1 #[doc = include_str!("panic.md")]
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"]
8 // Expands to either `$crate::panic::panic_2015` or `$crate::panic::panic_2021`
9 // depending on the edition of the caller.
11 /* compiler built-in */
15 /// Asserts that two expressions are equal to each other (using [`PartialEq`]).
17 /// On panic, this macro will print the values of the expressions with their
18 /// debug representations.
20 /// Like [`assert!`], this macro has a second form, where a custom
21 /// panic message can be provided.
30 /// assert_eq!(a, b, "we are testing addition with {} and {}", a, b);
33 #[stable(feature = "rust1", since = "1.0.0")]
34 #[cfg_attr(not(test), rustc_diagnostic_item = "assert_eq_macro")]
35 #[allow_internal_unstable(core_panic)]
36 macro_rules! assert_eq {
37 ($left:expr, $right:expr $(,)?) => {
38 match (&$left, &$right) {
39 (left_val, right_val) => {
40 if !(*left_val == *right_val) {
41 let kind = $crate::panicking::AssertKind::Eq;
42 // The reborrows below are intentional. Without them, the stack slot for the
43 // borrow is initialized even before the values are compared, leading to a
44 // noticeable slow down.
45 $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None);
50 ($left:expr, $right:expr, $($arg:tt)+) => {
51 match (&$left, &$right) {
52 (left_val, right_val) => {
53 if !(*left_val == *right_val) {
54 let kind = $crate::panicking::AssertKind::Eq;
55 // The reborrows below are intentional. Without them, the stack slot for the
56 // borrow is initialized even before the values are compared, leading to a
57 // noticeable slow down.
58 $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)));
65 /// Asserts that two expressions are not equal to each other (using [`PartialEq`]).
67 /// On panic, this macro will print the values of the expressions with their
68 /// debug representations.
70 /// Like [`assert!`], this macro has a second form, where a custom
71 /// panic message can be provided.
80 /// assert_ne!(a, b, "we are testing that the values are not equal");
83 #[stable(feature = "assert_ne", since = "1.13.0")]
84 #[cfg_attr(not(test), rustc_diagnostic_item = "assert_ne_macro")]
85 #[allow_internal_unstable(core_panic)]
86 macro_rules! assert_ne {
87 ($left:expr, $right:expr $(,)?) => {
88 match (&$left, &$right) {
89 (left_val, right_val) => {
90 if *left_val == *right_val {
91 let kind = $crate::panicking::AssertKind::Ne;
92 // The reborrows below are intentional. Without them, the stack slot for the
93 // borrow is initialized even before the values are compared, leading to a
94 // noticeable slow down.
95 $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None);
100 ($left:expr, $right:expr, $($arg:tt)+) => {
101 match (&($left), &($right)) {
102 (left_val, right_val) => {
103 if *left_val == *right_val {
104 let kind = $crate::panicking::AssertKind::Ne;
105 // The reborrows below are intentional. Without them, the stack slot for the
106 // borrow is initialized even before the values are compared, leading to a
107 // noticeable slow down.
108 $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)));
115 /// Asserts that an expression matches any of the given patterns.
117 /// Like in a `match` expression, the pattern can be optionally followed by `if`
118 /// and a guard expression that has access to names bound by the pattern.
120 /// On panic, this macro will print the value of the expression with its
121 /// debug representation.
123 /// Like [`assert!`], this macro has a second form, where a custom
124 /// panic message can be provided.
129 /// #![feature(assert_matches)]
131 /// use std::assert_matches::assert_matches;
133 /// let a = 1u32.checked_add(2);
134 /// let b = 1u32.checked_sub(2);
135 /// assert_matches!(a, Some(_));
136 /// assert_matches!(b, None);
138 /// let c = Ok("abc".to_string());
139 /// assert_matches!(c, Ok(x) | Err(x) if x.len() < 100);
141 #[unstable(feature = "assert_matches", issue = "82775")]
142 #[allow_internal_unstable(core_panic)]
143 #[rustc_macro_transparency = "semitransparent"]
144 pub macro assert_matches {
145 ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
147 $( $pattern )|+ $( if $guard )? => {}
149 $crate::panicking::assert_matches_failed(
151 $crate::stringify!($($pattern)|+ $(if $guard)?),
152 $crate::option::Option::None
157 ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )?, $($arg:tt)+) => {
159 $( $pattern )|+ $( if $guard )? => {}
161 $crate::panicking::assert_matches_failed(
163 $crate::stringify!($($pattern)|+ $(if $guard)?),
164 $crate::option::Option::Some($crate::format_args!($($arg)+))
171 /// Asserts that a boolean expression is `true` at runtime.
173 /// This will invoke the [`panic!`] macro if the provided expression cannot be
174 /// evaluated to `true` at runtime.
176 /// Like [`assert!`], this macro also has a second version, where a custom panic
177 /// message can be provided.
181 /// Unlike [`assert!`], `debug_assert!` statements are only enabled in non
182 /// optimized builds by default. An optimized build will not execute
183 /// `debug_assert!` statements unless `-C debug-assertions` is passed to the
184 /// compiler. This makes `debug_assert!` useful for checks that are too
185 /// expensive to be present in a release build but may be helpful during
186 /// development. The result of expanding `debug_assert!` is always type checked.
188 /// An unchecked assertion allows a program in an inconsistent state to keep
189 /// running, which might have unexpected consequences but does not introduce
190 /// unsafety as long as this only happens in safe code. The performance cost
191 /// of assertions, however, is not measurable in general. Replacing [`assert!`]
192 /// with `debug_assert!` is thus only encouraged after thorough profiling, and
193 /// more importantly, only in safe code!
198 /// // the panic message for these assertions is the stringified value of the
199 /// // expression given.
200 /// debug_assert!(true);
202 /// fn some_expensive_computation() -> bool { true } // a very simple function
203 /// debug_assert!(some_expensive_computation());
205 /// // assert with a custom message
207 /// debug_assert!(x, "x wasn't true!");
209 /// let a = 3; let b = 27;
210 /// debug_assert!(a + b == 30, "a = {}, b = {}", a, b);
213 #[stable(feature = "rust1", since = "1.0.0")]
214 #[rustc_diagnostic_item = "debug_assert_macro"]
215 #[allow_internal_unstable(edition_panic)]
216 macro_rules! debug_assert {
218 if $crate::cfg!(debug_assertions) {
219 $crate::assert!($($arg)*);
224 /// Asserts that two expressions are equal to each other.
226 /// On panic, this macro will print the values of the expressions with their
227 /// debug representations.
229 /// Unlike [`assert_eq!`], `debug_assert_eq!` statements are only enabled in non
230 /// optimized builds by default. An optimized build will not execute
231 /// `debug_assert_eq!` statements unless `-C debug-assertions` is passed to the
232 /// compiler. This makes `debug_assert_eq!` useful for checks that are too
233 /// expensive to be present in a release build but may be helpful during
234 /// development. The result of expanding `debug_assert_eq!` is always type checked.
241 /// debug_assert_eq!(a, b);
244 #[stable(feature = "rust1", since = "1.0.0")]
245 #[cfg_attr(not(test), rustc_diagnostic_item = "debug_assert_eq_macro")]
246 macro_rules! debug_assert_eq {
248 if $crate::cfg!(debug_assertions) {
249 $crate::assert_eq!($($arg)*);
254 /// Asserts that two expressions are not equal to each other.
256 /// On panic, this macro will print the values of the expressions with their
257 /// debug representations.
259 /// Unlike [`assert_ne!`], `debug_assert_ne!` statements are only enabled in non
260 /// optimized builds by default. An optimized build will not execute
261 /// `debug_assert_ne!` statements unless `-C debug-assertions` is passed to the
262 /// compiler. This makes `debug_assert_ne!` useful for checks that are too
263 /// expensive to be present in a release build but may be helpful during
264 /// development. The result of expanding `debug_assert_ne!` is always type checked.
271 /// debug_assert_ne!(a, b);
274 #[stable(feature = "assert_ne", since = "1.13.0")]
275 #[cfg_attr(not(test), rustc_diagnostic_item = "debug_assert_ne_macro")]
276 macro_rules! debug_assert_ne {
278 if $crate::cfg!(debug_assertions) {
279 $crate::assert_ne!($($arg)*);
284 /// Asserts that an expression matches any of the given patterns.
286 /// Like in a `match` expression, the pattern can be optionally followed by `if`
287 /// and a guard expression that has access to names bound by the pattern.
289 /// On panic, this macro will print the value of the expression with its
290 /// debug representation.
292 /// Unlike [`assert_matches!`], `debug_assert_matches!` statements are only
293 /// enabled in non optimized builds by default. An optimized build will not
294 /// execute `debug_assert_matches!` statements unless `-C debug-assertions` is
295 /// passed to the compiler. This makes `debug_assert_matches!` useful for
296 /// checks that are too expensive to be present in a release build but may be
297 /// helpful during development. The result of expanding `debug_assert_matches!`
298 /// is always type checked.
303 /// #![feature(assert_matches)]
305 /// use std::assert_matches::debug_assert_matches;
307 /// let a = 1u32.checked_add(2);
308 /// let b = 1u32.checked_sub(2);
309 /// debug_assert_matches!(a, Some(_));
310 /// debug_assert_matches!(b, None);
312 /// let c = Ok("abc".to_string());
313 /// debug_assert_matches!(c, Ok(x) | Err(x) if x.len() < 100);
316 #[unstable(feature = "assert_matches", issue = "82775")]
317 #[allow_internal_unstable(assert_matches)]
318 #[rustc_macro_transparency = "semitransparent"]
319 pub macro debug_assert_matches($($arg:tt)*) {
320 if $crate::cfg!(debug_assertions) {
321 $crate::assert_matches::assert_matches!($($arg)*);
325 /// Returns whether the given expression matches any of the given patterns.
327 /// Like in a `match` expression, the pattern can be optionally followed by `if`
328 /// and a guard expression that has access to names bound by the pattern.
334 /// assert!(matches!(foo, 'A'..='Z' | 'a'..='z'));
336 /// let bar = Some(4);
337 /// assert!(matches!(bar, Some(x) if x > 2));
340 #[stable(feature = "matches_macro", since = "1.42.0")]
341 #[cfg_attr(not(test), rustc_diagnostic_item = "matches_macro")]
342 macro_rules! matches {
343 ($expression:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
345 $( $pattern )|+ $( if $guard )? => true,
351 /// Unwraps a result or propagates its error.
353 /// The [`?` operator][propagating-errors] was added to replace `try!`
354 /// and should be used instead. Furthermore, `try` is a reserved word
355 /// in Rust 2018, so if you must use it, you will need to use the
356 /// [raw-identifier syntax][ris]: `r#try`.
358 /// [propagating-errors]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
359 /// [ris]: https://doc.rust-lang.org/nightly/rust-by-example/compatibility/raw_identifiers.html
361 /// `try!` matches the given [`Result`]. In case of the `Ok` variant, the
362 /// expression has the value of the wrapped value.
364 /// In case of the `Err` variant, it retrieves the inner error. `try!` then
365 /// performs conversion using `From`. This provides automatic conversion
366 /// between specialized errors and more general ones. The resulting
367 /// error is then immediately returned.
369 /// Because of the early return, `try!` can only be used in functions that
370 /// return [`Result`].
376 /// use std::fs::File;
377 /// use std::io::prelude::*;
383 /// impl From<io::Error> for MyError {
384 /// fn from(e: io::Error) -> MyError {
385 /// MyError::FileWriteError
389 /// // The preferred method of quick returning Errors
390 /// fn write_to_file_question() -> Result<(), MyError> {
391 /// let mut file = File::create("my_best_friends.txt")?;
392 /// file.write_all(b"This is a list of my best friends.")?;
396 /// // The previous method of quick returning Errors
397 /// fn write_to_file_using_try() -> Result<(), MyError> {
398 /// let mut file = r#try!(File::create("my_best_friends.txt"));
399 /// r#try!(file.write_all(b"This is a list of my best friends."));
403 /// // This is equivalent to:
404 /// fn write_to_file_using_match() -> Result<(), MyError> {
405 /// let mut file = r#try!(File::create("my_best_friends.txt"));
406 /// match file.write_all(b"This is a list of my best friends.") {
408 /// Err(e) => return Err(From::from(e)),
414 #[stable(feature = "rust1", since = "1.0.0")]
415 #[deprecated(since = "1.39.0", note = "use the `?` operator instead")]
418 ($expr:expr $(,)?) => {
420 $crate::result::Result::Ok(val) => val,
421 $crate::result::Result::Err(err) => {
422 return $crate::result::Result::Err($crate::convert::From::from(err));
428 /// Writes formatted data into a buffer.
430 /// This macro accepts a 'writer', a format string, and a list of arguments. Arguments will be
431 /// formatted according to the specified format string and the result will be passed to the writer.
432 /// The writer may be any value with a `write_fmt` method; generally this comes from an
433 /// implementation of either the [`fmt::Write`] or the [`io::Write`] trait. The macro
434 /// returns whatever the `write_fmt` method returns; commonly a [`fmt::Result`], or an
437 /// See [`std::fmt`] for more information on the format string syntax.
439 /// [`std::fmt`]: ../std/fmt/index.html
440 /// [`fmt::Write`]: crate::fmt::Write
441 /// [`io::Write`]: ../std/io/trait.Write.html
442 /// [`fmt::Result`]: crate::fmt::Result
443 /// [`io::Result`]: ../std/io/type.Result.html
448 /// use std::io::Write;
450 /// fn main() -> std::io::Result<()> {
451 /// let mut w = Vec::new();
452 /// write!(&mut w, "test")?;
453 /// write!(&mut w, "formatted {}", "arguments")?;
455 /// assert_eq!(w, b"testformatted arguments");
460 /// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
461 /// implementing either, as objects do not typically implement both. However, the module must
462 /// avoid conflict between the trait names, such as by importing them as `_` or otherwise renaming
466 /// use std::fmt::Write as _;
467 /// use std::io::Write as _;
469 /// fn main() -> Result<(), Box<dyn std::error::Error>> {
470 /// let mut s = String::new();
471 /// let mut v = Vec::new();
473 /// write!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
474 /// write!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
475 /// assert_eq!(v, b"s = \"abc 123\"");
480 /// If you also need the trait names themselves, such as to implement one or both on your types,
481 /// import the containing module and then name them with a prefix:
484 /// # #![allow(unused_imports)]
485 /// use std::fmt::{self, Write as _};
486 /// use std::io::{self, Write as _};
490 /// impl fmt::Write for Example {
491 /// fn write_str(&mut self, _s: &str) -> core::fmt::Result {
492 /// unimplemented!();
497 /// Note: This macro can be used in `no_std` setups as well.
498 /// In a `no_std` setup you are responsible for the implementation details of the components.
501 /// # extern crate core;
502 /// use core::fmt::Write;
506 /// impl Write for Example {
507 /// fn write_str(&mut self, _s: &str) -> core::fmt::Result {
508 /// unimplemented!();
512 /// let mut m = Example{};
513 /// write!(&mut m, "Hello World").expect("Not written");
516 #[stable(feature = "rust1", since = "1.0.0")]
517 #[cfg_attr(not(test), rustc_diagnostic_item = "write_macro")]
519 ($dst:expr, $($arg:tt)*) => {
520 $dst.write_fmt($crate::format_args!($($arg)*))
524 /// Write formatted data into a buffer, with a newline appended.
526 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
527 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
529 /// For more information, see [`write!`]. For information on the format string syntax, see
532 /// [`std::fmt`]: ../std/fmt/index.html
537 /// use std::io::{Write, Result};
539 /// fn main() -> Result<()> {
540 /// let mut w = Vec::new();
541 /// writeln!(&mut w)?;
542 /// writeln!(&mut w, "test")?;
543 /// writeln!(&mut w, "formatted {}", "arguments")?;
545 /// assert_eq!(&w[..], "\ntest\nformatted arguments\n".as_bytes());
550 #[stable(feature = "rust1", since = "1.0.0")]
551 #[cfg_attr(not(test), rustc_diagnostic_item = "writeln_macro")]
552 #[allow_internal_unstable(format_args_nl)]
553 macro_rules! writeln {
554 ($dst:expr $(,)?) => {
555 $crate::write!($dst, "\n")
557 ($dst:expr, $($arg:tt)*) => {
558 $dst.write_fmt($crate::format_args_nl!($($arg)*))
562 /// Indicates unreachable code.
564 /// This is useful any time that the compiler can't determine that some code is unreachable. For
567 /// * Match arms with guard conditions.
568 /// * Loops that dynamically terminate.
569 /// * Iterators that dynamically terminate.
571 /// If the determination that the code is unreachable proves incorrect, the
572 /// program immediately terminates with a [`panic!`].
574 /// The unsafe counterpart of this macro is the [`unreachable_unchecked`] function, which
575 /// will cause undefined behavior if the code is reached.
577 /// [`unreachable_unchecked`]: crate::hint::unreachable_unchecked
581 /// This will always [`panic!`] because `unreachable!` is just a shorthand for `panic!` with a
582 /// fixed, specific message.
584 /// Like `panic!`, this macro has a second form for displaying custom values.
591 /// # #[allow(dead_code)]
592 /// fn foo(x: Option<i32>) {
594 /// Some(n) if n >= 0 => println!("Some(Non-negative)"),
595 /// Some(n) if n < 0 => println!("Some(Negative)"),
596 /// Some(_) => unreachable!(), // compile error if commented out
597 /// None => println!("None")
605 /// # #[allow(dead_code)]
606 /// fn divide_by_three(x: u32) -> u32 { // one of the poorest implementations of x/3
608 /// if 3*i < i { panic!("u32 overflow"); }
609 /// if x < 3*i { return i-1; }
611 /// unreachable!("The loop should always return");
615 #[rustc_builtin_macro(unreachable)]
616 #[allow_internal_unstable(edition_panic)]
617 #[stable(feature = "rust1", since = "1.0.0")]
618 #[cfg_attr(not(test), rustc_diagnostic_item = "unreachable_macro")]
619 macro_rules! unreachable {
620 // Expands to either `$crate::panic::unreachable_2015` or `$crate::panic::unreachable_2021`
621 // depending on the edition of the caller.
623 /* compiler built-in */
627 /// Indicates unimplemented code by panicking with a message of "not implemented".
629 /// This allows your code to type-check, which is useful if you are prototyping or
630 /// implementing a trait that requires multiple methods which you don't plan to use all of.
632 /// The difference between `unimplemented!` and [`todo!`] is that while `todo!`
633 /// conveys an intent of implementing the functionality later and the message is "not yet
634 /// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
635 /// Also some IDEs will mark `todo!`s.
639 /// This will always [`panic!`] because `unimplemented!` is just a shorthand for `panic!` with a
640 /// fixed, specific message.
642 /// Like `panic!`, this macro has a second form for displaying custom values.
644 /// [`todo!`]: crate::todo
648 /// Say we have a trait `Foo`:
652 /// fn bar(&self) -> u8;
654 /// fn qux(&self) -> Result<u64, ()>;
658 /// We want to implement `Foo` for 'MyStruct', but for some reason it only makes sense
659 /// to implement the `bar()` function. `baz()` and `qux()` will still need to be defined
660 /// in our implementation of `Foo`, but we can use `unimplemented!` in their definitions
661 /// to allow our code to compile.
663 /// We still want to have our program stop running if the unimplemented methods are
668 /// # fn bar(&self) -> u8;
670 /// # fn qux(&self) -> Result<u64, ()>;
674 /// impl Foo for MyStruct {
675 /// fn bar(&self) -> u8 {
680 /// // It makes no sense to `baz` a `MyStruct`, so we have no logic here
682 /// // This will display "thread 'main' panicked at 'not implemented'".
683 /// unimplemented!();
686 /// fn qux(&self) -> Result<u64, ()> {
687 /// // We have some logic here,
688 /// // We can add a message to unimplemented! to display our omission.
689 /// // This will display:
690 /// // "thread 'main' panicked at 'not implemented: MyStruct isn't quxable'".
691 /// unimplemented!("MyStruct isn't quxable");
696 /// let s = MyStruct;
701 #[stable(feature = "rust1", since = "1.0.0")]
702 #[cfg_attr(not(test), rustc_diagnostic_item = "unimplemented_macro")]
703 #[allow_internal_unstable(core_panic)]
704 macro_rules! unimplemented {
706 $crate::panicking::panic("not implemented")
709 $crate::panic!("not implemented: {}", $crate::format_args!($($arg)+))
713 /// Indicates unfinished code.
715 /// This can be useful if you are prototyping and are just looking to have your
718 /// The difference between [`unimplemented!`] and `todo!` is that while `todo!` conveys
719 /// 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 /// Also some IDEs will mark `todo!`s.
725 /// This will always [`panic!`].
729 /// Here's an example of some in-progress code. We have a trait `Foo`:
738 /// We want to implement `Foo` on one of our types, but we also want to work on
739 /// just `bar()` first. In order for our code to compile, we need to implement
740 /// `baz()`, so we can use `todo!`:
749 /// impl Foo for MyStruct {
751 /// // implementation goes here
755 /// // let's not worry about implementing baz() for now
761 /// let s = MyStruct;
764 /// // we aren't even using baz(), so this is fine.
768 #[stable(feature = "todo_macro", since = "1.40.0")]
769 #[cfg_attr(not(test), rustc_diagnostic_item = "todo_macro")]
770 #[allow_internal_unstable(core_panic)]
773 $crate::panicking::panic("not yet implemented")
776 $crate::panic!("not yet implemented: {}", $crate::format_args!($($arg)+))
780 /// Definitions of built-in macros.
782 /// Most of the macro properties (stability, visibility, etc.) are taken from the source code here,
783 /// with exception of expansion functions transforming macro inputs into outputs,
784 /// those functions are provided by the compiler.
785 pub(crate) mod builtin {
787 /// Causes compilation to fail with the given error message when encountered.
789 /// This macro should be used when a crate uses a conditional compilation strategy to provide
790 /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
791 /// but emits an error during *compilation* rather than at *runtime*.
795 /// Two such examples are macros and `#[cfg]` environments.
797 /// Emit a better compiler error if a macro is passed invalid values. Without the final branch,
798 /// the compiler would still emit an error, but the error's message would not mention the two
802 /// macro_rules! give_me_foo_or_bar {
806 /// compile_error!("This macro only accepts `foo` or `bar`");
810 /// give_me_foo_or_bar!(neither);
811 /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
814 /// Emit a compiler error if one of a number of features isn't available.
817 /// #[cfg(not(any(feature = "foo", feature = "bar")))]
818 /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.");
820 #[stable(feature = "compile_error_macro", since = "1.20.0")]
821 #[rustc_builtin_macro]
823 macro_rules! compile_error {
824 ($msg:expr $(,)?) => {{ /* compiler built-in */ }};
827 /// Constructs parameters for the other string-formatting macros.
829 /// This macro functions by taking a formatting string literal containing
830 /// `{}` for each additional argument passed. `format_args!` prepares the
831 /// additional parameters to ensure the output can be interpreted as a string
832 /// and canonicalizes the arguments into a single type. Any value that implements
833 /// the [`Display`] trait can be passed to `format_args!`, as can any
834 /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
836 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
837 /// passed to the macros within [`std::fmt`] for performing useful redirection.
838 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
839 /// proxied through this one. `format_args!`, unlike its derived macros, avoids
840 /// heap allocations.
842 /// You can use the [`fmt::Arguments`] value that `format_args!` returns
843 /// in `Debug` and `Display` contexts as seen below. The example also shows
844 /// that `Debug` and `Display` format to the same thing: the interpolated
845 /// format string in `format_args!`.
848 /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
849 /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
850 /// assert_eq!("1 foo 2", display);
851 /// assert_eq!(display, debug);
854 /// For more information, see the documentation in [`std::fmt`].
856 /// [`Display`]: crate::fmt::Display
857 /// [`Debug`]: crate::fmt::Debug
858 /// [`fmt::Arguments`]: crate::fmt::Arguments
859 /// [`std::fmt`]: ../std/fmt/index.html
860 /// [`format!`]: ../std/macro.format.html
861 /// [`println!`]: ../std/macro.println.html
868 /// let s = fmt::format(format_args!("hello {}", "world"));
869 /// assert_eq!(s, format!("hello {}", "world"));
871 #[stable(feature = "rust1", since = "1.0.0")]
872 #[cfg_attr(not(test), rustc_diagnostic_item = "format_args_macro")]
873 #[allow_internal_unsafe]
874 #[allow_internal_unstable(fmt_internals)]
875 #[rustc_builtin_macro]
877 macro_rules! format_args {
878 ($fmt:expr) => {{ /* compiler built-in */ }};
879 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
882 /// Same as [`format_args`], but can be used in some const contexts.
884 /// This macro is used by the panic macros for the `const_panic` feature.
886 /// This macro will be removed once `format_args` is allowed in const contexts.
887 #[unstable(feature = "const_format_args", issue = "none")]
888 #[allow_internal_unstable(fmt_internals, const_fmt_arguments_new)]
889 #[rustc_builtin_macro]
891 macro_rules! const_format_args {
892 ($fmt:expr) => {{ /* compiler built-in */ }};
893 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
896 /// Same as [`format_args`], but adds a newline in the end.
898 feature = "format_args_nl",
900 reason = "`format_args_nl` is only for internal \
901 language use and is subject to change"
903 #[allow_internal_unstable(fmt_internals)]
904 #[rustc_builtin_macro]
906 macro_rules! format_args_nl {
907 ($fmt:expr) => {{ /* compiler built-in */ }};
908 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
911 /// Inspects an environment variable at compile time.
913 /// This macro will expand to the value of the named environment variable at
914 /// compile time, yielding an expression of type `&'static str`. Use
915 /// [`std::env::var`] instead if you want to read the value at runtime.
917 /// [`std::env::var`]: ../std/env/fn.var.html
919 /// If the environment variable is not defined, then a compilation error
920 /// will be emitted. To not emit a compile error, use the [`option_env!`]
926 /// let path: &'static str = env!("PATH");
927 /// println!("the $PATH variable at the time of compiling was: {path}");
930 /// You can customize the error message by passing a string as the second
934 /// let doc: &'static str = env!("documentation", "what's that?!");
937 /// If the `documentation` environment variable is not defined, you'll get
938 /// the following error:
941 /// error: what's that?!
943 #[stable(feature = "rust1", since = "1.0.0")]
944 #[rustc_builtin_macro]
947 ($name:expr $(,)?) => {{ /* compiler built-in */ }};
948 ($name:expr, $error_msg:expr $(,)?) => {{ /* compiler built-in */ }};
951 /// Optionally inspects an environment variable at compile time.
953 /// If the named environment variable is present at compile time, this will
954 /// expand into an expression of type `Option<&'static str>` whose value is
955 /// `Some` of the value of the environment variable. If the environment
956 /// variable is not present, then this will expand to `None`. See
957 /// [`Option<T>`][Option] for more information on this type. Use
958 /// [`std::env::var`] instead if you want to read the value at runtime.
960 /// [`std::env::var`]: ../std/env/fn.var.html
962 /// A compile time error is never emitted when using this macro regardless
963 /// of whether the environment variable is present or not.
968 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
969 /// println!("the secret key might be: {key:?}");
971 #[stable(feature = "rust1", since = "1.0.0")]
972 #[rustc_builtin_macro]
974 macro_rules! option_env {
975 ($name:expr $(,)?) => {{ /* compiler built-in */ }};
978 /// Concatenates identifiers into one identifier.
980 /// This macro takes any number of comma-separated identifiers, and
981 /// concatenates them all into one, yielding an expression which is a new
982 /// identifier. Note that hygiene makes it such that this macro cannot
983 /// capture local variables. Also, as a general rule, macros are only
984 /// allowed in item, statement or expression position. That means while
985 /// you may use this macro for referring to existing variables, functions or
986 /// modules etc, you cannot define a new one with it.
991 /// #![feature(concat_idents)]
994 /// fn foobar() -> u32 { 23 }
996 /// let f = concat_idents!(foo, bar);
997 /// println!("{}", f());
999 /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
1003 feature = "concat_idents",
1005 reason = "`concat_idents` is not stable enough for use and is subject to change"
1007 #[rustc_builtin_macro]
1009 macro_rules! concat_idents {
1010 ($($e:ident),+ $(,)?) => {{ /* compiler built-in */ }};
1013 /// Concatenates literals into a byte slice.
1015 /// This macro takes any number of comma-separated literals, and concatenates them all into
1016 /// one, yielding an expression of type `&[u8; _]`, which represents all of the literals
1017 /// concatenated left-to-right. The literals passed can be any combination of:
1019 /// - byte literals (`b'r'`)
1020 /// - byte strings (`b"Rust"`)
1021 /// - arrays of bytes/numbers (`[b'A', 66, b'C']`)
1026 /// #![feature(concat_bytes)]
1029 /// let s: &[u8; 6] = concat_bytes!(b'A', b"BC", [68, b'E', 70]);
1030 /// assert_eq!(s, b"ABCDEF");
1033 #[unstable(feature = "concat_bytes", issue = "87555")]
1034 #[rustc_builtin_macro]
1036 macro_rules! concat_bytes {
1037 ($($e:literal),+ $(,)?) => {{ /* compiler built-in */ }};
1040 /// Concatenates literals into a static string slice.
1042 /// This macro takes any number of comma-separated literals, yielding an
1043 /// expression of type `&'static str` which represents all of the literals
1044 /// concatenated left-to-right.
1046 /// Integer and floating point literals are stringified in order to be
1052 /// let s = concat!("test", 10, 'b', true);
1053 /// assert_eq!(s, "test10btrue");
1055 #[stable(feature = "rust1", since = "1.0.0")]
1056 #[rustc_builtin_macro]
1058 macro_rules! concat {
1059 ($($e:expr),* $(,)?) => {{ /* compiler built-in */ }};
1062 /// Expands to the line number on which it was invoked.
1064 /// With [`column!`] and [`file!`], these macros provide debugging information for
1065 /// developers about the location within the source.
1067 /// The expanded expression has type `u32` and is 1-based, so the first line
1068 /// in each file evaluates to 1, the second to 2, etc. This is consistent
1069 /// with error messages by common compilers or popular editors.
1070 /// The returned line is *not necessarily* the line of the `line!` invocation itself,
1071 /// but rather the first macro invocation leading up to the invocation
1072 /// of the `line!` macro.
1077 /// let current_line = line!();
1078 /// println!("defined on line: {current_line}");
1080 #[stable(feature = "rust1", since = "1.0.0")]
1081 #[rustc_builtin_macro]
1085 /* compiler built-in */
1089 /// Expands to the column number at which it was invoked.
1091 /// With [`line!`] and [`file!`], these macros provide debugging information for
1092 /// developers about the location within the source.
1094 /// The expanded expression has type `u32` and is 1-based, so the first column
1095 /// in each line evaluates to 1, the second to 2, etc. This is consistent
1096 /// with error messages by common compilers or popular editors.
1097 /// The returned column is *not necessarily* the line of the `column!` invocation itself,
1098 /// but rather the first macro invocation leading up to the invocation
1099 /// of the `column!` macro.
1104 /// let current_col = column!();
1105 /// println!("defined on column: {current_col}");
1108 /// `column!` counts Unicode code points, not bytes or graphemes. As a result, the first two
1109 /// invocations return the same value, but the third does not.
1112 /// let a = ("foobar", column!()).1;
1113 /// let b = ("人之初性本善", column!()).1;
1114 /// let c = ("f̅o̅o̅b̅a̅r̅", column!()).1; // Uses combining overline (U+0305)
1116 /// assert_eq!(a, b);
1117 /// assert_ne!(b, c);
1119 #[stable(feature = "rust1", since = "1.0.0")]
1120 #[rustc_builtin_macro]
1122 macro_rules! column {
1124 /* compiler built-in */
1128 /// Expands to the file name in which it was invoked.
1130 /// With [`line!`] and [`column!`], these macros provide debugging information for
1131 /// developers about the location within the source.
1133 /// The expanded expression has type `&'static str`, and the returned file
1134 /// is not the invocation of the `file!` macro itself, but rather the
1135 /// first macro invocation leading up to the invocation of the `file!`
1141 /// let this_file = file!();
1142 /// println!("defined in file: {this_file}");
1144 #[stable(feature = "rust1", since = "1.0.0")]
1145 #[rustc_builtin_macro]
1149 /* compiler built-in */
1153 /// Stringifies its arguments.
1155 /// This macro will yield an expression of type `&'static str` which is the
1156 /// stringification of all the tokens passed to the macro. No restrictions
1157 /// are placed on the syntax of the macro invocation itself.
1159 /// Note that the expanded results of the input tokens may change in the
1160 /// future. You should be careful if you rely on the output.
1165 /// let one_plus_one = stringify!(1 + 1);
1166 /// assert_eq!(one_plus_one, "1 + 1");
1168 #[stable(feature = "rust1", since = "1.0.0")]
1169 #[rustc_builtin_macro]
1171 macro_rules! stringify {
1173 /* compiler built-in */
1177 /// Includes a UTF-8 encoded file as a string.
1179 /// The file is located relative to the current file (similarly to how
1180 /// modules are found). The provided path is interpreted in a platform-specific
1181 /// way at compile time. So, for instance, an invocation with a Windows path
1182 /// containing backslashes `\` would not compile correctly on Unix.
1184 /// This macro will yield an expression of type `&'static str` which is the
1185 /// contents of the file.
1189 /// Assume there are two files in the same directory with the following
1192 /// File 'spanish.in':
1200 /// ```ignore (cannot-doctest-external-file-dependency)
1202 /// let my_str = include_str!("spanish.in");
1203 /// assert_eq!(my_str, "adiós\n");
1204 /// print!("{my_str}");
1208 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
1209 #[stable(feature = "rust1", since = "1.0.0")]
1210 #[rustc_builtin_macro]
1212 #[cfg_attr(not(test), rustc_diagnostic_item = "include_str_macro")]
1213 macro_rules! include_str {
1214 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1217 /// Includes a file as a reference to a byte array.
1219 /// The file is located relative to the current file (similarly to how
1220 /// modules are found). The provided path is interpreted in a platform-specific
1221 /// way at compile time. So, for instance, an invocation with a Windows path
1222 /// containing backslashes `\` would not compile correctly on Unix.
1224 /// This macro will yield an expression of type `&'static [u8; N]` which is
1225 /// the contents of the file.
1229 /// Assume there are two files in the same directory with the following
1232 /// File 'spanish.in':
1240 /// ```ignore (cannot-doctest-external-file-dependency)
1242 /// let bytes = include_bytes!("spanish.in");
1243 /// assert_eq!(bytes, b"adi\xc3\xb3s\n");
1244 /// print!("{}", String::from_utf8_lossy(bytes));
1248 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
1249 #[stable(feature = "rust1", since = "1.0.0")]
1250 #[rustc_builtin_macro]
1252 #[cfg_attr(not(test), rustc_diagnostic_item = "include_bytes_macro")]
1253 macro_rules! include_bytes {
1254 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1257 /// Expands to a string that represents the current module path.
1259 /// The current module path can be thought of as the hierarchy of modules
1260 /// leading back up to the crate root. The first component of the path
1261 /// returned is the name of the crate currently being compiled.
1268 /// assert!(module_path!().ends_with("test"));
1274 #[stable(feature = "rust1", since = "1.0.0")]
1275 #[rustc_builtin_macro]
1277 macro_rules! module_path {
1279 /* compiler built-in */
1283 /// Evaluates boolean combinations of configuration flags at compile-time.
1285 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
1286 /// boolean expression evaluation of configuration flags. This frequently
1287 /// leads to less duplicated code.
1289 /// The syntax given to this macro is the same syntax as the [`cfg`]
1292 /// `cfg!`, unlike `#[cfg]`, does not remove any code and only evaluates to true or false. For
1293 /// example, all blocks in an if/else expression need to be valid when `cfg!` is used for
1294 /// the condition, regardless of what `cfg!` is evaluating.
1296 /// [`cfg`]: ../reference/conditional-compilation.html#the-cfg-attribute
1301 /// let my_directory = if cfg!(windows) {
1302 /// "windows-specific-directory"
1304 /// "unix-directory"
1307 #[stable(feature = "rust1", since = "1.0.0")]
1308 #[rustc_builtin_macro]
1312 /* compiler built-in */
1316 /// Parses a file as an expression or an item according to the context.
1318 /// The file is located relative to the current file (similarly to how
1319 /// modules are found). The provided path is interpreted in a platform-specific
1320 /// way at compile time. So, for instance, an invocation with a Windows path
1321 /// containing backslashes `\` would not compile correctly on Unix.
1323 /// Using this macro is often a bad idea, because if the file is
1324 /// parsed as an expression, it is going to be placed in the
1325 /// surrounding code unhygienically. This could result in variables
1326 /// or functions being different from what the file expected if
1327 /// there are variables or functions that have the same name in
1328 /// the current file.
1332 /// Assume there are two files in the same directory with the following
1335 /// File 'monkeys.in':
1337 /// ```ignore (only-for-syntax-highlight)
1342 /// .collect::<String>()
1347 /// ```ignore (cannot-doctest-external-file-dependency)
1349 /// let my_string = include!("monkeys.in");
1350 /// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
1351 /// println!("{my_string}");
1355 /// Compiling 'main.rs' and running the resulting binary will print
1357 #[stable(feature = "rust1", since = "1.0.0")]
1358 #[rustc_builtin_macro]
1360 macro_rules! include {
1361 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1364 /// Asserts that a boolean expression is `true` at runtime.
1366 /// This will invoke the [`panic!`] macro if the provided expression cannot be
1367 /// evaluated to `true` at runtime.
1371 /// Assertions are always checked in both debug and release builds, and cannot
1372 /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
1373 /// release builds by default.
1375 /// Unsafe code may rely on `assert!` to enforce run-time invariants that, if
1376 /// violated could lead to unsafety.
1378 /// Other use-cases of `assert!` include testing and enforcing run-time
1379 /// invariants in safe code (whose violation cannot result in unsafety).
1381 /// # Custom Messages
1383 /// This macro has a second form, where a custom panic message can
1384 /// be provided with or without arguments for formatting. See [`std::fmt`]
1385 /// for syntax for this form. Expressions used as format arguments will only
1386 /// be evaluated if the assertion fails.
1388 /// [`std::fmt`]: ../std/fmt/index.html
1393 /// // the panic message for these assertions is the stringified value of the
1394 /// // expression given.
1397 /// fn some_computation() -> bool { true } // a very simple function
1399 /// assert!(some_computation());
1401 /// // assert with a custom message
1403 /// assert!(x, "x wasn't true!");
1405 /// let a = 3; let b = 27;
1406 /// assert!(a + b == 30, "a = {}, b = {}", a, b);
1408 #[stable(feature = "rust1", since = "1.0.0")]
1409 #[rustc_builtin_macro]
1411 #[rustc_diagnostic_item = "assert_macro"]
1412 #[allow_internal_unstable(core_panic, edition_panic)]
1413 macro_rules! assert {
1414 ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
1415 ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};
1418 /// Prints passed tokens into the standard output.
1420 feature = "log_syntax",
1422 reason = "`log_syntax!` is not stable enough for use and is subject to change"
1424 #[rustc_builtin_macro]
1426 macro_rules! log_syntax {
1428 /* compiler built-in */
1432 /// Enables or disables tracing functionality used for debugging other macros.
1434 feature = "trace_macros",
1436 reason = "`trace_macros` is not stable enough for use and is subject to change"
1438 #[rustc_builtin_macro]
1440 macro_rules! trace_macros {
1441 (true) => {{ /* compiler built-in */ }};
1442 (false) => {{ /* compiler built-in */ }};
1445 /// Attribute macro used to apply derive macros.
1447 /// See [the reference] for more info.
1449 /// [the reference]: ../../../reference/attributes/derive.html
1450 #[stable(feature = "rust1", since = "1.0.0")]
1451 #[rustc_builtin_macro]
1452 pub macro derive($item:item) {
1453 /* compiler built-in */
1456 /// Attribute macro used to apply derive macros for implementing traits
1457 /// in a const context.
1459 /// See [the reference] for more info.
1461 /// [the reference]: ../../../reference/attributes/derive.html
1462 #[unstable(feature = "derive_const", issue = "none")]
1463 #[rustc_builtin_macro]
1464 pub macro derive_const($item:item) {
1465 /* compiler built-in */
1468 /// Attribute macro applied to a function to turn it into a unit test.
1470 /// See [the reference] for more info.
1472 /// [the reference]: ../../../reference/attributes/testing.html#the-test-attribute
1473 #[stable(feature = "rust1", since = "1.0.0")]
1474 #[allow_internal_unstable(test, rustc_attrs)]
1475 #[rustc_builtin_macro]
1476 pub macro test($item:item) {
1477 /* compiler built-in */
1480 /// Attribute macro applied to a function to turn it into a benchmark test.
1485 reason = "`bench` is a part of custom test frameworks which are unstable"
1487 #[allow_internal_unstable(test, rustc_attrs)]
1488 #[rustc_builtin_macro]
1489 pub macro bench($item:item) {
1490 /* compiler built-in */
1493 /// An implementation detail of the `#[test]` and `#[bench]` macros.
1495 feature = "custom_test_frameworks",
1497 reason = "custom test frameworks are an unstable feature"
1499 #[allow_internal_unstable(test, rustc_attrs)]
1500 #[rustc_builtin_macro]
1501 pub macro test_case($item:item) {
1502 /* compiler built-in */
1505 /// Attribute macro applied to a static to register it as a global allocator.
1507 /// See also [`std::alloc::GlobalAlloc`](../../../std/alloc/trait.GlobalAlloc.html).
1508 #[stable(feature = "global_allocator", since = "1.28.0")]
1509 #[allow_internal_unstable(rustc_attrs)]
1510 #[rustc_builtin_macro]
1511 pub macro global_allocator($item:item) {
1512 /* compiler built-in */
1515 /// Attribute macro applied to a function to register it as a handler for allocation failure.
1517 /// See also [`std::alloc::handle_alloc_error`](../../../std/alloc/fn.handle_alloc_error.html).
1518 #[unstable(feature = "alloc_error_handler", issue = "51540")]
1519 #[allow_internal_unstable(rustc_attrs)]
1520 #[rustc_builtin_macro]
1521 pub macro alloc_error_handler($item:item) {
1522 /* compiler built-in */
1525 /// Keeps the item it's applied to if the passed path is accessible, and removes it otherwise.
1527 feature = "cfg_accessible",
1529 reason = "`cfg_accessible` is not fully implemented"
1531 #[rustc_builtin_macro]
1532 pub macro cfg_accessible($item:item) {
1533 /* compiler built-in */
1536 /// Expands all `#[cfg]` and `#[cfg_attr]` attributes in the code fragment it's applied to.
1538 feature = "cfg_eval",
1540 reason = "`cfg_eval` is a recently implemented feature"
1542 #[rustc_builtin_macro]
1543 pub macro cfg_eval($($tt:tt)*) {
1544 /* compiler built-in */
1547 /// Unstable placeholder for type ascription.
1548 #[rustc_builtin_macro]
1550 feature = "type_ascription",
1552 reason = "placeholder syntax for type ascription"
1554 pub macro type_ascribe($expr:expr, $ty:ty) {
1555 /* compiler built-in */
1558 /// Unstable implementation detail of the `rustc` compiler, do not use.
1559 #[rustc_builtin_macro]
1560 #[stable(feature = "rust1", since = "1.0.0")]
1561 #[allow_internal_unstable(core_intrinsics, libstd_sys_internals, rt)]
1562 #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
1563 #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
1564 pub macro RustcDecodable($item:item) {
1565 /* compiler built-in */
1568 /// Unstable implementation detail of the `rustc` compiler, do not use.
1569 #[rustc_builtin_macro]
1570 #[stable(feature = "rust1", since = "1.0.0")]
1571 #[allow_internal_unstable(core_intrinsics, rt)]
1572 #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
1573 #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
1574 pub macro RustcEncodable($item:item) {
1575 /* compiler built-in */