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