) => { $($initial_tokens:tt)* })+
234 _ => { $($extra_tokens:tt)* }
235 ) => {
236 cfg_match! {
237 @__items ();
238 $((($initial_meta) ($($initial_tokens)*)),)+
239 (() ($($extra_tokens)*)),
240 }
241 },
242
243 // without a final wildcard
244 (
245 $(cfg($extra_meta:meta) => { $($extra_tokens:tt)* })*
246 ) => {
247 cfg_match! {
248 @__items ();
249 $((($extra_meta) ($($extra_tokens)*)),)*
250 }
251 },
252
253 // Internal and recursive macro to emit all the items
254 //
255 // Collects all the previous cfgs in a list at the beginning, so they can be
256 // negated. After the semicolon is all the remaining items.
257 (@__items ($($_:meta,)*);) => {},
258 (
259 @__items ($($no:meta,)*);
260 (($($yes:meta)?) ($($tokens:tt)*)),
261 $($rest:tt,)*
262 ) => {
263 // Emit all items within one block, applying an appropriate #[cfg]. The
264 // #[cfg] will require all `$yes` matchers specified and must also negate
265 // all previous matchers.
266 #[cfg(all(
267 $($yes,)?
268 not(any($($no),*))
269 ))]
270 cfg_match! { @__identity $($tokens)* }
271
272 // Recurse to emit all other items in `$rest`, and when we do so add all
273 // our `$yes` matchers to the list of `$no` matchers as future emissions
274 // will have to negate everything we just matched as well.
275 cfg_match! {
276 @__items ($($no,)* $($yes,)?);
277 $($rest,)*
278 }
279 },
280
281 // Internal macro to make __apply work out right for different match types,
282 // because of how macros match/expand stuff.
283 (@__identity $($tokens:tt)*) => {
284 $($tokens)*
285 }
286}
287
288/// A macro for defining `#[cfg]` match-like statements.
289///
290/// It is similar to the `if/elif` C preprocessor macro by allowing definition of a cascade of
291/// `#[cfg]` cases, emitting the implementation which matches first.
292///
293/// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
294/// without having to rewrite each clause multiple times.
295///
296/// Trailing `_` wildcard match arms are **optional** and they indicate a fallback branch when
297/// all previous declarations do not evaluate to true.
298///
299/// # Example
300///
301/// ```
302/// #![feature(cfg_match)]
303///
304/// cfg_match! {
305/// unix => {
306/// fn foo() { /* unix specific functionality */ }
307/// }
308/// target_pointer_width = "32" => {
309/// fn foo() { /* non-unix, 32-bit functionality */ }
310/// }
311/// _ => {
312/// fn foo() { /* fallback implementation */ }
313/// }
314/// }
315/// ```
316///
317/// If desired, it is possible to return expressions through the use of surrounding braces:
318///
319/// ```
320/// #![feature(cfg_match)]
321///
322/// let _some_string = cfg_match! {{
323/// unix => { "With great power comes great electricity bills" }
324/// _ => { "Behind every successful diet is an unwatched pizza" }
325/// }};
326/// ```
327#[cfg(not(bootstrap))]
328#[unstable(feature = "cfg_match", issue = "115585")]
329#[rustc_diagnostic_item = "cfg_match"]
330pub macro cfg_match {
331 ({ $($tt:tt)* }) => {{
332 cfg_match! { $($tt)* }
333 }},
334 (_ => { $($output:tt)* }) => {
335 $($output)*
336 },
337 (
338 $cfg:meta => $output:tt
339 $($( $rest:tt )+)?
340 ) => {
341 #[cfg($cfg)]
342 cfg_match! { _ => $output }
343 $(
344 #[cfg(not($cfg))]
345 cfg_match! { $($rest)+ }
346 )?
347 },
348}
349
350/// Asserts that a boolean expression is `true` at runtime.
351///
352/// This will invoke the [`panic!`] macro if the provided expression cannot be
353/// evaluated to `true` at runtime.
354///
355/// Like [`assert!`], this macro also has a second version, where a custom panic
356/// message can be provided.
357///
358/// # Uses
359///
360/// Unlike [`assert!`], `debug_assert!` statements are only enabled in non
361/// optimized builds by default. An optimized build will not execute
362/// `debug_assert!` statements unless `-C debug-assertions` is passed to the
363/// compiler. This makes `debug_assert!` useful for checks that are too
364/// expensive to be present in a release build but may be helpful during
365/// development. The result of expanding `debug_assert!` is always type checked.
366///
367/// An unchecked assertion allows a program in an inconsistent state to keep
368/// running, which might have unexpected consequences but does not introduce
369/// unsafety as long as this only happens in safe code. The performance cost
370/// of assertions, however, is not measurable in general. Replacing [`assert!`]
371/// with `debug_assert!` is thus only encouraged after thorough profiling, and
372/// more importantly, only in safe code!
373///
374/// # Examples
375///
376/// ```
377/// // the panic message for these assertions is the stringified value of the
378/// // expression given.
379/// debug_assert!(true);
380///
381/// fn some_expensive_computation() -> bool { true } // a very simple function
382/// debug_assert!(some_expensive_computation());
383///
384/// // assert with a custom message
385/// let x = true;
386/// debug_assert!(x, "x wasn't true!");
387///
388/// let a = 3; let b = 27;
389/// debug_assert!(a + b == 30, "a = {}, b = {}", a, b);
390/// ```
391#[macro_export]
392#[stable(feature = "rust1", since = "1.0.0")]
393#[rustc_diagnostic_item = "debug_assert_macro"]
394#[allow_internal_unstable(edition_panic)]
395macro_rules! debug_assert {
396 ($($arg:tt)*) => {
397 if $crate::cfg!(debug_assertions) {
398 $crate::assert!($($arg)*);
399 }
400 };
401}
402
403/// Asserts that two expressions are equal to each other.
404///
405/// On panic, this macro will print the values of the expressions with their
406/// debug representations.
407///
408/// Unlike [`assert_eq!`], `debug_assert_eq!` statements are only enabled in non
409/// optimized builds by default. An optimized build will not execute
410/// `debug_assert_eq!` statements unless `-C debug-assertions` is passed to the
411/// compiler. This makes `debug_assert_eq!` useful for checks that are too
412/// expensive to be present in a release build but may be helpful during
413/// development. The result of expanding `debug_assert_eq!` is always type checked.
414///
415/// # Examples
416///
417/// ```
418/// let a = 3;
419/// let b = 1 + 2;
420/// debug_assert_eq!(a, b);
421/// ```
422#[macro_export]
423#[stable(feature = "rust1", since = "1.0.0")]
424#[cfg_attr(not(test), rustc_diagnostic_item = "debug_assert_eq_macro")]
425macro_rules! debug_assert_eq {
426 ($($arg:tt)*) => {
427 if $crate::cfg!(debug_assertions) {
428 $crate::assert_eq!($($arg)*);
429 }
430 };
431}
432
433/// Asserts that two expressions are not equal to each other.
434///
435/// On panic, this macro will print the values of the expressions with their
436/// debug representations.
437///
438/// Unlike [`assert_ne!`], `debug_assert_ne!` statements are only enabled in non
439/// optimized builds by default. An optimized build will not execute
440/// `debug_assert_ne!` statements unless `-C debug-assertions` is passed to the
441/// compiler. This makes `debug_assert_ne!` useful for checks that are too
442/// expensive to be present in a release build but may be helpful during
443/// development. The result of expanding `debug_assert_ne!` is always type checked.
444///
445/// # Examples
446///
447/// ```
448/// let a = 3;
449/// let b = 2;
450/// debug_assert_ne!(a, b);
451/// ```
452#[macro_export]
453#[stable(feature = "assert_ne", since = "1.13.0")]
454#[cfg_attr(not(test), rustc_diagnostic_item = "debug_assert_ne_macro")]
455macro_rules! debug_assert_ne {
456 ($($arg:tt)*) => {
457 if $crate::cfg!(debug_assertions) {
458 $crate::assert_ne!($($arg)*);
459 }
460 };
461}
462
463/// Asserts that an expression matches the provided pattern.
464///
465/// This macro is generally preferable to `debug_assert!(matches!(value, pattern))`, because it can
466/// print the debug representation of the actual value shape that did not meet expectations. In
467/// contrast, using [`debug_assert!`] will only print that expectations were not met, but not why.
468///
469/// The pattern syntax is exactly the same as found in a match arm and the `matches!` macro. The
470/// optional if guard can be used to add additional checks that must be true for the matched value,
471/// otherwise this macro will panic.
472///
473/// On panic, this macro will print the value of the expression with its debug representation.
474///
475/// Like [`assert!`], this macro has a second form, where a custom panic message can be provided.
476///
477/// Unlike [`assert_matches!`], `debug_assert_matches!` statements are only enabled in non optimized
478/// builds by default. An optimized build will not execute `debug_assert_matches!` statements unless
479/// `-C debug-assertions` is passed to the compiler. This makes `debug_assert_matches!` useful for
480/// checks that are too expensive to be present in a release build but may be helpful during
481/// development. The result of expanding `debug_assert_matches!` is always type checked.
482///
483/// # Examples
484///
485/// ```
486/// #![feature(assert_matches)]
487///
488/// use std::assert_matches::debug_assert_matches;
489///
490/// let a = Some(345);
491/// let b = Some(56);
492/// debug_assert_matches!(a, Some(_));
493/// debug_assert_matches!(b, Some(_));
494///
495/// debug_assert_matches!(a, Some(345));
496/// debug_assert_matches!(a, Some(345) | None);
497///
498/// // debug_assert_matches!(a, None); // panics
499/// // debug_assert_matches!(b, Some(345)); // panics
500/// // debug_assert_matches!(b, Some(345) | None); // panics
501///
502/// debug_assert_matches!(a, Some(x) if x > 100);
503/// // debug_assert_matches!(a, Some(x) if x < 100); // panics
504/// ```
505#[unstable(feature = "assert_matches", issue = "82775")]
506#[allow_internal_unstable(assert_matches)]
507#[rustc_macro_transparency = "semitransparent"]
508pub macro debug_assert_matches($($arg:tt)*) {
509 if $crate::cfg!(debug_assertions) {
510 $crate::assert_matches::assert_matches!($($arg)*);
511 }
512}
513
514/// Returns whether the given expression matches the provided pattern.
515///
516/// The pattern syntax is exactly the same as found in a match arm. The optional if guard can be
517/// used to add additional checks that must be true for the matched value, otherwise this macro will
518/// return `false`.
519///
520/// When testing that a value matches a pattern, it's generally preferable to use
521/// [`assert_matches!`] as it will print the debug representation of the value if the assertion
522/// fails.
523///
524/// # Examples
525///
526/// ```
527/// let foo = 'f';
528/// assert!(matches!(foo, 'A'..='Z' | 'a'..='z'));
529///
530/// let bar = Some(4);
531/// assert!(matches!(bar, Some(x) if x > 2));
532/// ```
533#[macro_export]
534#[stable(feature = "matches_macro", since = "1.42.0")]
535#[cfg_attr(not(test), rustc_diagnostic_item = "matches_macro")]
536macro_rules! matches {
537 ($expression:expr, $pattern:pat $(if $guard:expr)? $(,)?) => {
538 match $expression {
539 $pattern $(if $guard)? => true,
540 _ => false
541 }
542 };
543}
544
545/// Unwraps a result or propagates its error.
546///
547/// The [`?` operator][propagating-errors] was added to replace `try!`
548/// and should be used instead. Furthermore, `try` is a reserved word
549/// in Rust 2018, so if you must use it, you will need to use the
550/// [raw-identifier syntax][ris]: `r#try`.
551///
552/// [propagating-errors]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
553/// [ris]: https://doc.rust-lang.org/nightly/rust-by-example/compatibility/raw_identifiers.html
554///
555/// `try!` matches the given [`Result`]. In case of the `Ok` variant, the
556/// expression has the value of the wrapped value.
557///
558/// In case of the `Err` variant, it retrieves the inner error. `try!` then
559/// performs conversion using `From`. This provides automatic conversion
560/// between specialized errors and more general ones. The resulting
561/// error is then immediately returned.
562///
563/// Because of the early return, `try!` can only be used in functions that
564/// return [`Result`].
565///
566/// # Examples
567///
568/// ```
569/// use std::io;
570/// use std::fs::File;
571/// use std::io::prelude::*;
572///
573/// enum MyError {
574/// FileWriteError
575/// }
576///
577/// impl From<io::Error> for MyError {
578/// fn from(e: io::Error) -> MyError {
579/// MyError::FileWriteError
580/// }
581/// }
582///
583/// // The preferred method of quick returning Errors
584/// fn write_to_file_question() -> Result<(), MyError> {
585/// let mut file = File::create("my_best_friends.txt")?;
586/// file.write_all(b"This is a list of my best friends.")?;
587/// Ok(())
588/// }
589///
590/// // The previous method of quick returning Errors
591/// fn write_to_file_using_try() -> Result<(), MyError> {
592/// let mut file = r#try!(File::create("my_best_friends.txt"));
593/// r#try!(file.write_all(b"This is a list of my best friends."));
594/// Ok(())
595/// }
596///
597/// // This is equivalent to:
598/// fn write_to_file_using_match() -> Result<(), MyError> {
599/// let mut file = r#try!(File::create("my_best_friends.txt"));
600/// match file.write_all(b"This is a list of my best friends.") {
601/// Ok(v) => v,
602/// Err(e) => return Err(From::from(e)),
603/// }
604/// Ok(())
605/// }
606/// ```
607#[macro_export]
608#[stable(feature = "rust1", since = "1.0.0")]
609#[deprecated(since = "1.39.0", note = "use the `?` operator instead")]
610#[doc(alias = "?")]
611macro_rules! r#try {
612 ($expr:expr $(,)?) => {
613 match $expr {
614 $crate::result::Result::Ok(val) => val,
615 $crate::result::Result::Err(err) => {
616 return $crate::result::Result::Err($crate::convert::From::from(err));
617 }
618 }
619 };
620}
621
622/// Writes formatted data into a buffer.
623///
624/// This macro accepts a 'writer', a format string, and a list of arguments. Arguments will be
625/// formatted according to the specified format string and the result will be passed to the writer.
626/// The writer may be any value with a `write_fmt` method; generally this comes from an
627/// implementation of either the [`fmt::Write`] or the [`io::Write`] trait. The macro
628/// returns whatever the `write_fmt` method returns; commonly a [`fmt::Result`], or an
629/// [`io::Result`].
630///
631/// See [`std::fmt`] for more information on the format string syntax.
632///
633/// [`std::fmt`]: ../std/fmt/index.html
634/// [`fmt::Write`]: crate::fmt::Write
635/// [`io::Write`]: ../std/io/trait.Write.html
636/// [`fmt::Result`]: crate::fmt::Result
637/// [`io::Result`]: ../std/io/type.Result.html
638///
639/// # Examples
640///
641/// ```
642/// use std::io::Write;
643///
644/// fn main() -> std::io::Result<()> {
645/// let mut w = Vec::new();
646/// write!(&mut w, "test")?;
647/// write!(&mut w, "formatted {}", "arguments")?;
648///
649/// assert_eq!(w, b"testformatted arguments");
650/// Ok(())
651/// }
652/// ```
653///
654/// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
655/// implementing either, as objects do not typically implement both. However, the module must
656/// avoid conflict between the trait names, such as by importing them as `_` or otherwise renaming
657/// them:
658///
659/// ```
660/// use std::fmt::Write as _;
661/// use std::io::Write as _;
662///
663/// fn main() -> Result<(), Box<dyn std::error::Error>> {
664/// let mut s = String::new();
665/// let mut v = Vec::new();
666///
667/// write!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
668/// write!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
669/// assert_eq!(v, b"s = \"abc 123\"");
670/// Ok(())
671/// }
672/// ```
673///
674/// If you also need the trait names themselves, such as to implement one or both on your types,
675/// import the containing module and then name them with a prefix:
676///
677/// ```
678/// # #![allow(unused_imports)]
679/// use std::fmt::{self, Write as _};
680/// use std::io::{self, Write as _};
681///
682/// struct Example;
683///
684/// impl fmt::Write for Example {
685/// fn write_str(&mut self, _s: &str) -> core::fmt::Result {
686/// unimplemented!();
687/// }
688/// }
689/// ```
690///
691/// Note: This macro can be used in `no_std` setups as well.
692/// In a `no_std` setup you are responsible for the implementation details of the components.
693///
694/// ```no_run
695/// use core::fmt::Write;
696///
697/// struct Example;
698///
699/// impl Write for Example {
700/// fn write_str(&mut self, _s: &str) -> core::fmt::Result {
701/// unimplemented!();
702/// }
703/// }
704///
705/// let mut m = Example{};
706/// write!(&mut m, "Hello World").expect("Not written");
707/// ```
708#[macro_export]
709#[stable(feature = "rust1", since = "1.0.0")]
710#[cfg_attr(not(test), rustc_diagnostic_item = "write_macro")]
711macro_rules! write {
712 ($dst:expr, $($arg:tt)*) => {
713 $dst.write_fmt($crate::format_args!($($arg)*))
714 };
715}
716
717/// Writes formatted data into a buffer, with a newline appended.
718///
719/// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
720/// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
721///
722/// For more information, see [`write!`]. For information on the format string syntax, see
723/// [`std::fmt`].
724///
725/// [`std::fmt`]: ../std/fmt/index.html
726///
727/// # Examples
728///
729/// ```
730/// use std::io::{Write, Result};
731///
732/// fn main() -> Result<()> {
733/// let mut w = Vec::new();
734/// writeln!(&mut w)?;
735/// writeln!(&mut w, "test")?;
736/// writeln!(&mut w, "formatted {}", "arguments")?;
737///
738/// assert_eq!(&w[..], "\ntest\nformatted arguments\n".as_bytes());
739/// Ok(())
740/// }
741/// ```
742#[macro_export]
743#[stable(feature = "rust1", since = "1.0.0")]
744#[cfg_attr(not(test), rustc_diagnostic_item = "writeln_macro")]
745#[allow_internal_unstable(format_args_nl)]
746macro_rules! writeln {
747 ($dst:expr $(,)?) => {
748 $crate::write!($dst, "\n")
749 };
750 ($dst:expr, $($arg:tt)*) => {
751 $dst.write_fmt($crate::format_args_nl!($($arg)*))
752 };
753}
754
755/// Indicates unreachable code.
756///
757/// This is useful any time that the compiler can't determine that some code is unreachable. For
758/// example:
759///
760/// * Match arms with guard conditions.
761/// * Loops that dynamically terminate.
762/// * Iterators that dynamically terminate.
763///
764/// If the determination that the code is unreachable proves incorrect, the
765/// program immediately terminates with a [`panic!`].
766///
767/// The unsafe counterpart of this macro is the [`unreachable_unchecked`] function, which
768/// will cause undefined behavior if the code is reached.
769///
770/// [`unreachable_unchecked`]: crate::hint::unreachable_unchecked
771///
772/// # Panics
773///
774/// This will always [`panic!`] because `unreachable!` is just a shorthand for `panic!` with a
775/// fixed, specific message.
776///
777/// Like `panic!`, this macro has a second form for displaying custom values.
778///
779/// # Examples
780///
781/// Match arms:
782///
783/// ```
784/// # #[allow(dead_code)]
785/// fn foo(x: Option<i32>) {
786/// match x {
787/// Some(n) if n >= 0 => println!("Some(Non-negative)"),
788/// Some(n) if n < 0 => println!("Some(Negative)"),
789/// Some(_) => unreachable!(), // compile error if commented out
790/// None => println!("None")
791/// }
792/// }
793/// ```
794///
795/// Iterators:
796///
797/// ```
798/// # #[allow(dead_code)]
799/// fn divide_by_three(x: u32) -> u32 { // one of the poorest implementations of x/3
800/// for i in 0.. {
801/// if 3*i < i { panic!("u32 overflow"); }
802/// if x < 3*i { return i-1; }
803/// }
804/// unreachable!("The loop should always return");
805/// }
806/// ```
807#[macro_export]
808#[rustc_builtin_macro(unreachable)]
809#[allow_internal_unstable(edition_panic)]
810#[stable(feature = "rust1", since = "1.0.0")]
811#[cfg_attr(not(test), rustc_diagnostic_item = "unreachable_macro")]
812macro_rules! unreachable {
813 // Expands to either `$crate::panic::unreachable_2015` or `$crate::panic::unreachable_2021`
814 // depending on the edition of the caller.
815 ($($arg:tt)*) => {
816 /* compiler built-in */
817 };
818}
819
820/// Indicates unimplemented code by panicking with a message of "not implemented".
821///
822/// This allows your code to type-check, which is useful if you are prototyping or
823/// implementing a trait that requires multiple methods which you don't plan to use all of.
824///
825/// The difference between `unimplemented!` and [`todo!`] is that while `todo!`
826/// conveys an intent of implementing the functionality later and the message is "not yet
827/// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
828///
829/// Also, some IDEs will mark `todo!`s.
830///
831/// # Panics
832///
833/// This will always [`panic!`] because `unimplemented!` is just a shorthand for `panic!` with a
834/// fixed, specific message.
835///
836/// Like `panic!`, this macro has a second form for displaying custom values.
837///
838/// [`todo!`]: crate::todo
839///
840/// # Examples
841///
842/// Say we have a trait `Foo`:
843///
844/// ```
845/// trait Foo {
846/// fn bar(&self) -> u8;
847/// fn baz(&self);
848/// fn qux(&self) -> Result<u64, ()>;
849/// }
850/// ```
851///
852/// We want to implement `Foo` for 'MyStruct', but for some reason it only makes sense
853/// to implement the `bar()` function. `baz()` and `qux()` will still need to be defined
854/// in our implementation of `Foo`, but we can use `unimplemented!` in their definitions
855/// to allow our code to compile.
856///
857/// We still want to have our program stop running if the unimplemented methods are
858/// reached.
859///
860/// ```
861/// # trait Foo {
862/// # fn bar(&self) -> u8;
863/// # fn baz(&self);
864/// # fn qux(&self) -> Result<u64, ()>;
865/// # }
866/// struct MyStruct;
867///
868/// impl Foo for MyStruct {
869/// fn bar(&self) -> u8 {
870/// 1 + 1
871/// }
872///
873/// fn baz(&self) {
874/// // It makes no sense to `baz` a `MyStruct`, so we have no logic here
875/// // at all.
876/// // This will display "thread 'main' panicked at 'not implemented'".
877/// unimplemented!();
878/// }
879///
880/// fn qux(&self) -> Result<u64, ()> {
881/// // We have some logic here,
882/// // We can add a message to unimplemented! to display our omission.
883/// // This will display:
884/// // "thread 'main' panicked at 'not implemented: MyStruct isn't quxable'".
885/// unimplemented!("MyStruct isn't quxable");
886/// }
887/// }
888///
889/// fn main() {
890/// let s = MyStruct;
891/// s.bar();
892/// }
893/// ```
894#[macro_export]
895#[stable(feature = "rust1", since = "1.0.0")]
896#[cfg_attr(not(test), rustc_diagnostic_item = "unimplemented_macro")]
897#[allow_internal_unstable(panic_internals)]
898macro_rules! unimplemented {
899 () => {
900 $crate::panicking::panic("not implemented")
901 };
902 ($($arg:tt)+) => {
903 $crate::panic!("not implemented: {}", $crate::format_args!($($arg)+))
904 };
905}
906
907/// Indicates unfinished code.
908///
909/// This can be useful if you are prototyping and just
910/// want a placeholder to let your code pass type analysis.
911///
912/// The difference between [`unimplemented!`] and `todo!` is that while `todo!` conveys
913/// an intent of implementing the functionality later and the message is "not yet
914/// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
915///
916/// Also, some IDEs will mark `todo!`s.
917///
918/// # Panics
919///
920/// This will always [`panic!`] because `todo!` is just a shorthand for `panic!` with a
921/// fixed, specific message.
922///
923/// Like `panic!`, this macro has a second form for displaying custom values.
924///
925/// # Examples
926///
927/// Here's an example of some in-progress code. We have a trait `Foo`:
928///
929/// ```
930/// trait Foo {
931/// fn bar(&self) -> u8;
932/// fn baz(&self);
933/// fn qux(&self) -> Result<u64, ()>;
934/// }
935/// ```
936///
937/// We want to implement `Foo` on one of our types, but we also want to work on
938/// just `bar()` first. In order for our code to compile, we need to implement
939/// `baz()` and `qux()`, so we can use `todo!`:
940///
941/// ```
942/// # trait Foo {
943/// # fn bar(&self) -> u8;
944/// # fn baz(&self);
945/// # fn qux(&self) -> Result<u64, ()>;
946/// # }
947/// struct MyStruct;
948///
949/// impl Foo for MyStruct {
950/// fn bar(&self) -> u8 {
951/// 1 + 1
952/// }
953///
954/// fn baz(&self) {
955/// // Let's not worry about implementing baz() for now
956/// todo!();
957/// }
958///
959/// fn qux(&self) -> Result<u64, ()> {
960/// // We can add a message to todo! to display our omission.
961/// // This will display:
962/// // "thread 'main' panicked at 'not yet implemented: MyStruct is not yet quxable'".
963/// todo!("MyStruct is not yet quxable");
964/// }
965/// }
966///
967/// fn main() {
968/// let s = MyStruct;
969/// s.bar();
970///
971/// // We aren't even using baz() or qux(), so this is fine.
972/// }
973/// ```
974#[macro_export]
975#[stable(feature = "todo_macro", since = "1.40.0")]
976#[cfg_attr(not(test), rustc_diagnostic_item = "todo_macro")]
977#[allow_internal_unstable(panic_internals)]
978macro_rules! todo {
979 () => {
980 $crate::panicking::panic("not yet implemented")
981 };
982 ($($arg:tt)+) => {
983 $crate::panic!("not yet implemented: {}", $crate::format_args!($($arg)+))
984 };
985}
986
987/// Definitions of built-in macros.
988///
989/// Most of the macro properties (stability, visibility, etc.) are taken from the source code here,
990/// with exception of expansion functions transforming macro inputs into outputs,
991/// those functions are provided by the compiler.
992pub(crate) mod builtin {
993
994 /// Causes compilation to fail with the given error message when encountered.
995 ///
996 /// This macro should be used when a crate uses a conditional compilation strategy to provide
997 /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
998 /// but emits an error during *compilation* rather than at *runtime*.
999 ///
1000 /// # Examples
1001 ///
1002 /// Two such examples are macros and `#[cfg]` environments.
1003 ///
1004 /// Emit a better compiler error if a macro is passed invalid values. Without the final branch,
1005 /// the compiler would still emit an error, but the error's message would not mention the two
1006 /// valid values.
1007 ///
1008 /// ```compile_fail
1009 /// macro_rules! give_me_foo_or_bar {
1010 /// (foo) => {};
1011 /// (bar) => {};
1012 /// ($x:ident) => {
1013 /// compile_error!("This macro only accepts `foo` or `bar`");
1014 /// }
1015 /// }
1016 ///
1017 /// give_me_foo_or_bar!(neither);
1018 /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
1019 /// ```
1020 ///
1021 /// Emit a compiler error if one of a number of features isn't available.
1022 ///
1023 /// ```compile_fail
1024 /// #[cfg(not(any(feature = "foo", feature = "bar")))]
1025 /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.");
1026 /// ```
1027 #[stable(feature = "compile_error_macro", since = "1.20.0")]
1028 #[rustc_builtin_macro]
1029 #[macro_export]
1030 macro_rules! compile_error {
1031 ($msg:expr $(,)?) => {{ /* compiler built-in */ }};
1032 }
1033
1034 /// Constructs parameters for the other string-formatting macros.
1035 ///
1036 /// This macro functions by taking a formatting string literal containing
1037 /// `{}` for each additional argument passed. `format_args!` prepares the
1038 /// additional parameters to ensure the output can be interpreted as a string
1039 /// and canonicalizes the arguments into a single type. Any value that implements
1040 /// the [`Display`] trait can be passed to `format_args!`, as can any
1041 /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
1042 ///
1043 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
1044 /// passed to the macros within [`std::fmt`] for performing useful redirection.
1045 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
1046 /// proxied through this one. `format_args!`, unlike its derived macros, avoids
1047 /// heap allocations.
1048 ///
1049 /// You can use the [`fmt::Arguments`] value that `format_args!` returns
1050 /// in `Debug` and `Display` contexts as seen below. The example also shows
1051 /// that `Debug` and `Display` format to the same thing: the interpolated
1052 /// format string in `format_args!`.
1053 ///
1054 /// ```rust
1055 /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
1056 /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
1057 /// assert_eq!("1 foo 2", display);
1058 /// assert_eq!(display, debug);
1059 /// ```
1060 ///
1061 /// See [the formatting documentation in `std::fmt`](../std/fmt/index.html)
1062 /// for details of the macro argument syntax, and further information.
1063 ///
1064 /// [`Display`]: crate::fmt::Display
1065 /// [`Debug`]: crate::fmt::Debug
1066 /// [`fmt::Arguments`]: crate::fmt::Arguments
1067 /// [`std::fmt`]: ../std/fmt/index.html
1068 /// [`format!`]: ../std/macro.format.html
1069 /// [`println!`]: ../std/macro.println.html
1070 ///
1071 /// # Examples
1072 ///
1073 /// ```
1074 /// use std::fmt;
1075 ///
1076 /// let s = fmt::format(format_args!("hello {}", "world"));
1077 /// assert_eq!(s, format!("hello {}", "world"));
1078 /// ```
1079 ///
1080 /// # Lifetime limitation
1081 ///
1082 /// Except when no formatting arguments are used,
1083 /// the produced `fmt::Arguments` value borrows temporary values,
1084 /// which means it can only be used within the same expression
1085 /// and cannot be stored for later use.
1086 /// This is a known limitation, see [#92698](https://github.com/rust-lang/rust/issues/92698).
1087 #[stable(feature = "rust1", since = "1.0.0")]
1088 #[cfg_attr(not(test), rustc_diagnostic_item = "format_args_macro")]
1089 #[allow_internal_unsafe]
1090 #[allow_internal_unstable(fmt_internals)]
1091 #[rustc_builtin_macro]
1092 #[macro_export]
1093 macro_rules! format_args {
1094 ($fmt:expr) => {{ /* compiler built-in */ }};
1095 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
1096 }
1097
1098 /// Same as [`format_args`], but can be used in some const contexts.
1099 ///
1100 /// This macro is used by the panic macros for the `const_panic` feature.
1101 ///
1102 /// This macro will be removed once `format_args` is allowed in const contexts.
1103 #[unstable(feature = "const_format_args", issue = "none")]
1104 #[allow_internal_unstable(fmt_internals, const_fmt_arguments_new)]
1105 #[rustc_builtin_macro]
1106 #[macro_export]
1107 macro_rules! const_format_args {
1108 ($fmt:expr) => {{ /* compiler built-in */ }};
1109 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
1110 }
1111
1112 /// Same as [`format_args`], but adds a newline in the end.
1113 #[unstable(
1114 feature = "format_args_nl",
1115 issue = "none",
1116 reason = "`format_args_nl` is only for internal \
1117 language use and is subject to change"
1118 )]
1119 #[allow_internal_unstable(fmt_internals)]
1120 #[rustc_builtin_macro]
1121 #[macro_export]
1122 macro_rules! format_args_nl {
1123 ($fmt:expr) => {{ /* compiler built-in */ }};
1124 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
1125 }
1126
1127 /// Inspects an environment variable at compile time.
1128 ///
1129 /// This macro will expand to the value of the named environment variable at
1130 /// compile time, yielding an expression of type `&'static str`. Use
1131 /// [`std::env::var`] instead if you want to read the value at runtime.
1132 ///
1133 /// [`std::env::var`]: ../std/env/fn.var.html
1134 ///
1135 /// If the environment variable is not defined, then a compilation error
1136 /// will be emitted. To not emit a compile error, use the [`option_env!`]
1137 /// macro instead. A compilation error will also be emitted if the
1138 /// environment variable is not a valid Unicode string.
1139 ///
1140 /// # Examples
1141 ///
1142 /// ```
1143 /// let path: &'static str = env!("PATH");
1144 /// println!("the $PATH variable at the time of compiling was: {path}");
1145 /// ```
1146 ///
1147 /// You can customize the error message by passing a string as the second
1148 /// parameter:
1149 ///
1150 /// ```compile_fail
1151 /// let doc: &'static str = env!("documentation", "what's that?!");
1152 /// ```
1153 ///
1154 /// If the `documentation` environment variable is not defined, you'll get
1155 /// the following error:
1156 ///
1157 /// ```text
1158 /// error: what's that?!
1159 /// ```
1160 #[stable(feature = "rust1", since = "1.0.0")]
1161 #[rustc_builtin_macro]
1162 #[macro_export]
1163 #[rustc_diagnostic_item = "env_macro"] // useful for external lints
1164 macro_rules! env {
1165 ($name:expr $(,)?) => {{ /* compiler built-in */ }};
1166 ($name:expr, $error_msg:expr $(,)?) => {{ /* compiler built-in */ }};
1167 }
1168
1169 /// Optionally inspects an environment variable at compile time.
1170 ///
1171 /// If the named environment variable is present at compile time, this will
1172 /// expand into an expression of type `Option<&'static str>` whose value is
1173 /// `Some` of the value of the environment variable (a compilation error
1174 /// will be emitted if the environment variable is not a valid Unicode
1175 /// string). If the environment variable is not present, then this will
1176 /// expand to `None`. See [`Option<T>`][Option] for more information on this
1177 /// type. Use [`std::env::var`] instead if you want to read the value at
1178 /// runtime.
1179 ///
1180 /// [`std::env::var`]: ../std/env/fn.var.html
1181 ///
1182 /// A compile time error is only emitted when using this macro if the
1183 /// environment variable exists and is not a valid Unicode string. To also
1184 /// emit a compile error if the environment variable is not present, use the
1185 /// [`env!`] macro instead.
1186 ///
1187 /// # Examples
1188 ///
1189 /// ```
1190 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
1191 /// println!("the secret key might be: {key:?}");
1192 /// ```
1193 #[stable(feature = "rust1", since = "1.0.0")]
1194 #[rustc_builtin_macro]
1195 #[macro_export]
1196 #[rustc_diagnostic_item = "option_env_macro"] // useful for external lints
1197 macro_rules! option_env {
1198 ($name:expr $(,)?) => {{ /* compiler built-in */ }};
1199 }
1200
1201 /// Concatenates identifiers into one identifier.
1202 ///
1203 /// This macro takes any number of comma-separated identifiers, and
1204 /// concatenates them all into one, yielding an expression which is a new
1205 /// identifier. Note that hygiene makes it such that this macro cannot
1206 /// capture local variables. Also, as a general rule, macros are only
1207 /// allowed in item, statement or expression position. That means while
1208 /// you may use this macro for referring to existing variables, functions or
1209 /// modules etc, you cannot define a new one with it.
1210 ///
1211 /// # Examples
1212 ///
1213 /// ```
1214 /// #![feature(concat_idents)]
1215 ///
1216 /// # fn main() {
1217 /// fn foobar() -> u32 { 23 }
1218 ///
1219 /// let f = concat_idents!(foo, bar);
1220 /// println!("{}", f());
1221 ///
1222 /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
1223 /// # }
1224 /// ```
1225 #[unstable(
1226 feature = "concat_idents",
1227 issue = "29599",
1228 reason = "`concat_idents` is not stable enough for use and is subject to change"
1229 )]
1230 #[rustc_builtin_macro]
1231 #[macro_export]
1232 macro_rules! concat_idents {
1233 ($($e:ident),+ $(,)?) => {{ /* compiler built-in */ }};
1234 }
1235
1236 /// Concatenates literals into a byte slice.
1237 ///
1238 /// This macro takes any number of comma-separated literals, and concatenates them all into
1239 /// one, yielding an expression of type `&[u8; _]`, which represents all of the literals
1240 /// concatenated left-to-right. The literals passed can be any combination of:
1241 ///
1242 /// - byte literals (`b'r'`)
1243 /// - byte strings (`b"Rust"`)
1244 /// - arrays of bytes/numbers (`[b'A', 66, b'C']`)
1245 ///
1246 /// # Examples
1247 ///
1248 /// ```
1249 /// #![feature(concat_bytes)]
1250 ///
1251 /// # fn main() {
1252 /// let s: &[u8; 6] = concat_bytes!(b'A', b"BC", [68, b'E', 70]);
1253 /// assert_eq!(s, b"ABCDEF");
1254 /// # }
1255 /// ```
1256 #[unstable(feature = "concat_bytes", issue = "87555")]
1257 #[rustc_builtin_macro]
1258 #[macro_export]
1259 macro_rules! concat_bytes {
1260 ($($e:literal),+ $(,)?) => {{ /* compiler built-in */ }};
1261 }
1262
1263 /// Concatenates literals into a static string slice.
1264 ///
1265 /// This macro takes any number of comma-separated literals, yielding an
1266 /// expression of type `&'static str` which represents all of the literals
1267 /// concatenated left-to-right.
1268 ///
1269 /// Integer and floating point literals are [stringified](core::stringify) in order to be
1270 /// concatenated.
1271 ///
1272 /// # Examples
1273 ///
1274 /// ```
1275 /// let s = concat!("test", 10, 'b', true);
1276 /// assert_eq!(s, "test10btrue");
1277 /// ```
1278 #[stable(feature = "rust1", since = "1.0.0")]
1279 #[rustc_builtin_macro]
1280 #[macro_export]
1281 macro_rules! concat {
1282 ($($e:expr),* $(,)?) => {{ /* compiler built-in */ }};
1283 }
1284
1285 /// Expands to the line number on which it was invoked.
1286 ///
1287 /// With [`column!`] and [`file!`], these macros provide debugging information for
1288 /// developers about the location within the source.
1289 ///
1290 /// The expanded expression has type `u32` and is 1-based, so the first line
1291 /// in each file evaluates to 1, the second to 2, etc. This is consistent
1292 /// with error messages by common compilers or popular editors.
1293 /// The returned line is *not necessarily* the line of the `line!` invocation itself,
1294 /// but rather the first macro invocation leading up to the invocation
1295 /// of the `line!` macro.
1296 ///
1297 /// # Examples
1298 ///
1299 /// ```
1300 /// let current_line = line!();
1301 /// println!("defined on line: {current_line}");
1302 /// ```
1303 #[stable(feature = "rust1", since = "1.0.0")]
1304 #[rustc_builtin_macro]
1305 #[macro_export]
1306 macro_rules! line {
1307 () => {
1308 /* compiler built-in */
1309 };
1310 }
1311
1312 /// Expands to the column number at which it was invoked.
1313 ///
1314 /// With [`line!`] and [`file!`], these macros provide debugging information for
1315 /// developers about the location within the source.
1316 ///
1317 /// The expanded expression has type `u32` and is 1-based, so the first column
1318 /// in each line evaluates to 1, the second to 2, etc. This is consistent
1319 /// with error messages by common compilers or popular editors.
1320 /// The returned column is *not necessarily* the line of the `column!` invocation itself,
1321 /// but rather the first macro invocation leading up to the invocation
1322 /// of the `column!` macro.
1323 ///
1324 /// # Examples
1325 ///
1326 /// ```
1327 /// let current_col = column!();
1328 /// println!("defined on column: {current_col}");
1329 /// ```
1330 ///
1331 /// `column!` counts Unicode code points, not bytes or graphemes. As a result, the first two
1332 /// invocations return the same value, but the third does not.
1333 ///
1334 /// ```
1335 /// let a = ("foobar", column!()).1;
1336 /// let b = ("人之初性本善", column!()).1;
1337 /// let c = ("f̅o̅o̅b̅a̅r̅", column!()).1; // Uses combining overline (U+0305)
1338 ///
1339 /// assert_eq!(a, b);
1340 /// assert_ne!(b, c);
1341 /// ```
1342 #[stable(feature = "rust1", since = "1.0.0")]
1343 #[rustc_builtin_macro]
1344 #[macro_export]
1345 macro_rules! column {
1346 () => {
1347 /* compiler built-in */
1348 };
1349 }
1350
1351 /// Expands to the file name in which it was invoked.
1352 ///
1353 /// With [`line!`] and [`column!`], these macros provide debugging information for
1354 /// developers about the location within the source.
1355 ///
1356 /// The expanded expression has type `&'static str`, and the returned file
1357 /// is not the invocation of the `file!` macro itself, but rather the
1358 /// first macro invocation leading up to the invocation of the `file!`
1359 /// macro.
1360 ///
1361 /// # Examples
1362 ///
1363 /// ```
1364 /// let this_file = file!();
1365 /// println!("defined in file: {this_file}");
1366 /// ```
1367 #[stable(feature = "rust1", since = "1.0.0")]
1368 #[rustc_builtin_macro]
1369 #[macro_export]
1370 macro_rules! file {
1371 () => {
1372 /* compiler built-in */
1373 };
1374 }
1375
1376 /// Stringifies its arguments.
1377 ///
1378 /// This macro will yield an expression of type `&'static str` which is the
1379 /// stringification of all the tokens passed to the macro. No restrictions
1380 /// are placed on the syntax of the macro invocation itself.
1381 ///
1382 /// Note that the expanded results of the input tokens may change in the
1383 /// future. You should be careful if you rely on the output.
1384 ///
1385 /// # Examples
1386 ///
1387 /// ```
1388 /// let one_plus_one = stringify!(1 + 1);
1389 /// assert_eq!(one_plus_one, "1 + 1");
1390 /// ```
1391 #[stable(feature = "rust1", since = "1.0.0")]
1392 #[rustc_builtin_macro]
1393 #[macro_export]
1394 macro_rules! stringify {
1395 ($($t:tt)*) => {
1396 /* compiler built-in */
1397 };
1398 }
1399
1400 /// Includes a UTF-8 encoded file as a string.
1401 ///
1402 /// The file is located relative to the current file (similarly to how
1403 /// modules are found). The provided path is interpreted in a platform-specific
1404 /// way at compile time. So, for instance, an invocation with a Windows path
1405 /// containing backslashes `\` would not compile correctly on Unix.
1406 ///
1407 /// This macro will yield an expression of type `&'static str` which is the
1408 /// contents of the file.
1409 ///
1410 /// # Examples
1411 ///
1412 /// Assume there are two files in the same directory with the following
1413 /// contents:
1414 ///
1415 /// File 'spanish.in':
1416 ///
1417 /// ```text
1418 /// adiós
1419 /// ```
1420 ///
1421 /// File 'main.rs':
1422 ///
1423 /// ```ignore (cannot-doctest-external-file-dependency)
1424 /// fn main() {
1425 /// let my_str = include_str!("spanish.in");
1426 /// assert_eq!(my_str, "adiós\n");
1427 /// print!("{my_str}");
1428 /// }
1429 /// ```
1430 ///
1431 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
1432 #[stable(feature = "rust1", since = "1.0.0")]
1433 #[rustc_builtin_macro]
1434 #[macro_export]
1435 #[cfg_attr(not(test), rustc_diagnostic_item = "include_str_macro")]
1436 macro_rules! include_str {
1437 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1438 }
1439
1440 /// Includes a file as a reference to a byte array.
1441 ///
1442 /// The file is located relative to the current file (similarly to how
1443 /// modules are found). The provided path is interpreted in a platform-specific
1444 /// way at compile time. So, for instance, an invocation with a Windows path
1445 /// containing backslashes `\` would not compile correctly on Unix.
1446 ///
1447 /// This macro will yield an expression of type `&'static [u8; N]` which is
1448 /// the contents of the file.
1449 ///
1450 /// # Examples
1451 ///
1452 /// Assume there are two files in the same directory with the following
1453 /// contents:
1454 ///
1455 /// File 'spanish.in':
1456 ///
1457 /// ```text
1458 /// adiós
1459 /// ```
1460 ///
1461 /// File 'main.rs':
1462 ///
1463 /// ```ignore (cannot-doctest-external-file-dependency)
1464 /// fn main() {
1465 /// let bytes = include_bytes!("spanish.in");
1466 /// assert_eq!(bytes, b"adi\xc3\xb3s\n");
1467 /// print!("{}", String::from_utf8_lossy(bytes));
1468 /// }
1469 /// ```
1470 ///
1471 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
1472 #[stable(feature = "rust1", since = "1.0.0")]
1473 #[rustc_builtin_macro]
1474 #[macro_export]
1475 #[cfg_attr(not(test), rustc_diagnostic_item = "include_bytes_macro")]
1476 macro_rules! include_bytes {
1477 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1478 }
1479
1480 /// Expands to a string that represents the current module path.
1481 ///
1482 /// The current module path can be thought of as the hierarchy of modules
1483 /// leading back up to the crate root. The first component of the path
1484 /// returned is the name of the crate currently being compiled.
1485 ///
1486 /// # Examples
1487 ///
1488 /// ```
1489 /// mod test {
1490 /// pub fn foo() {
1491 /// assert!(module_path!().ends_with("test"));
1492 /// }
1493 /// }
1494 ///
1495 /// test::foo();
1496 /// ```
1497 #[stable(feature = "rust1", since = "1.0.0")]
1498 #[rustc_builtin_macro]
1499 #[macro_export]
1500 macro_rules! module_path {
1501 () => {
1502 /* compiler built-in */
1503 };
1504 }
1505
1506 /// Evaluates boolean combinations of configuration flags at compile-time.
1507 ///
1508 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
1509 /// boolean expression evaluation of configuration flags. This frequently
1510 /// leads to less duplicated code.
1511 ///
1512 /// The syntax given to this macro is the same syntax as the [`cfg`]
1513 /// attribute.
1514 ///
1515 /// `cfg!`, unlike `#[cfg]`, does not remove any code and only evaluates to true or false. For
1516 /// example, all blocks in an if/else expression need to be valid when `cfg!` is used for
1517 /// the condition, regardless of what `cfg!` is evaluating.
1518 ///
1519 /// [`cfg`]: ../reference/conditional-compilation.html#the-cfg-attribute
1520 ///
1521 /// # Examples
1522 ///
1523 /// ```
1524 /// let my_directory = if cfg!(windows) {
1525 /// "windows-specific-directory"
1526 /// } else {
1527 /// "unix-directory"
1528 /// };
1529 /// ```
1530 #[stable(feature = "rust1", since = "1.0.0")]
1531 #[rustc_builtin_macro]
1532 #[macro_export]
1533 macro_rules! cfg {
1534 ($($cfg:tt)*) => {
1535 /* compiler built-in */
1536 };
1537 }
1538
1539 /// Parses a file as an expression or an item according to the context.
1540 ///
1541 /// **Warning**: For multi-file Rust projects, the `include!` macro is probably not what you
1542 /// are looking for. Usually, multi-file Rust projects use
1543 /// [modules](https://doc.rust-lang.org/reference/items/modules.html). Multi-file projects and
1544 /// modules are explained in the Rust-by-Example book
1545 /// [here](https://doc.rust-lang.org/rust-by-example/mod/split.html) and the module system is
1546 /// explained in the Rust Book
1547 /// [here](https://doc.rust-lang.org/book/ch07-02-defining-modules-to-control-scope-and-privacy.html).
1548 ///
1549 /// The included file is placed in the surrounding code
1550 /// [unhygienically](https://doc.rust-lang.org/reference/macros-by-example.html#hygiene). If
1551 /// the included file is parsed as an expression and variables or functions share names across
1552 /// both files, it could result in variables or functions being different from what the
1553 /// included file expected.
1554 ///
1555 /// The included file is located relative to the current file (similarly to how modules are
1556 /// found). The provided path is interpreted in a platform-specific way at compile time. So,
1557 /// for instance, an invocation with a Windows path containing backslashes `\` would not
1558 /// compile correctly on Unix.
1559 ///
1560 /// # Uses
1561 ///
1562 /// The `include!` macro is primarily used for two purposes. It is used to include
1563 /// documentation that is written in a separate file and it is used to include [build artifacts
1564 /// usually as a result from the `build.rs`
1565 /// script](https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script).
1566 ///
1567 /// When using the `include` macro to include stretches of documentation, remember that the
1568 /// included file still needs to be a valid Rust syntax. It is also possible to
1569 /// use the [`include_str`] macro as `#![doc = include_str!("...")]` (at the module level) or
1570 /// `#[doc = include_str!("...")]` (at the item level) to include documentation from a plain
1571 /// text or markdown file.
1572 ///
1573 /// # Examples
1574 ///
1575 /// Assume there are two files in the same directory with the following contents:
1576 ///
1577 /// File 'monkeys.in':
1578 ///
1579 /// ```ignore (only-for-syntax-highlight)
1580 /// ['🙈', '🙊', '🙉']
1581 /// .iter()
1582 /// .cycle()
1583 /// .take(6)
1584 /// .collect::<String>()
1585 /// ```
1586 ///
1587 /// File 'main.rs':
1588 ///
1589 /// ```ignore (cannot-doctest-external-file-dependency)
1590 /// fn main() {
1591 /// let my_string = include!("monkeys.in");
1592 /// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
1593 /// println!("{my_string}");
1594 /// }
1595 /// ```
1596 ///
1597 /// Compiling 'main.rs' and running the resulting binary will print
1598 /// "🙈🙊🙉🙈🙊🙉".
1599 #[stable(feature = "rust1", since = "1.0.0")]
1600 #[rustc_builtin_macro]
1601 #[macro_export]
1602 #[rustc_diagnostic_item = "include_macro"] // useful for external lints
1603 macro_rules! include {
1604 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1605 }
1606
1607 /// Automatic Differentiation macro which allows generating a new function to compute
1608 /// the derivative of a given function. It may only be applied to a function.
1609 /// The expected usage syntax is
1610 /// `#[autodiff(NAME, MODE, INPUT_ACTIVITIES, OUTPUT_ACTIVITY)]`
1611 /// where:
1612 /// NAME is a string that represents a valid function name.
1613 /// MODE is any of Forward, Reverse, ForwardFirst, ReverseFirst.
1614 /// INPUT_ACTIVITIES consists of one valid activity for each input parameter.
1615 /// OUTPUT_ACTIVITY must not be set if we implicitly return nothing (or explicitly return
1616 /// `-> ()`). Otherwise it must be set to one of the allowed activities.
1617 #[unstable(feature = "autodiff", issue = "124509")]
1618 #[allow_internal_unstable(rustc_attrs)]
1619 #[rustc_builtin_macro]
1620 pub macro autodiff($item:item) {
1621 /* compiler built-in */
1622 }
1623
1624 /// Asserts that a boolean expression is `true` at runtime.
1625 ///
1626 /// This will invoke the [`panic!`] macro if the provided expression cannot be
1627 /// evaluated to `true` at runtime.
1628 ///
1629 /// # Uses
1630 ///
1631 /// Assertions are always checked in both debug and release builds, and cannot
1632 /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
1633 /// release builds by default.
1634 ///
1635 /// Unsafe code may rely on `assert!` to enforce run-time invariants that, if
1636 /// violated could lead to unsafety.
1637 ///
1638 /// Other use-cases of `assert!` include testing and enforcing run-time
1639 /// invariants in safe code (whose violation cannot result in unsafety).
1640 ///
1641 /// # Custom Messages
1642 ///
1643 /// This macro has a second form, where a custom panic message can
1644 /// be provided with or without arguments for formatting. See [`std::fmt`]
1645 /// for syntax for this form. Expressions used as format arguments will only
1646 /// be evaluated if the assertion fails.
1647 ///
1648 /// [`std::fmt`]: ../std/fmt/index.html
1649 ///
1650 /// # Examples
1651 ///
1652 /// ```
1653 /// // the panic message for these assertions is the stringified value of the
1654 /// // expression given.
1655 /// assert!(true);
1656 ///
1657 /// fn some_computation() -> bool { true } // a very simple function
1658 ///
1659 /// assert!(some_computation());
1660 ///
1661 /// // assert with a custom message
1662 /// let x = true;
1663 /// assert!(x, "x wasn't true!");
1664 ///
1665 /// let a = 3; let b = 27;
1666 /// assert!(a + b == 30, "a = {}, b = {}", a, b);
1667 /// ```
1668 #[stable(feature = "rust1", since = "1.0.0")]
1669 #[rustc_builtin_macro]
1670 #[macro_export]
1671 #[rustc_diagnostic_item = "assert_macro"]
1672 #[allow_internal_unstable(
1673 core_intrinsics,
1674 panic_internals,
1675 edition_panic,
1676 generic_assert_internals
1677 )]
1678 macro_rules! assert {
1679 ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
1680 ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};
1681 }
1682
1683 /// Prints passed tokens into the standard output.
1684 #[unstable(
1685 feature = "log_syntax",
1686 issue = "29598",
1687 reason = "`log_syntax!` is not stable enough for use and is subject to change"
1688 )]
1689 #[rustc_builtin_macro]
1690 #[macro_export]
1691 macro_rules! log_syntax {
1692 ($($arg:tt)*) => {
1693 /* compiler built-in */
1694 };
1695 }
1696
1697 /// Enables or disables tracing functionality used for debugging other macros.
1698 #[unstable(
1699 feature = "trace_macros",
1700 issue = "29598",
1701 reason = "`trace_macros` is not stable enough for use and is subject to change"
1702 )]
1703 #[rustc_builtin_macro]
1704 #[macro_export]
1705 macro_rules! trace_macros {
1706 (true) => {{ /* compiler built-in */ }};
1707 (false) => {{ /* compiler built-in */ }};
1708 }
1709
1710 /// Attribute macro used to apply derive macros.
1711 ///
1712 /// See [the reference] for more info.
1713 ///
1714 /// [the reference]: ../../../reference/attributes/derive.html
1715 #[stable(feature = "rust1", since = "1.0.0")]
1716 #[rustc_builtin_macro]
1717 pub macro derive($item:item) {
1718 /* compiler built-in */
1719 }
1720
1721 /// Attribute macro used to apply derive macros for implementing traits
1722 /// in a const context.
1723 ///
1724 /// See [the reference] for more info.
1725 ///
1726 /// [the reference]: ../../../reference/attributes/derive.html
1727 #[unstable(feature = "derive_const", issue = "none")]
1728 #[rustc_builtin_macro]
1729 pub macro derive_const($item:item) {
1730 /* compiler built-in */
1731 }
1732
1733 /// Attribute macro applied to a function to turn it into a unit test.
1734 ///
1735 /// See [the reference] for more info.
1736 ///
1737 /// [the reference]: ../../../reference/attributes/testing.html#the-test-attribute
1738 #[stable(feature = "rust1", since = "1.0.0")]
1739 #[allow_internal_unstable(test, rustc_attrs, coverage_attribute)]
1740 #[rustc_builtin_macro]
1741 pub macro test($item:item) {
1742 /* compiler built-in */
1743 }
1744
1745 /// Attribute macro applied to a function to turn it into a benchmark test.
1746 #[unstable(
1747 feature = "test",
1748 issue = "50297",
1749 soft,
1750 reason = "`bench` is a part of custom test frameworks which are unstable"
1751 )]
1752 #[allow_internal_unstable(test, rustc_attrs, coverage_attribute)]
1753 #[rustc_builtin_macro]
1754 pub macro bench($item:item) {
1755 /* compiler built-in */
1756 }
1757
1758 /// An implementation detail of the `#[test]` and `#[bench]` macros.
1759 #[unstable(
1760 feature = "custom_test_frameworks",
1761 issue = "50297",
1762 reason = "custom test frameworks are an unstable feature"
1763 )]
1764 #[allow_internal_unstable(test, rustc_attrs)]
1765 #[rustc_builtin_macro]
1766 pub macro test_case($item:item) {
1767 /* compiler built-in */
1768 }
1769
1770 /// Attribute macro applied to a static to register it as a global allocator.
1771 ///
1772 /// See also [`std::alloc::GlobalAlloc`](../../../std/alloc/trait.GlobalAlloc.html).
1773 #[stable(feature = "global_allocator", since = "1.28.0")]
1774 #[allow_internal_unstable(rustc_attrs)]
1775 #[rustc_builtin_macro]
1776 pub macro global_allocator($item:item) {
1777 /* compiler built-in */
1778 }
1779
1780 /// Attribute macro applied to a function to give it a post-condition.
1781 ///
1782 /// The attribute carries an argument token-tree which is
1783 /// eventually parsed as a unary closure expression that is
1784 /// invoked on a reference to the return value.
1785 #[cfg(not(bootstrap))]
1786 #[unstable(feature = "contracts", issue = "128044")]
1787 #[allow_internal_unstable(contracts_internals)]
1788 #[rustc_builtin_macro]
1789 pub macro contracts_ensures($item:item) {
1790 /* compiler built-in */
1791 }
1792
1793 /// Attribute macro applied to a function to give it a precondition.
1794 ///
1795 /// The attribute carries an argument token-tree which is
1796 /// eventually parsed as an boolean expression with access to the
1797 /// function's formal parameters
1798 #[cfg(not(bootstrap))]
1799 #[unstable(feature = "contracts", issue = "128044")]
1800 #[allow_internal_unstable(contracts_internals)]
1801 #[rustc_builtin_macro]
1802 pub macro contracts_requires($item:item) {
1803 /* compiler built-in */
1804 }
1805
1806 /// Attribute macro applied to a function to register it as a handler for allocation failure.
1807 ///
1808 /// See also [`std::alloc::handle_alloc_error`](../../../std/alloc/fn.handle_alloc_error.html).
1809 #[unstable(feature = "alloc_error_handler", issue = "51540")]
1810 #[allow_internal_unstable(rustc_attrs)]
1811 #[rustc_builtin_macro]
1812 pub macro alloc_error_handler($item:item) {
1813 /* compiler built-in */
1814 }
1815
1816 /// Keeps the item it's applied to if the passed path is accessible, and removes it otherwise.
1817 #[unstable(
1818 feature = "cfg_accessible",
1819 issue = "64797",
1820 reason = "`cfg_accessible` is not fully implemented"
1821 )]
1822 #[rustc_builtin_macro]
1823 pub macro cfg_accessible($item:item) {
1824 /* compiler built-in */
1825 }
1826
1827 /// Expands all `#[cfg]` and `#[cfg_attr]` attributes in the code fragment it's applied to.
1828 #[unstable(
1829 feature = "cfg_eval",
1830 issue = "82679",
1831 reason = "`cfg_eval` is a recently implemented feature"
1832 )]
1833 #[rustc_builtin_macro]
1834 pub macro cfg_eval($($tt:tt)*) {
1835 /* compiler built-in */
1836 }
1837
1838 /// Unstable placeholder for type ascription.
1839 #[allow_internal_unstable(builtin_syntax)]
1840 #[unstable(
1841 feature = "type_ascription",
1842 issue = "23416",
1843 reason = "placeholder syntax for type ascription"
1844 )]
1845 #[rustfmt::skip]
1846 pub macro type_ascribe($expr:expr, $ty:ty) {
1847 builtin # type_ascribe($expr, $ty)
1848 }
1849
1850 /// Unstable placeholder for deref patterns.
1851 #[allow_internal_unstable(builtin_syntax)]
1852 #[unstable(
1853 feature = "deref_patterns",
1854 issue = "87121",
1855 reason = "placeholder syntax for deref patterns"
1856 )]
1857 pub macro deref($pat:pat) {
1858 builtin # deref($pat)
1859 }
1860}