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#[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#[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/// Selects code at compile-time based on `cfg` predicates.
200///
201/// This macro evaluates, at compile-time, a series of `cfg` predicates,
202/// selects the first that is true, and emits the code guarded by that
203/// predicate. The code guarded by other predicates is not emitted.
204///
205/// An optional trailing `_` wildcard can be used to specify a fallback. If
206/// none of the predicates are true, a [`compile_error`] is emitted.
207///
208/// # Example
209///
210/// ```
211/// #![feature(cfg_select)]
212///
213/// cfg_select! {
214/// unix => {
215/// fn foo() { /* unix specific functionality */ }
216/// }
217/// target_pointer_width = "32" => {
218/// fn foo() { /* non-unix, 32-bit functionality */ }
219/// }
220/// _ => {
221/// fn foo() { /* fallback implementation */ }
222/// }
223/// }
224/// ```
225///
226/// The `cfg_select!` macro can also be used in expression position:
227///
228/// ```
229/// #![feature(cfg_select)]
230///
231/// let _some_string = cfg_select! {
232/// unix => { "With great power comes great electricity bills" }
233/// _ => { "Behind every successful diet is an unwatched pizza" }
234/// };
235/// ```
236#[unstable(feature = "cfg_select", issue = "115585")]
237#[rustc_diagnostic_item = "cfg_select"]
238#[rustc_builtin_macro]
239pub macro cfg_select($($tt:tt)*) {
240 /* compiler built-in */
241}
242
243/// Asserts that a boolean expression is `true` at runtime.
244///
245/// This will invoke the [`panic!`] macro if the provided expression cannot be
246/// evaluated to `true` at runtime.
247///
248/// Like [`assert!`], this macro also has a second version, where a custom panic
249/// message can be provided.
250///
251/// # Uses
252///
253/// Unlike [`assert!`], `debug_assert!` statements are only enabled in non
254/// optimized builds by default. An optimized build will not execute
255/// `debug_assert!` statements unless `-C debug-assertions` is passed to the
256/// compiler. This makes `debug_assert!` useful for checks that are too
257/// expensive to be present in a release build but may be helpful during
258/// development. The result of expanding `debug_assert!` is always type checked.
259///
260/// An unchecked assertion allows a program in an inconsistent state to keep
261/// running, which might have unexpected consequences but does not introduce
262/// unsafety as long as this only happens in safe code. The performance cost
263/// of assertions, however, is not measurable in general. Replacing [`assert!`]
264/// with `debug_assert!` is thus only encouraged after thorough profiling, and
265/// more importantly, only in safe code!
266///
267/// # Examples
268///
269/// ```
270/// // the panic message for these assertions is the stringified value of the
271/// // expression given.
272/// debug_assert!(true);
273///
274/// fn some_expensive_computation() -> bool { true } // a very simple function
275/// debug_assert!(some_expensive_computation());
276///
277/// // assert with a custom message
278/// let x = true;
279/// debug_assert!(x, "x wasn't true!");
280///
281/// let a = 3; let b = 27;
282/// debug_assert!(a + b == 30, "a = {}, b = {}", a, b);
283/// ```
284#[macro_export]
285#[stable(feature = "rust1", since = "1.0.0")]
286#[rustc_diagnostic_item = "debug_assert_macro"]
287#[allow_internal_unstable(edition_panic)]
288macro_rules! debug_assert {
289 ($($arg:tt)*) => {
290 if $crate::cfg!(debug_assertions) {
291 $crate::assert!($($arg)*);
292 }
293 };
294}
295
296/// Asserts that two expressions are equal to each other.
297///
298/// On panic, this macro will print the values of the expressions with their
299/// debug representations.
300///
301/// Unlike [`assert_eq!`], `debug_assert_eq!` statements are only enabled in non
302/// optimized builds by default. An optimized build will not execute
303/// `debug_assert_eq!` statements unless `-C debug-assertions` is passed to the
304/// compiler. This makes `debug_assert_eq!` useful for checks that are too
305/// expensive to be present in a release build but may be helpful during
306/// development. The result of expanding `debug_assert_eq!` is always type checked.
307///
308/// # Examples
309///
310/// ```
311/// let a = 3;
312/// let b = 1 + 2;
313/// debug_assert_eq!(a, b);
314/// ```
315#[macro_export]
316#[stable(feature = "rust1", since = "1.0.0")]
317#[rustc_diagnostic_item = "debug_assert_eq_macro"]
318macro_rules! debug_assert_eq {
319 ($($arg:tt)*) => {
320 if $crate::cfg!(debug_assertions) {
321 $crate::assert_eq!($($arg)*);
322 }
323 };
324}
325
326/// Asserts that two expressions are not equal to each other.
327///
328/// On panic, this macro will print the values of the expressions with their
329/// debug representations.
330///
331/// Unlike [`assert_ne!`], `debug_assert_ne!` statements are only enabled in non
332/// optimized builds by default. An optimized build will not execute
333/// `debug_assert_ne!` statements unless `-C debug-assertions` is passed to the
334/// compiler. This makes `debug_assert_ne!` useful for checks that are too
335/// expensive to be present in a release build but may be helpful during
336/// development. The result of expanding `debug_assert_ne!` is always type checked.
337///
338/// # Examples
339///
340/// ```
341/// let a = 3;
342/// let b = 2;
343/// debug_assert_ne!(a, b);
344/// ```
345#[macro_export]
346#[stable(feature = "assert_ne", since = "1.13.0")]
347#[rustc_diagnostic_item = "debug_assert_ne_macro"]
348macro_rules! debug_assert_ne {
349 ($($arg:tt)*) => {
350 if $crate::cfg!(debug_assertions) {
351 $crate::assert_ne!($($arg)*);
352 }
353 };
354}
355
356/// Asserts that an expression matches the provided pattern.
357///
358/// This macro is generally preferable to `debug_assert!(matches!(value, pattern))`, because it can
359/// print the debug representation of the actual value shape that did not meet expectations. In
360/// contrast, using [`debug_assert!`] will only print that expectations were not met, but not why.
361///
362/// The pattern syntax is exactly the same as found in a match arm and the `matches!` macro. The
363/// optional if guard can be used to add additional checks that must be true for the matched value,
364/// otherwise this macro will panic.
365///
366/// On panic, this macro will print the value of the expression with its debug representation.
367///
368/// Like [`assert!`], this macro has a second form, where a custom panic message can be provided.
369///
370/// Unlike [`assert_matches!`], `debug_assert_matches!` statements are only enabled in non optimized
371/// builds by default. An optimized build will not execute `debug_assert_matches!` statements unless
372/// `-C debug-assertions` is passed to the compiler. This makes `debug_assert_matches!` useful for
373/// checks that are too expensive to be present in a release build but may be helpful during
374/// development. The result of expanding `debug_assert_matches!` is always type checked.
375///
376/// # Examples
377///
378/// ```
379/// #![feature(assert_matches)]
380///
381/// use std::assert_matches::debug_assert_matches;
382///
383/// let a = Some(345);
384/// let b = Some(56);
385/// debug_assert_matches!(a, Some(_));
386/// debug_assert_matches!(b, Some(_));
387///
388/// debug_assert_matches!(a, Some(345));
389/// debug_assert_matches!(a, Some(345) | None);
390///
391/// // debug_assert_matches!(a, None); // panics
392/// // debug_assert_matches!(b, Some(345)); // panics
393/// // debug_assert_matches!(b, Some(345) | None); // panics
394///
395/// debug_assert_matches!(a, Some(x) if x > 100);
396/// // debug_assert_matches!(a, Some(x) if x < 100); // panics
397/// ```
398#[unstable(feature = "assert_matches", issue = "82775")]
399#[allow_internal_unstable(assert_matches)]
400#[rustc_macro_transparency = "semitransparent"]
401pub macro debug_assert_matches($($arg:tt)*) {
402 if $crate::cfg!(debug_assertions) {
403 $crate::assert_matches::assert_matches!($($arg)*);
404 }
405}
406
407/// Returns whether the given expression matches the provided pattern.
408///
409/// The pattern syntax is exactly the same as found in a match arm. The optional if guard can be
410/// used to add additional checks that must be true for the matched value, otherwise this macro will
411/// return `false`.
412///
413/// When testing that a value matches a pattern, it's generally preferable to use
414/// [`assert_matches!`] as it will print the debug representation of the value if the assertion
415/// fails.
416///
417/// # Examples
418///
419/// ```
420/// let foo = 'f';
421/// assert!(matches!(foo, 'A'..='Z' | 'a'..='z'));
422///
423/// let bar = Some(4);
424/// assert!(matches!(bar, Some(x) if x > 2));
425/// ```
426#[macro_export]
427#[stable(feature = "matches_macro", since = "1.42.0")]
428#[rustc_diagnostic_item = "matches_macro"]
429#[allow_internal_unstable(non_exhaustive_omitted_patterns_lint, stmt_expr_attributes)]
430macro_rules! matches {
431 ($expression:expr, $pattern:pat $(if $guard:expr)? $(,)?) => {
432 #[allow(non_exhaustive_omitted_patterns)]
433 match $expression {
434 $pattern $(if $guard)? => true,
435 _ => false
436 }
437 };
438}
439
440/// Unwraps a result or propagates its error.
441///
442/// The [`?` operator][propagating-errors] was added to replace `try!`
443/// and should be used instead. Furthermore, `try` is a reserved word
444/// in Rust 2018, so if you must use it, you will need to use the
445/// [raw-identifier syntax][ris]: `r#try`.
446///
447/// [propagating-errors]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
448/// [ris]: https://doc.rust-lang.org/nightly/rust-by-example/compatibility/raw_identifiers.html
449///
450/// `try!` matches the given [`Result`]. In case of the `Ok` variant, the
451/// expression has the value of the wrapped value.
452///
453/// In case of the `Err` variant, it retrieves the inner error. `try!` then
454/// performs conversion using `From`. This provides automatic conversion
455/// between specialized errors and more general ones. The resulting
456/// error is then immediately returned.
457///
458/// Because of the early return, `try!` can only be used in functions that
459/// return [`Result`].
460///
461/// # Examples
462///
463/// ```
464/// use std::io;
465/// use std::fs::File;
466/// use std::io::prelude::*;
467///
468/// enum MyError {
469/// FileWriteError
470/// }
471///
472/// impl From<io::Error> for MyError {
473/// fn from(e: io::Error) -> MyError {
474/// MyError::FileWriteError
475/// }
476/// }
477///
478/// // The preferred method of quick returning Errors
479/// fn write_to_file_question() -> Result<(), MyError> {
480/// let mut file = File::create("my_best_friends.txt")?;
481/// file.write_all(b"This is a list of my best friends.")?;
482/// Ok(())
483/// }
484///
485/// // The previous method of quick returning Errors
486/// fn write_to_file_using_try() -> Result<(), MyError> {
487/// let mut file = r#try!(File::create("my_best_friends.txt"));
488/// r#try!(file.write_all(b"This is a list of my best friends."));
489/// Ok(())
490/// }
491///
492/// // This is equivalent to:
493/// fn write_to_file_using_match() -> Result<(), MyError> {
494/// let mut file = r#try!(File::create("my_best_friends.txt"));
495/// match file.write_all(b"This is a list of my best friends.") {
496/// Ok(v) => v,
497/// Err(e) => return Err(From::from(e)),
498/// }
499/// Ok(())
500/// }
501/// ```
502#[macro_export]
503#[stable(feature = "rust1", since = "1.0.0")]
504#[deprecated(since = "1.39.0", note = "use the `?` operator instead")]
505#[doc(alias = "?")]
506macro_rules! r#try {
507 ($expr:expr $(,)?) => {
508 match $expr {
509 $crate::result::Result::Ok(val) => val,
510 $crate::result::Result::Err(err) => {
511 return $crate::result::Result::Err($crate::convert::From::from(err));
512 }
513 }
514 };
515}
516
517/// Writes formatted data into a buffer.
518///
519/// This macro accepts a 'writer', a format string, and a list of arguments. Arguments will be
520/// formatted according to the specified format string and the result will be passed to the writer.
521/// The writer may be any value with a `write_fmt` method; generally this comes from an
522/// implementation of either the [`fmt::Write`] or the [`io::Write`] trait. The macro
523/// returns whatever the `write_fmt` method returns; commonly a [`fmt::Result`], or an
524/// [`io::Result`].
525///
526/// See [`std::fmt`] for more information on the format string syntax.
527///
528/// [`std::fmt`]: ../std/fmt/index.html
529/// [`fmt::Write`]: crate::fmt::Write
530/// [`io::Write`]: ../std/io/trait.Write.html
531/// [`fmt::Result`]: crate::fmt::Result
532/// [`io::Result`]: ../std/io/type.Result.html
533///
534/// # Examples
535///
536/// ```
537/// use std::io::Write;
538///
539/// fn main() -> std::io::Result<()> {
540/// let mut w = Vec::new();
541/// write!(&mut w, "test")?;
542/// write!(&mut w, "formatted {}", "arguments")?;
543///
544/// assert_eq!(w, b"testformatted arguments");
545/// Ok(())
546/// }
547/// ```
548///
549/// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
550/// implementing either, as objects do not typically implement both. However, the module must
551/// avoid conflict between the trait names, such as by importing them as `_` or otherwise renaming
552/// them:
553///
554/// ```
555/// use std::fmt::Write as _;
556/// use std::io::Write as _;
557///
558/// fn main() -> Result<(), Box<dyn std::error::Error>> {
559/// let mut s = String::new();
560/// let mut v = Vec::new();
561///
562/// write!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
563/// write!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
564/// assert_eq!(v, b"s = \"abc 123\"");
565/// Ok(())
566/// }
567/// ```
568///
569/// If you also need the trait names themselves, such as to implement one or both on your types,
570/// import the containing module and then name them with a prefix:
571///
572/// ```
573/// # #![allow(unused_imports)]
574/// use std::fmt::{self, Write as _};
575/// use std::io::{self, Write as _};
576///
577/// struct Example;
578///
579/// impl fmt::Write for Example {
580/// fn write_str(&mut self, _s: &str) -> core::fmt::Result {
581/// unimplemented!();
582/// }
583/// }
584/// ```
585///
586/// Note: This macro can be used in `no_std` setups as well.
587/// In a `no_std` setup you are responsible for the implementation details of the components.
588///
589/// ```no_run
590/// use core::fmt::Write;
591///
592/// struct Example;
593///
594/// impl Write for Example {
595/// fn write_str(&mut self, _s: &str) -> core::fmt::Result {
596/// unimplemented!();
597/// }
598/// }
599///
600/// let mut m = Example{};
601/// write!(&mut m, "Hello World").expect("Not written");
602/// ```
603#[macro_export]
604#[stable(feature = "rust1", since = "1.0.0")]
605#[rustc_diagnostic_item = "write_macro"]
606macro_rules! write {
607 ($dst:expr, $($arg:tt)*) => {
608 $dst.write_fmt($crate::format_args!($($arg)*))
609 };
610}
611
612/// Writes formatted data into a buffer, with a newline appended.
613///
614/// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
615/// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
616///
617/// For more information, see [`write!`]. For information on the format string syntax, see
618/// [`std::fmt`].
619///
620/// [`std::fmt`]: ../std/fmt/index.html
621///
622/// # Examples
623///
624/// ```
625/// use std::io::{Write, Result};
626///
627/// fn main() -> Result<()> {
628/// let mut w = Vec::new();
629/// writeln!(&mut w)?;
630/// writeln!(&mut w, "test")?;
631/// writeln!(&mut w, "formatted {}", "arguments")?;
632///
633/// assert_eq!(&w[..], "\ntest\nformatted arguments\n".as_bytes());
634/// Ok(())
635/// }
636/// ```
637#[macro_export]
638#[stable(feature = "rust1", since = "1.0.0")]
639#[rustc_diagnostic_item = "writeln_macro"]
640#[allow_internal_unstable(format_args_nl)]
641macro_rules! writeln {
642 ($dst:expr $(,)?) => {
643 $crate::write!($dst, "\n")
644 };
645 ($dst:expr, $($arg:tt)*) => {
646 $dst.write_fmt($crate::format_args_nl!($($arg)*))
647 };
648}
649
650/// Indicates unreachable code.
651///
652/// This is useful any time that the compiler can't determine that some code is unreachable. For
653/// example:
654///
655/// * Match arms with guard conditions.
656/// * Loops that dynamically terminate.
657/// * Iterators that dynamically terminate.
658///
659/// If the determination that the code is unreachable proves incorrect, the
660/// program immediately terminates with a [`panic!`].
661///
662/// The unsafe counterpart of this macro is the [`unreachable_unchecked`] function, which
663/// will cause undefined behavior if the code is reached.
664///
665/// [`unreachable_unchecked`]: crate::hint::unreachable_unchecked
666///
667/// # Panics
668///
669/// This will always [`panic!`] because `unreachable!` is just a shorthand for `panic!` with a
670/// fixed, specific message.
671///
672/// Like `panic!`, this macro has a second form for displaying custom values.
673///
674/// # Examples
675///
676/// Match arms:
677///
678/// ```
679/// # #[allow(dead_code)]
680/// fn foo(x: Option<i32>) {
681/// match x {
682/// Some(n) if n >= 0 => println!("Some(Non-negative)"),
683/// Some(n) if n < 0 => println!("Some(Negative)"),
684/// Some(_) => unreachable!(), // compile error if commented out
685/// None => println!("None")
686/// }
687/// }
688/// ```
689///
690/// Iterators:
691///
692/// ```
693/// # #[allow(dead_code)]
694/// fn divide_by_three(x: u32) -> u32 { // one of the poorest implementations of x/3
695/// for i in 0.. {
696/// if 3*i < i { panic!("u32 overflow"); }
697/// if x < 3*i { return i-1; }
698/// }
699/// unreachable!("The loop should always return");
700/// }
701/// ```
702#[macro_export]
703#[rustc_builtin_macro(unreachable)]
704#[allow_internal_unstable(edition_panic)]
705#[stable(feature = "rust1", since = "1.0.0")]
706#[rustc_diagnostic_item = "unreachable_macro"]
707macro_rules! unreachable {
708 // Expands to either `$crate::panic::unreachable_2015` or `$crate::panic::unreachable_2021`
709 // depending on the edition of the caller.
710 ($($arg:tt)*) => {
711 /* compiler built-in */
712 };
713}
714
715/// Indicates unimplemented code by panicking with a message of "not implemented".
716///
717/// This allows your code to type-check, which is useful if you are prototyping or
718/// implementing a trait that requires multiple methods which you don't plan to use all of.
719///
720/// The difference between `unimplemented!` and [`todo!`] is that while `todo!`
721/// conveys an intent of implementing the functionality later and the message is "not yet
722/// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
723///
724/// Also, some IDEs will mark `todo!`s.
725///
726/// # Panics
727///
728/// This will always [`panic!`] because `unimplemented!` is just a shorthand for `panic!` with a
729/// fixed, specific message.
730///
731/// Like `panic!`, this macro has a second form for displaying custom values.
732///
733/// [`todo!`]: crate::todo
734///
735/// # Examples
736///
737/// Say we have a trait `Foo`:
738///
739/// ```
740/// trait Foo {
741/// fn bar(&self) -> u8;
742/// fn baz(&self);
743/// fn qux(&self) -> Result<u64, ()>;
744/// }
745/// ```
746///
747/// We want to implement `Foo` for 'MyStruct', but for some reason it only makes sense
748/// to implement the `bar()` function. `baz()` and `qux()` will still need to be defined
749/// in our implementation of `Foo`, but we can use `unimplemented!` in their definitions
750/// to allow our code to compile.
751///
752/// We still want to have our program stop running if the unimplemented methods are
753/// reached.
754///
755/// ```
756/// # trait Foo {
757/// # fn bar(&self) -> u8;
758/// # fn baz(&self);
759/// # fn qux(&self) -> Result<u64, ()>;
760/// # }
761/// struct MyStruct;
762///
763/// impl Foo for MyStruct {
764/// fn bar(&self) -> u8 {
765/// 1 + 1
766/// }
767///
768/// fn baz(&self) {
769/// // It makes no sense to `baz` a `MyStruct`, so we have no logic here
770/// // at all.
771/// // This will display "thread 'main' panicked at 'not implemented'".
772/// unimplemented!();
773/// }
774///
775/// fn qux(&self) -> Result<u64, ()> {
776/// // We have some logic here,
777/// // We can add a message to unimplemented! to display our omission.
778/// // This will display:
779/// // "thread 'main' panicked at 'not implemented: MyStruct isn't quxable'".
780/// unimplemented!("MyStruct isn't quxable");
781/// }
782/// }
783///
784/// fn main() {
785/// let s = MyStruct;
786/// s.bar();
787/// }
788/// ```
789#[macro_export]
790#[stable(feature = "rust1", since = "1.0.0")]
791#[rustc_diagnostic_item = "unimplemented_macro"]
792#[allow_internal_unstable(panic_internals)]
793macro_rules! unimplemented {
794 () => {
795 $crate::panicking::panic("not implemented")
796 };
797 ($($arg:tt)+) => {
798 $crate::panic!("not implemented: {}", $crate::format_args!($($arg)+))
799 };
800}
801
802/// Indicates unfinished code.
803///
804/// This can be useful if you are prototyping and just
805/// want a placeholder to let your code pass type analysis.
806///
807/// The difference between [`unimplemented!`] and `todo!` is that while `todo!` conveys
808/// an intent of implementing the functionality later and the message is "not yet
809/// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
810///
811/// Also, some IDEs will mark `todo!`s.
812///
813/// # Panics
814///
815/// This will always [`panic!`] because `todo!` is just a shorthand for `panic!` with a
816/// fixed, specific message.
817///
818/// Like `panic!`, this macro has a second form for displaying custom values.
819///
820/// # Examples
821///
822/// Here's an example of some in-progress code. We have a trait `Foo`:
823///
824/// ```
825/// trait Foo {
826/// fn bar(&self) -> u8;
827/// fn baz(&self);
828/// fn qux(&self) -> Result<u64, ()>;
829/// }
830/// ```
831///
832/// We want to implement `Foo` on one of our types, but we also want to work on
833/// just `bar()` first. In order for our code to compile, we need to implement
834/// `baz()` and `qux()`, so we can use `todo!`:
835///
836/// ```
837/// # trait Foo {
838/// # fn bar(&self) -> u8;
839/// # fn baz(&self);
840/// # fn qux(&self) -> Result<u64, ()>;
841/// # }
842/// struct MyStruct;
843///
844/// impl Foo for MyStruct {
845/// fn bar(&self) -> u8 {
846/// 1 + 1
847/// }
848///
849/// fn baz(&self) {
850/// // Let's not worry about implementing baz() for now
851/// todo!();
852/// }
853///
854/// fn qux(&self) -> Result<u64, ()> {
855/// // We can add a message to todo! to display our omission.
856/// // This will display:
857/// // "thread 'main' panicked at 'not yet implemented: MyStruct is not yet quxable'".
858/// todo!("MyStruct is not yet quxable");
859/// }
860/// }
861///
862/// fn main() {
863/// let s = MyStruct;
864/// s.bar();
865///
866/// // We aren't even using baz() or qux(), so this is fine.
867/// }
868/// ```
869#[macro_export]
870#[stable(feature = "todo_macro", since = "1.40.0")]
871#[rustc_diagnostic_item = "todo_macro"]
872#[allow_internal_unstable(panic_internals)]
873macro_rules! todo {
874 () => {
875 $crate::panicking::panic("not yet implemented")
876 };
877 ($($arg:tt)+) => {
878 $crate::panic!("not yet implemented: {}", $crate::format_args!($($arg)+))
879 };
880}
881
882/// Definitions of built-in macros.
883///
884/// Most of the macro properties (stability, visibility, etc.) are taken from the source code here,
885/// with exception of expansion functions transforming macro inputs into outputs,
886/// those functions are provided by the compiler.
887pub(crate) mod builtin {
888
889 /// Causes compilation to fail with the given error message when encountered.
890 ///
891 /// This macro should be used when a crate uses a conditional compilation strategy to provide
892 /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
893 /// but emits an error during *compilation* rather than at *runtime*.
894 ///
895 /// # Examples
896 ///
897 /// Two such examples are macros and `#[cfg]` environments.
898 ///
899 /// Emit a better compiler error if a macro is passed invalid values. Without the final branch,
900 /// the compiler would still emit an error, but the error's message would not mention the two
901 /// valid values.
902 ///
903 /// ```compile_fail
904 /// macro_rules! give_me_foo_or_bar {
905 /// (foo) => {};
906 /// (bar) => {};
907 /// ($x:ident) => {
908 /// compile_error!("This macro only accepts `foo` or `bar`");
909 /// }
910 /// }
911 ///
912 /// give_me_foo_or_bar!(neither);
913 /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
914 /// ```
915 ///
916 /// Emit a compiler error if one of a number of features isn't available.
917 ///
918 /// ```compile_fail
919 /// #[cfg(not(any(feature = "foo", feature = "bar")))]
920 /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.");
921 /// ```
922 #[stable(feature = "compile_error_macro", since = "1.20.0")]
923 #[rustc_builtin_macro]
924 #[macro_export]
925 macro_rules! compile_error {
926 ($msg:expr $(,)?) => {{ /* compiler built-in */ }};
927 }
928
929 /// Constructs parameters for the other string-formatting macros.
930 ///
931 /// This macro functions by taking a formatting string literal containing
932 /// `{}` for each additional argument passed. `format_args!` prepares the
933 /// additional parameters to ensure the output can be interpreted as a string
934 /// and canonicalizes the arguments into a single type. Any value that implements
935 /// the [`Display`] trait can be passed to `format_args!`, as can any
936 /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
937 ///
938 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
939 /// passed to the macros within [`std::fmt`] for performing useful redirection.
940 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
941 /// proxied through this one. `format_args!`, unlike its derived macros, avoids
942 /// heap allocations.
943 ///
944 /// You can use the [`fmt::Arguments`] value that `format_args!` returns
945 /// in `Debug` and `Display` contexts as seen below. The example also shows
946 /// that `Debug` and `Display` format to the same thing: the interpolated
947 /// format string in `format_args!`.
948 ///
949 /// ```rust
950 /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
951 /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
952 /// assert_eq!("1 foo 2", display);
953 /// assert_eq!(display, debug);
954 /// ```
955 ///
956 /// See [the formatting documentation in `std::fmt`](../std/fmt/index.html)
957 /// for details of the macro argument syntax, and further information.
958 ///
959 /// [`Display`]: crate::fmt::Display
960 /// [`Debug`]: crate::fmt::Debug
961 /// [`fmt::Arguments`]: crate::fmt::Arguments
962 /// [`std::fmt`]: ../std/fmt/index.html
963 /// [`format!`]: ../std/macro.format.html
964 /// [`println!`]: ../std/macro.println.html
965 ///
966 /// # Examples
967 ///
968 /// ```
969 /// use std::fmt;
970 ///
971 /// let s = fmt::format(format_args!("hello {}", "world"));
972 /// assert_eq!(s, format!("hello {}", "world"));
973 /// ```
974 ///
975 /// # Lifetime limitation
976 ///
977 /// Except when no formatting arguments are used,
978 /// the produced `fmt::Arguments` value borrows temporary values,
979 /// which means it can only be used within the same expression
980 /// and cannot be stored for later use.
981 /// This is a known limitation, see [#92698](https://github.com/rust-lang/rust/issues/92698).
982 #[stable(feature = "rust1", since = "1.0.0")]
983 #[rustc_diagnostic_item = "format_args_macro"]
984 #[allow_internal_unsafe]
985 #[allow_internal_unstable(fmt_internals)]
986 #[rustc_builtin_macro]
987 #[macro_export]
988 macro_rules! format_args {
989 ($fmt:expr) => {{ /* compiler built-in */ }};
990 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
991 }
992
993 /// Same as [`format_args`], but can be used in some const contexts.
994 ///
995 /// This macro is used by the panic macros for the `const_panic` feature.
996 ///
997 /// This macro will be removed once `format_args` is allowed in const contexts.
998 #[unstable(feature = "const_format_args", issue = "none")]
999 #[allow_internal_unstable(fmt_internals, const_fmt_arguments_new)]
1000 #[rustc_builtin_macro]
1001 #[macro_export]
1002 macro_rules! const_format_args {
1003 ($fmt:expr) => {{ /* compiler built-in */ }};
1004 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
1005 }
1006
1007 /// Same as [`format_args`], but adds a newline in the end.
1008 #[unstable(
1009 feature = "format_args_nl",
1010 issue = "none",
1011 reason = "`format_args_nl` is only for internal \
1012 language use and is subject to change"
1013 )]
1014 #[allow_internal_unstable(fmt_internals)]
1015 #[rustc_builtin_macro]
1016 #[macro_export]
1017 macro_rules! format_args_nl {
1018 ($fmt:expr) => {{ /* compiler built-in */ }};
1019 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
1020 }
1021
1022 /// Inspects an environment variable at compile time.
1023 ///
1024 /// This macro will expand to the value of the named environment variable at
1025 /// compile time, yielding an expression of type `&'static str`. Use
1026 /// [`std::env::var`] instead if you want to read the value at runtime.
1027 ///
1028 /// [`std::env::var`]: ../std/env/fn.var.html
1029 ///
1030 /// If the environment variable is not defined, then a compilation error
1031 /// will be emitted. To not emit a compile error, use the [`option_env!`]
1032 /// macro instead. A compilation error will also be emitted if the
1033 /// environment variable is not a valid Unicode string.
1034 ///
1035 /// # Examples
1036 ///
1037 /// ```
1038 /// let path: &'static str = env!("PATH");
1039 /// println!("the $PATH variable at the time of compiling was: {path}");
1040 /// ```
1041 ///
1042 /// You can customize the error message by passing a string as the second
1043 /// parameter:
1044 ///
1045 /// ```compile_fail
1046 /// let doc: &'static str = env!("documentation", "what's that?!");
1047 /// ```
1048 ///
1049 /// If the `documentation` environment variable is not defined, you'll get
1050 /// the following error:
1051 ///
1052 /// ```text
1053 /// error: what's that?!
1054 /// ```
1055 #[stable(feature = "rust1", since = "1.0.0")]
1056 #[rustc_builtin_macro]
1057 #[macro_export]
1058 #[rustc_diagnostic_item = "env_macro"] // useful for external lints
1059 macro_rules! env {
1060 ($name:expr $(,)?) => {{ /* compiler built-in */ }};
1061 ($name:expr, $error_msg:expr $(,)?) => {{ /* compiler built-in */ }};
1062 }
1063
1064 /// Optionally inspects an environment variable at compile time.
1065 ///
1066 /// If the named environment variable is present at compile time, this will
1067 /// expand into an expression of type `Option<&'static str>` whose value is
1068 /// `Some` of the value of the environment variable (a compilation error
1069 /// will be emitted if the environment variable is not a valid Unicode
1070 /// string). If the environment variable is not present, then this will
1071 /// expand to `None`. See [`Option<T>`][Option] for more information on this
1072 /// type. Use [`std::env::var`] instead if you want to read the value at
1073 /// runtime.
1074 ///
1075 /// [`std::env::var`]: ../std/env/fn.var.html
1076 ///
1077 /// A compile time error is only emitted when using this macro if the
1078 /// environment variable exists and is not a valid Unicode string. To also
1079 /// emit a compile error if the environment variable is not present, use the
1080 /// [`env!`] macro instead.
1081 ///
1082 /// # Examples
1083 ///
1084 /// ```
1085 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
1086 /// println!("the secret key might be: {key:?}");
1087 /// ```
1088 #[stable(feature = "rust1", since = "1.0.0")]
1089 #[rustc_builtin_macro]
1090 #[macro_export]
1091 #[rustc_diagnostic_item = "option_env_macro"] // useful for external lints
1092 macro_rules! option_env {
1093 ($name:expr $(,)?) => {{ /* compiler built-in */ }};
1094 }
1095
1096 /// Concatenates literals into a byte slice.
1097 ///
1098 /// This macro takes any number of comma-separated literals, and concatenates them all into
1099 /// one, yielding an expression of type `&[u8; _]`, which represents all of the literals
1100 /// concatenated left-to-right. The literals passed can be any combination of:
1101 ///
1102 /// - byte literals (`b'r'`)
1103 /// - byte strings (`b"Rust"`)
1104 /// - arrays of bytes/numbers (`[b'A', 66, b'C']`)
1105 ///
1106 /// # Examples
1107 ///
1108 /// ```
1109 /// #![feature(concat_bytes)]
1110 ///
1111 /// # fn main() {
1112 /// let s: &[u8; 6] = concat_bytes!(b'A', b"BC", [68, b'E', 70]);
1113 /// assert_eq!(s, b"ABCDEF");
1114 /// # }
1115 /// ```
1116 #[unstable(feature = "concat_bytes", issue = "87555")]
1117 #[rustc_builtin_macro]
1118 #[macro_export]
1119 macro_rules! concat_bytes {
1120 ($($e:literal),+ $(,)?) => {{ /* compiler built-in */ }};
1121 }
1122
1123 /// Concatenates literals into a static string slice.
1124 ///
1125 /// This macro takes any number of comma-separated literals, yielding an
1126 /// expression of type `&'static str` which represents all of the literals
1127 /// concatenated left-to-right.
1128 ///
1129 /// Integer and floating point literals are [stringified](core::stringify) in order to be
1130 /// concatenated.
1131 ///
1132 /// # Examples
1133 ///
1134 /// ```
1135 /// let s = concat!("test", 10, 'b', true);
1136 /// assert_eq!(s, "test10btrue");
1137 /// ```
1138 #[stable(feature = "rust1", since = "1.0.0")]
1139 #[rustc_builtin_macro]
1140 #[rustc_diagnostic_item = "macro_concat"]
1141 #[macro_export]
1142 macro_rules! concat {
1143 ($($e:expr),* $(,)?) => {{ /* compiler built-in */ }};
1144 }
1145
1146 /// Expands to the line number on which it was invoked.
1147 ///
1148 /// With [`column!`] and [`file!`], these macros provide debugging information for
1149 /// developers about the location within the source.
1150 ///
1151 /// The expanded expression has type `u32` and is 1-based, so the first line
1152 /// in each file evaluates to 1, the second to 2, etc. This is consistent
1153 /// with error messages by common compilers or popular editors.
1154 /// The returned line is *not necessarily* the line of the `line!` invocation itself,
1155 /// but rather the first macro invocation leading up to the invocation
1156 /// of the `line!` macro.
1157 ///
1158 /// # Examples
1159 ///
1160 /// ```
1161 /// let current_line = line!();
1162 /// println!("defined on line: {current_line}");
1163 /// ```
1164 #[stable(feature = "rust1", since = "1.0.0")]
1165 #[rustc_builtin_macro]
1166 #[macro_export]
1167 macro_rules! line {
1168 () => {
1169 /* compiler built-in */
1170 };
1171 }
1172
1173 /// Expands to the column number at which it was invoked.
1174 ///
1175 /// With [`line!`] and [`file!`], these macros provide debugging information for
1176 /// developers about the location within the source.
1177 ///
1178 /// The expanded expression has type `u32` and is 1-based, so the first column
1179 /// in each line evaluates to 1, the second to 2, etc. This is consistent
1180 /// with error messages by common compilers or popular editors.
1181 /// The returned column is *not necessarily* the line of the `column!` invocation itself,
1182 /// but rather the first macro invocation leading up to the invocation
1183 /// of the `column!` macro.
1184 ///
1185 /// # Examples
1186 ///
1187 /// ```
1188 /// let current_col = column!();
1189 /// println!("defined on column: {current_col}");
1190 /// ```
1191 ///
1192 /// `column!` counts Unicode code points, not bytes or graphemes. As a result, the first two
1193 /// invocations return the same value, but the third does not.
1194 ///
1195 /// ```
1196 /// let a = ("foobar", column!()).1;
1197 /// let b = ("人之初性本善", column!()).1;
1198 /// let c = ("f̅o̅o̅b̅a̅r̅", column!()).1; // Uses combining overline (U+0305)
1199 ///
1200 /// assert_eq!(a, b);
1201 /// assert_ne!(b, c);
1202 /// ```
1203 #[stable(feature = "rust1", since = "1.0.0")]
1204 #[rustc_builtin_macro]
1205 #[macro_export]
1206 macro_rules! column {
1207 () => {
1208 /* compiler built-in */
1209 };
1210 }
1211
1212 /// Expands to the file name in which it was invoked.
1213 ///
1214 /// With [`line!`] and [`column!`], these macros provide debugging information for
1215 /// developers about the location within the source.
1216 ///
1217 /// The expanded expression has type `&'static str`, and the returned file
1218 /// is not the invocation of the `file!` macro itself, but rather the
1219 /// first macro invocation leading up to the invocation of the `file!`
1220 /// macro.
1221 ///
1222 /// The file name is derived from the crate root's source path passed to the Rust compiler
1223 /// and the sequence the compiler takes to get from the crate root to the
1224 /// module containing `file!`, modified by any flags passed to the Rust compiler (e.g.
1225 /// `--remap-path-prefix`). If the crate's source path is relative, the initial base
1226 /// directory will be the working directory of the Rust compiler. For example, if the source
1227 /// path passed to the compiler is `./src/lib.rs` which has a `mod foo;` with a source path of
1228 /// `src/foo/mod.rs`, then calling `file!` inside `mod foo;` will return `./src/foo/mod.rs`.
1229 ///
1230 /// Future compiler options might make further changes to the behavior of `file!`,
1231 /// including potentially making it entirely empty. Code (e.g. test libraries)
1232 /// relying on `file!` producing an openable file path would be incompatible
1233 /// with such options, and might wish to recommend not using those options.
1234 ///
1235 /// # Examples
1236 ///
1237 /// ```
1238 /// let this_file = file!();
1239 /// println!("defined in file: {this_file}");
1240 /// ```
1241 #[stable(feature = "rust1", since = "1.0.0")]
1242 #[rustc_builtin_macro]
1243 #[macro_export]
1244 macro_rules! file {
1245 () => {
1246 /* compiler built-in */
1247 };
1248 }
1249
1250 /// Stringifies its arguments.
1251 ///
1252 /// This macro will yield an expression of type `&'static str` which is the
1253 /// stringification of all the tokens passed to the macro. No restrictions
1254 /// are placed on the syntax of the macro invocation itself.
1255 ///
1256 /// Note that the expanded results of the input tokens may change in the
1257 /// future. You should be careful if you rely on the output.
1258 ///
1259 /// # Examples
1260 ///
1261 /// ```
1262 /// let one_plus_one = stringify!(1 + 1);
1263 /// assert_eq!(one_plus_one, "1 + 1");
1264 /// ```
1265 #[stable(feature = "rust1", since = "1.0.0")]
1266 #[rustc_builtin_macro]
1267 #[macro_export]
1268 macro_rules! stringify {
1269 ($($t:tt)*) => {
1270 /* compiler built-in */
1271 };
1272 }
1273
1274 /// Includes a UTF-8 encoded file as a string.
1275 ///
1276 /// The file is located relative to the current file (similarly to how
1277 /// modules are found). The provided path is interpreted in a platform-specific
1278 /// way at compile time. So, for instance, an invocation with a Windows path
1279 /// containing backslashes `\` would not compile correctly on Unix.
1280 ///
1281 /// This macro will yield an expression of type `&'static str` which is the
1282 /// contents of the file.
1283 ///
1284 /// # Examples
1285 ///
1286 /// Assume there are two files in the same directory with the following
1287 /// contents:
1288 ///
1289 /// File 'spanish.in':
1290 ///
1291 /// ```text
1292 /// adiós
1293 /// ```
1294 ///
1295 /// File 'main.rs':
1296 ///
1297 /// ```ignore (cannot-doctest-external-file-dependency)
1298 /// fn main() {
1299 /// let my_str = include_str!("spanish.in");
1300 /// assert_eq!(my_str, "adiós\n");
1301 /// print!("{my_str}");
1302 /// }
1303 /// ```
1304 ///
1305 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
1306 #[stable(feature = "rust1", since = "1.0.0")]
1307 #[rustc_builtin_macro]
1308 #[macro_export]
1309 #[rustc_diagnostic_item = "include_str_macro"]
1310 macro_rules! include_str {
1311 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1312 }
1313
1314 /// Includes a file as a reference to a byte array.
1315 ///
1316 /// The file is located relative to the current file (similarly to how
1317 /// modules are found). The provided path is interpreted in a platform-specific
1318 /// way at compile time. So, for instance, an invocation with a Windows path
1319 /// containing backslashes `\` would not compile correctly on Unix.
1320 ///
1321 /// This macro will yield an expression of type `&'static [u8; N]` which is
1322 /// the contents of the file.
1323 ///
1324 /// # Examples
1325 ///
1326 /// Assume there are two files in the same directory with the following
1327 /// contents:
1328 ///
1329 /// File 'spanish.in':
1330 ///
1331 /// ```text
1332 /// adiós
1333 /// ```
1334 ///
1335 /// File 'main.rs':
1336 ///
1337 /// ```ignore (cannot-doctest-external-file-dependency)
1338 /// fn main() {
1339 /// let bytes = include_bytes!("spanish.in");
1340 /// assert_eq!(bytes, b"adi\xc3\xb3s\n");
1341 /// print!("{}", String::from_utf8_lossy(bytes));
1342 /// }
1343 /// ```
1344 ///
1345 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
1346 #[stable(feature = "rust1", since = "1.0.0")]
1347 #[rustc_builtin_macro]
1348 #[macro_export]
1349 #[rustc_diagnostic_item = "include_bytes_macro"]
1350 macro_rules! include_bytes {
1351 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1352 }
1353
1354 /// Expands to a string that represents the current module path.
1355 ///
1356 /// The current module path can be thought of as the hierarchy of modules
1357 /// leading back up to the crate root. The first component of the path
1358 /// returned is the name of the crate currently being compiled.
1359 ///
1360 /// # Examples
1361 ///
1362 /// ```
1363 /// mod test {
1364 /// pub fn foo() {
1365 /// assert!(module_path!().ends_with("test"));
1366 /// }
1367 /// }
1368 ///
1369 /// test::foo();
1370 /// ```
1371 #[stable(feature = "rust1", since = "1.0.0")]
1372 #[rustc_builtin_macro]
1373 #[macro_export]
1374 macro_rules! module_path {
1375 () => {
1376 /* compiler built-in */
1377 };
1378 }
1379
1380 /// Evaluates boolean combinations of configuration flags at compile-time.
1381 ///
1382 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
1383 /// boolean expression evaluation of configuration flags. This frequently
1384 /// leads to less duplicated code.
1385 ///
1386 /// The syntax given to this macro is the same syntax as the [`cfg`]
1387 /// attribute.
1388 ///
1389 /// `cfg!`, unlike `#[cfg]`, does not remove any code and only evaluates to true or false. For
1390 /// example, all blocks in an if/else expression need to be valid when `cfg!` is used for
1391 /// the condition, regardless of what `cfg!` is evaluating.
1392 ///
1393 /// [`cfg`]: ../reference/conditional-compilation.html#the-cfg-attribute
1394 ///
1395 /// # Examples
1396 ///
1397 /// ```
1398 /// let my_directory = if cfg!(windows) {
1399 /// "windows-specific-directory"
1400 /// } else {
1401 /// "unix-directory"
1402 /// };
1403 /// ```
1404 #[stable(feature = "rust1", since = "1.0.0")]
1405 #[rustc_builtin_macro]
1406 #[macro_export]
1407 macro_rules! cfg {
1408 ($($cfg:tt)*) => {
1409 /* compiler built-in */
1410 };
1411 }
1412
1413 /// Parses a file as an expression or an item according to the context.
1414 ///
1415 /// **Warning**: For multi-file Rust projects, the `include!` macro is probably not what you
1416 /// are looking for. Usually, multi-file Rust projects use
1417 /// [modules](https://doc.rust-lang.org/reference/items/modules.html). Multi-file projects and
1418 /// modules are explained in the Rust-by-Example book
1419 /// [here](https://doc.rust-lang.org/rust-by-example/mod/split.html) and the module system is
1420 /// explained in the Rust Book
1421 /// [here](https://doc.rust-lang.org/book/ch07-02-defining-modules-to-control-scope-and-privacy.html).
1422 ///
1423 /// The included file is placed in the surrounding code
1424 /// [unhygienically](https://doc.rust-lang.org/reference/macros-by-example.html#hygiene). If
1425 /// the included file is parsed as an expression and variables or functions share names across
1426 /// both files, it could result in variables or functions being different from what the
1427 /// included file expected.
1428 ///
1429 /// The included file is located relative to the current file (similarly to how modules are
1430 /// found). The provided path is interpreted in a platform-specific way at compile time. So,
1431 /// for instance, an invocation with a Windows path containing backslashes `\` would not
1432 /// compile correctly on Unix.
1433 ///
1434 /// # Uses
1435 ///
1436 /// The `include!` macro is primarily used for two purposes. It is used to include
1437 /// documentation that is written in a separate file and it is used to include [build artifacts
1438 /// usually as a result from the `build.rs`
1439 /// script](https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script).
1440 ///
1441 /// When using the `include` macro to include stretches of documentation, remember that the
1442 /// included file still needs to be a valid Rust syntax. It is also possible to
1443 /// use the [`include_str`] macro as `#![doc = include_str!("...")]` (at the module level) or
1444 /// `#[doc = include_str!("...")]` (at the item level) to include documentation from a plain
1445 /// text or markdown file.
1446 ///
1447 /// # Examples
1448 ///
1449 /// Assume there are two files in the same directory with the following contents:
1450 ///
1451 /// File 'monkeys.in':
1452 ///
1453 /// ```ignore (only-for-syntax-highlight)
1454 /// ['🙈', '🙊', '🙉']
1455 /// .iter()
1456 /// .cycle()
1457 /// .take(6)
1458 /// .collect::<String>()
1459 /// ```
1460 ///
1461 /// File 'main.rs':
1462 ///
1463 /// ```ignore (cannot-doctest-external-file-dependency)
1464 /// fn main() {
1465 /// let my_string = include!("monkeys.in");
1466 /// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
1467 /// println!("{my_string}");
1468 /// }
1469 /// ```
1470 ///
1471 /// Compiling 'main.rs' and running the resulting binary will print
1472 /// "🙈🙊🙉🙈🙊🙉".
1473 #[stable(feature = "rust1", since = "1.0.0")]
1474 #[rustc_builtin_macro]
1475 #[macro_export]
1476 #[rustc_diagnostic_item = "include_macro"] // useful for external lints
1477 macro_rules! include {
1478 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1479 }
1480
1481 /// This macro uses forward-mode automatic differentiation to generate a new function.
1482 /// It may only be applied to a function. The new function will compute the derivative
1483 /// of the function to which the macro was applied.
1484 ///
1485 /// The expected usage syntax is:
1486 /// `#[autodiff_forward(NAME, INPUT_ACTIVITIES, OUTPUT_ACTIVITY)]`
1487 ///
1488 /// - `NAME`: A string that represents a valid function name.
1489 /// - `INPUT_ACTIVITIES`: Specifies one valid activity for each input parameter.
1490 /// - `OUTPUT_ACTIVITY`: Must not be set if the function implicitly returns nothing
1491 /// (or explicitly returns `-> ()`). Otherwise, it must be set to one of the allowed activities.
1492 #[unstable(feature = "autodiff", issue = "124509")]
1493 #[allow_internal_unstable(rustc_attrs)]
1494 #[rustc_builtin_macro]
1495 pub macro autodiff_forward($item:item) {
1496 /* compiler built-in */
1497 }
1498
1499 /// This macro uses reverse-mode automatic differentiation to generate a new function.
1500 /// It may only be applied to a function. The new function will compute the derivative
1501 /// of the function to which the macro was applied.
1502 ///
1503 /// The expected usage syntax is:
1504 /// `#[autodiff_reverse(NAME, INPUT_ACTIVITIES, OUTPUT_ACTIVITY)]`
1505 ///
1506 /// - `NAME`: A string that represents a valid function name.
1507 /// - `INPUT_ACTIVITIES`: Specifies one valid activity for each input parameter.
1508 /// - `OUTPUT_ACTIVITY`: Must not be set if the function implicitly returns nothing
1509 /// (or explicitly returns `-> ()`). Otherwise, it must be set to one of the allowed activities.
1510 #[unstable(feature = "autodiff", issue = "124509")]
1511 #[allow_internal_unstable(rustc_attrs)]
1512 #[rustc_builtin_macro]
1513 pub macro autodiff_reverse($item:item) {
1514 /* compiler built-in */
1515 }
1516
1517 /// Asserts that a boolean expression is `true` at runtime.
1518 ///
1519 /// This will invoke the [`panic!`] macro if the provided expression cannot be
1520 /// evaluated to `true` at runtime.
1521 ///
1522 /// # Uses
1523 ///
1524 /// Assertions are always checked in both debug and release builds, and cannot
1525 /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
1526 /// release builds by default.
1527 ///
1528 /// Unsafe code may rely on `assert!` to enforce run-time invariants that, if
1529 /// violated could lead to unsafety.
1530 ///
1531 /// Other use-cases of `assert!` include testing and enforcing run-time
1532 /// invariants in safe code (whose violation cannot result in unsafety).
1533 ///
1534 /// # Custom Messages
1535 ///
1536 /// This macro has a second form, where a custom panic message can
1537 /// be provided with or without arguments for formatting. See [`std::fmt`]
1538 /// for syntax for this form. Expressions used as format arguments will only
1539 /// be evaluated if the assertion fails.
1540 ///
1541 /// [`std::fmt`]: ../std/fmt/index.html
1542 ///
1543 /// # Examples
1544 ///
1545 /// ```
1546 /// // the panic message for these assertions is the stringified value of the
1547 /// // expression given.
1548 /// assert!(true);
1549 ///
1550 /// fn some_computation() -> bool { true } // a very simple function
1551 ///
1552 /// assert!(some_computation());
1553 ///
1554 /// // assert with a custom message
1555 /// let x = true;
1556 /// assert!(x, "x wasn't true!");
1557 ///
1558 /// let a = 3; let b = 27;
1559 /// assert!(a + b == 30, "a = {}, b = {}", a, b);
1560 /// ```
1561 #[stable(feature = "rust1", since = "1.0.0")]
1562 #[rustc_builtin_macro]
1563 #[macro_export]
1564 #[rustc_diagnostic_item = "assert_macro"]
1565 #[allow_internal_unstable(
1566 core_intrinsics,
1567 panic_internals,
1568 edition_panic,
1569 generic_assert_internals
1570 )]
1571 macro_rules! assert {
1572 ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
1573 ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};
1574 }
1575
1576 /// Prints passed tokens into the standard output.
1577 #[unstable(
1578 feature = "log_syntax",
1579 issue = "29598",
1580 reason = "`log_syntax!` is not stable enough for use and is subject to change"
1581 )]
1582 #[rustc_builtin_macro]
1583 #[macro_export]
1584 macro_rules! log_syntax {
1585 ($($arg:tt)*) => {
1586 /* compiler built-in */
1587 };
1588 }
1589
1590 /// Enables or disables tracing functionality used for debugging other macros.
1591 #[unstable(
1592 feature = "trace_macros",
1593 issue = "29598",
1594 reason = "`trace_macros` is not stable enough for use and is subject to change"
1595 )]
1596 #[rustc_builtin_macro]
1597 #[macro_export]
1598 macro_rules! trace_macros {
1599 (true) => {{ /* compiler built-in */ }};
1600 (false) => {{ /* compiler built-in */ }};
1601 }
1602
1603 /// Attribute macro used to apply derive macros.
1604 ///
1605 /// See [the reference] for more info.
1606 ///
1607 /// [the reference]: ../../../reference/attributes/derive.html
1608 #[stable(feature = "rust1", since = "1.0.0")]
1609 #[rustc_builtin_macro]
1610 pub macro derive($item:item) {
1611 /* compiler built-in */
1612 }
1613
1614 /// Attribute macro used to apply derive macros for implementing traits
1615 /// in a const context.
1616 ///
1617 /// See [the reference] for more info.
1618 ///
1619 /// [the reference]: ../../../reference/attributes/derive.html
1620 #[unstable(feature = "derive_const", issue = "118304")]
1621 #[rustc_builtin_macro]
1622 pub macro derive_const($item:item) {
1623 /* compiler built-in */
1624 }
1625
1626 /// Attribute macro applied to a function to turn it into a unit test.
1627 ///
1628 /// See [the reference] for more info.
1629 ///
1630 /// [the reference]: ../../../reference/attributes/testing.html#the-test-attribute
1631 #[stable(feature = "rust1", since = "1.0.0")]
1632 #[allow_internal_unstable(test, rustc_attrs, coverage_attribute)]
1633 #[rustc_builtin_macro]
1634 pub macro test($item:item) {
1635 /* compiler built-in */
1636 }
1637
1638 /// Attribute macro applied to a function to turn it into a benchmark test.
1639 #[unstable(
1640 feature = "test",
1641 issue = "50297",
1642 reason = "`bench` is a part of custom test frameworks which are unstable"
1643 )]
1644 #[allow_internal_unstable(test, rustc_attrs, coverage_attribute)]
1645 #[rustc_builtin_macro]
1646 pub macro bench($item:item) {
1647 /* compiler built-in */
1648 }
1649
1650 /// An implementation detail of the `#[test]` and `#[bench]` macros.
1651 #[unstable(
1652 feature = "custom_test_frameworks",
1653 issue = "50297",
1654 reason = "custom test frameworks are an unstable feature"
1655 )]
1656 #[allow_internal_unstable(test, rustc_attrs)]
1657 #[rustc_builtin_macro]
1658 pub macro test_case($item:item) {
1659 /* compiler built-in */
1660 }
1661
1662 /// Attribute macro applied to a static to register it as a global allocator.
1663 ///
1664 /// See also [`std::alloc::GlobalAlloc`](../../../std/alloc/trait.GlobalAlloc.html).
1665 #[stable(feature = "global_allocator", since = "1.28.0")]
1666 #[allow_internal_unstable(rustc_attrs)]
1667 #[rustc_builtin_macro]
1668 pub macro global_allocator($item:item) {
1669 /* compiler built-in */
1670 }
1671
1672 /// Attribute macro applied to a function to give it a post-condition.
1673 ///
1674 /// The attribute carries an argument token-tree which is
1675 /// eventually parsed as a unary closure expression that is
1676 /// invoked on a reference to the return value.
1677 #[unstable(feature = "contracts", issue = "128044")]
1678 #[allow_internal_unstable(contracts_internals)]
1679 #[rustc_builtin_macro]
1680 pub macro contracts_ensures($item:item) {
1681 /* compiler built-in */
1682 }
1683
1684 /// Attribute macro applied to a function to give it a precondition.
1685 ///
1686 /// The attribute carries an argument token-tree which is
1687 /// eventually parsed as an boolean expression with access to the
1688 /// function's formal parameters
1689 #[unstable(feature = "contracts", issue = "128044")]
1690 #[allow_internal_unstable(contracts_internals)]
1691 #[rustc_builtin_macro]
1692 pub macro contracts_requires($item:item) {
1693 /* compiler built-in */
1694 }
1695
1696 /// Attribute macro applied to a function to register it as a handler for allocation failure.
1697 ///
1698 /// See also [`std::alloc::handle_alloc_error`](../../../std/alloc/fn.handle_alloc_error.html).
1699 #[unstable(feature = "alloc_error_handler", issue = "51540")]
1700 #[allow_internal_unstable(rustc_attrs)]
1701 #[rustc_builtin_macro]
1702 pub macro alloc_error_handler($item:item) {
1703 /* compiler built-in */
1704 }
1705
1706 /// Keeps the item it's applied to if the passed path is accessible, and removes it otherwise.
1707 #[unstable(
1708 feature = "cfg_accessible",
1709 issue = "64797",
1710 reason = "`cfg_accessible` is not fully implemented"
1711 )]
1712 #[rustc_builtin_macro]
1713 pub macro cfg_accessible($item:item) {
1714 /* compiler built-in */
1715 }
1716
1717 /// Expands all `#[cfg]` and `#[cfg_attr]` attributes in the code fragment it's applied to.
1718 #[unstable(
1719 feature = "cfg_eval",
1720 issue = "82679",
1721 reason = "`cfg_eval` is a recently implemented feature"
1722 )]
1723 #[rustc_builtin_macro]
1724 pub macro cfg_eval($($tt:tt)*) {
1725 /* compiler built-in */
1726 }
1727
1728 /// Provide a list of type aliases and other opaque-type-containing type definitions
1729 /// to an item with a body. This list will be used in that body to define opaque
1730 /// types' hidden types.
1731 /// Can only be applied to things that have bodies.
1732 #[unstable(
1733 feature = "type_alias_impl_trait",
1734 issue = "63063",
1735 reason = "`type_alias_impl_trait` has open design concerns"
1736 )]
1737 #[rustc_builtin_macro]
1738 pub macro define_opaque($($tt:tt)*) {
1739 /* compiler built-in */
1740 }
1741
1742 /// Unstable placeholder for type ascription.
1743 #[allow_internal_unstable(builtin_syntax)]
1744 #[unstable(
1745 feature = "type_ascription",
1746 issue = "23416",
1747 reason = "placeholder syntax for type ascription"
1748 )]
1749 #[rustfmt::skip]
1750 pub macro type_ascribe($expr:expr, $ty:ty) {
1751 builtin # type_ascribe($expr, $ty)
1752 }
1753
1754 /// Unstable placeholder for deref patterns.
1755 #[allow_internal_unstable(builtin_syntax)]
1756 #[unstable(
1757 feature = "deref_patterns",
1758 issue = "87121",
1759 reason = "placeholder syntax for deref patterns"
1760 )]
1761 pub macro deref($pat:pat) {
1762 builtin # deref($pat)
1763 }
1764}