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 $crate::assert_matches::assert_matches!($($arg)*);
325 /// Returns whether the given expression matches any of the given patterns.
327 /// Like in a `match` expression, the pattern can be optionally followed by `if`
328 /// and a guard expression that has access to names bound by the pattern.
334 /// assert!(matches!(foo, 'A'..='Z' | 'a'..='z'));
336 /// let bar = Some(4);
337 /// assert!(matches!(bar, Some(x) if x > 2));
340 #[stable(feature = "matches_macro", since = "1.42.0")]
341 macro_rules! matches {
342 ($expression:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
344 $( $pattern )|+ $( if $guard )? => true,
350 /// Unwraps a result or propagates its error.
352 /// The [`?` operator][propagating-errors] was added to replace `try!`
353 /// and should be used instead. Furthermore, `try` is a reserved word
354 /// in Rust 2018, so if you must use it, you will need to use the
355 /// [raw-identifier syntax][ris]: `r#try`.
357 /// [propagating-errors]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
358 /// [ris]: https://doc.rust-lang.org/nightly/rust-by-example/compatibility/raw_identifiers.html
360 /// `try!` matches the given [`Result`]. In case of the `Ok` variant, the
361 /// expression has the value of the wrapped value.
363 /// In case of the `Err` variant, it retrieves the inner error. `try!` then
364 /// performs conversion using `From`. This provides automatic conversion
365 /// between specialized errors and more general ones. The resulting
366 /// error is then immediately returned.
368 /// Because of the early return, `try!` can only be used in functions that
369 /// return [`Result`].
375 /// use std::fs::File;
376 /// use std::io::prelude::*;
382 /// impl From<io::Error> for MyError {
383 /// fn from(e: io::Error) -> MyError {
384 /// MyError::FileWriteError
388 /// // The preferred method of quick returning Errors
389 /// fn write_to_file_question() -> Result<(), MyError> {
390 /// let mut file = File::create("my_best_friends.txt")?;
391 /// file.write_all(b"This is a list of my best friends.")?;
395 /// // The previous method of quick returning Errors
396 /// fn write_to_file_using_try() -> Result<(), MyError> {
397 /// let mut file = r#try!(File::create("my_best_friends.txt"));
398 /// r#try!(file.write_all(b"This is a list of my best friends."));
402 /// // This is equivalent to:
403 /// fn write_to_file_using_match() -> Result<(), MyError> {
404 /// let mut file = r#try!(File::create("my_best_friends.txt"));
405 /// match file.write_all(b"This is a list of my best friends.") {
407 /// Err(e) => return Err(From::from(e)),
413 #[stable(feature = "rust1", since = "1.0.0")]
414 #[deprecated(since = "1.39.0", note = "use the `?` operator instead")]
417 ($expr:expr $(,)?) => {
419 $crate::result::Result::Ok(val) => val,
420 $crate::result::Result::Err(err) => {
421 return $crate::result::Result::Err($crate::convert::From::from(err));
427 /// Writes formatted data into a buffer.
429 /// This macro accepts a 'writer', a format string, and a list of arguments. Arguments will be
430 /// formatted according to the specified format string and the result will be passed to the writer.
431 /// The writer may be any value with a `write_fmt` method; generally this comes from an
432 /// implementation of either the [`fmt::Write`] or the [`io::Write`] trait. The macro
433 /// returns whatever the `write_fmt` method returns; commonly a [`fmt::Result`], or an
436 /// See [`std::fmt`] for more information on the format string syntax.
438 /// [`std::fmt`]: ../std/fmt/index.html
439 /// [`fmt::Write`]: crate::fmt::Write
440 /// [`io::Write`]: ../std/io/trait.Write.html
441 /// [`fmt::Result`]: crate::fmt::Result
442 /// [`io::Result`]: ../std/io/type.Result.html
447 /// use std::io::Write;
449 /// fn main() -> std::io::Result<()> {
450 /// let mut w = Vec::new();
451 /// write!(&mut w, "test")?;
452 /// write!(&mut w, "formatted {}", "arguments")?;
454 /// assert_eq!(w, b"testformatted arguments");
459 /// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
460 /// implementing either, as objects do not typically implement both. However, the module must
461 /// avoid conflict between the trait names, such as by importing them as `_` or otherwise renaming
465 /// use std::fmt::Write as _;
466 /// use std::io::Write as _;
468 /// fn main() -> Result<(), Box<dyn std::error::Error>> {
469 /// let mut s = String::new();
470 /// let mut v = Vec::new();
472 /// write!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
473 /// write!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
474 /// assert_eq!(v, b"s = \"abc 123\"");
479 /// If you also need the trait names themselves, such as to implement one or both on your types,
480 /// import the containing module and then name them with a prefix:
483 /// # #![allow(unused_imports)]
484 /// use std::fmt::{self, Write as _};
485 /// use std::io::{self, Write as _};
489 /// impl fmt::Write for Example {
490 /// fn write_str(&mut self, _s: &str) -> core::fmt::Result {
491 /// unimplemented!();
496 /// Note: This macro can be used in `no_std` setups as well.
497 /// In a `no_std` setup you are responsible for the implementation details of the components.
500 /// # extern crate core;
501 /// use core::fmt::Write;
505 /// impl Write for Example {
506 /// fn write_str(&mut self, _s: &str) -> core::fmt::Result {
507 /// unimplemented!();
511 /// let mut m = Example{};
512 /// write!(&mut m, "Hello World").expect("Not written");
515 #[stable(feature = "rust1", since = "1.0.0")]
516 #[cfg_attr(not(test), rustc_diagnostic_item = "write_macro")]
518 ($dst:expr, $($arg:tt)*) => {
519 $dst.write_fmt($crate::format_args!($($arg)*))
523 /// Write formatted data into a buffer, with a newline appended.
525 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
526 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
528 /// For more information, see [`write!`]. For information on the format string syntax, see
531 /// [`std::fmt`]: ../std/fmt/index.html
536 /// use std::io::{Write, Result};
538 /// fn main() -> Result<()> {
539 /// let mut w = Vec::new();
540 /// writeln!(&mut w)?;
541 /// writeln!(&mut w, "test")?;
542 /// writeln!(&mut w, "formatted {}", "arguments")?;
544 /// assert_eq!(&w[..], "\ntest\nformatted arguments\n".as_bytes());
549 #[stable(feature = "rust1", since = "1.0.0")]
550 #[cfg_attr(not(test), rustc_diagnostic_item = "writeln_macro")]
551 #[allow_internal_unstable(format_args_nl)]
552 macro_rules! writeln {
553 ($dst:expr $(,)?) => {
554 $crate::write!($dst, "\n")
556 ($dst:expr, $($arg:tt)*) => {
557 $dst.write_fmt($crate::format_args_nl!($($arg)*))
561 /// Indicates unreachable code.
563 /// This is useful any time that the compiler can't determine that some code is unreachable. For
566 /// * Match arms with guard conditions.
567 /// * Loops that dynamically terminate.
568 /// * Iterators that dynamically terminate.
570 /// If the determination that the code is unreachable proves incorrect, the
571 /// program immediately terminates with a [`panic!`].
573 /// The unsafe counterpart of this macro is the [`unreachable_unchecked`] function, which
574 /// will cause undefined behavior if the code is reached.
576 /// [`unreachable_unchecked`]: crate::hint::unreachable_unchecked
580 /// This will always [`panic!`] because `unreachable!` is just a shorthand for `panic!` with a
581 /// fixed, specific message.
583 /// Like `panic!`, this macro has a second form for displaying custom values.
590 /// # #[allow(dead_code)]
591 /// fn foo(x: Option<i32>) {
593 /// Some(n) if n >= 0 => println!("Some(Non-negative)"),
594 /// Some(n) if n < 0 => println!("Some(Negative)"),
595 /// Some(_) => unreachable!(), // compile error if commented out
596 /// None => println!("None")
604 /// # #[allow(dead_code)]
605 /// fn divide_by_three(x: u32) -> u32 { // one of the poorest implementations of x/3
607 /// if 3*i < i { panic!("u32 overflow"); }
608 /// if x < 3*i { return i-1; }
610 /// unreachable!("The loop should always return");
614 #[rustc_builtin_macro(unreachable)]
615 #[allow_internal_unstable(edition_panic)]
616 #[stable(feature = "rust1", since = "1.0.0")]
617 #[cfg_attr(not(test), rustc_diagnostic_item = "unreachable_macro")]
618 macro_rules! unreachable {
619 // Expands to either `$crate::panic::unreachable_2015` or `$crate::panic::unreachable_2021`
620 // depending on the edition of the caller.
622 /* compiler built-in */
626 /// Indicates unimplemented code by panicking with a message of "not implemented".
628 /// This allows your code to type-check, which is useful if you are prototyping or
629 /// implementing a trait that requires multiple methods which you don't plan to use all of.
631 /// The difference between `unimplemented!` and [`todo!`] is that while `todo!`
632 /// conveys an intent of implementing the functionality later and the message is "not yet
633 /// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
634 /// Also some IDEs will mark `todo!`s.
638 /// This will always [`panic!`] because `unimplemented!` is just a shorthand for `panic!` with a
639 /// fixed, specific message.
641 /// Like `panic!`, this macro has a second form for displaying custom values.
643 /// [`todo!`]: crate::todo
647 /// Say we have a trait `Foo`:
651 /// fn bar(&self) -> u8;
653 /// fn qux(&self) -> Result<u64, ()>;
657 /// We want to implement `Foo` for 'MyStruct', but for some reason it only makes sense
658 /// to implement the `bar()` function. `baz()` and `qux()` will still need to be defined
659 /// in our implementation of `Foo`, but we can use `unimplemented!` in their definitions
660 /// to allow our code to compile.
662 /// We still want to have our program stop running if the unimplemented methods are
667 /// # fn bar(&self) -> u8;
669 /// # fn qux(&self) -> Result<u64, ()>;
673 /// impl Foo for MyStruct {
674 /// fn bar(&self) -> u8 {
679 /// // It makes no sense to `baz` a `MyStruct`, so we have no logic here
681 /// // This will display "thread 'main' panicked at 'not implemented'".
682 /// unimplemented!();
685 /// fn qux(&self) -> Result<u64, ()> {
686 /// // We have some logic here,
687 /// // We can add a message to unimplemented! to display our omission.
688 /// // This will display:
689 /// // "thread 'main' panicked at 'not implemented: MyStruct isn't quxable'".
690 /// unimplemented!("MyStruct isn't quxable");
695 /// let s = MyStruct;
700 #[stable(feature = "rust1", since = "1.0.0")]
701 #[cfg_attr(not(test), rustc_diagnostic_item = "unimplemented_macro")]
702 #[allow_internal_unstable(core_panic)]
703 macro_rules! unimplemented {
705 $crate::panicking::panic("not implemented")
708 $crate::panic!("not implemented: {}", $crate::format_args!($($arg)+))
712 /// Indicates unfinished code.
714 /// This can be useful if you are prototyping and are just looking to have your
717 /// The difference between [`unimplemented!`] and `todo!` is that while `todo!` conveys
718 /// an intent of implementing the functionality later and the message is "not yet
719 /// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
720 /// Also some IDEs will mark `todo!`s.
724 /// This will always [`panic!`].
728 /// Here's an example of some in-progress code. We have a trait `Foo`:
737 /// We want to implement `Foo` on one of our types, but we also want to work on
738 /// just `bar()` first. In order for our code to compile, we need to implement
739 /// `baz()`, so we can use `todo!`:
748 /// impl Foo for MyStruct {
750 /// // implementation goes here
754 /// // let's not worry about implementing baz() for now
760 /// let s = MyStruct;
763 /// // we aren't even using baz(), so this is fine.
767 #[stable(feature = "todo_macro", since = "1.40.0")]
768 #[cfg_attr(not(test), rustc_diagnostic_item = "todo_macro")]
769 #[allow_internal_unstable(core_panic)]
772 $crate::panicking::panic("not yet implemented")
775 $crate::panic!("not yet implemented: {}", $crate::format_args!($($arg)+))
779 /// Definitions of built-in macros.
781 /// Most of the macro properties (stability, visibility, etc.) are taken from the source code here,
782 /// with exception of expansion functions transforming macro inputs into outputs,
783 /// those functions are provided by the compiler.
784 pub(crate) mod builtin {
786 /// Causes compilation to fail with the given error message when encountered.
788 /// This macro should be used when a crate uses a conditional compilation strategy to provide
789 /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
790 /// but emits an error during *compilation* rather than at *runtime*.
794 /// Two such examples are macros and `#[cfg]` environments.
796 /// Emit a better compiler error if a macro is passed invalid values. Without the final branch,
797 /// the compiler would still emit an error, but the error's message would not mention the two
801 /// macro_rules! give_me_foo_or_bar {
805 /// compile_error!("This macro only accepts `foo` or `bar`");
809 /// give_me_foo_or_bar!(neither);
810 /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
813 /// Emit a compiler error if one of a number of features isn't available.
816 /// #[cfg(not(any(feature = "foo", feature = "bar")))]
817 /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.");
819 #[stable(feature = "compile_error_macro", since = "1.20.0")]
820 #[rustc_builtin_macro]
822 macro_rules! compile_error {
823 ($msg:expr $(,)?) => {{ /* compiler built-in */ }};
826 /// Constructs parameters for the other string-formatting macros.
828 /// This macro functions by taking a formatting string literal containing
829 /// `{}` for each additional argument passed. `format_args!` prepares the
830 /// additional parameters to ensure the output can be interpreted as a string
831 /// and canonicalizes the arguments into a single type. Any value that implements
832 /// the [`Display`] trait can be passed to `format_args!`, as can any
833 /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
835 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
836 /// passed to the macros within [`std::fmt`] for performing useful redirection.
837 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
838 /// proxied through this one. `format_args!`, unlike its derived macros, avoids
839 /// heap allocations.
841 /// You can use the [`fmt::Arguments`] value that `format_args!` returns
842 /// in `Debug` and `Display` contexts as seen below. The example also shows
843 /// that `Debug` and `Display` format to the same thing: the interpolated
844 /// format string in `format_args!`.
847 /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
848 /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
849 /// assert_eq!("1 foo 2", display);
850 /// assert_eq!(display, debug);
853 /// For more information, see the documentation in [`std::fmt`].
855 /// [`Display`]: crate::fmt::Display
856 /// [`Debug`]: crate::fmt::Debug
857 /// [`fmt::Arguments`]: crate::fmt::Arguments
858 /// [`std::fmt`]: ../std/fmt/index.html
859 /// [`format!`]: ../std/macro.format.html
860 /// [`println!`]: ../std/macro.println.html
867 /// let s = fmt::format(format_args!("hello {}", "world"));
868 /// assert_eq!(s, format!("hello {}", "world"));
870 #[stable(feature = "rust1", since = "1.0.0")]
871 #[cfg_attr(not(test), rustc_diagnostic_item = "format_args_macro")]
872 #[allow_internal_unsafe]
873 #[allow_internal_unstable(fmt_internals)]
874 #[rustc_builtin_macro]
876 macro_rules! format_args {
877 ($fmt:expr) => {{ /* compiler built-in */ }};
878 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
881 /// Same as [`format_args`], but can be used in some const contexts.
883 /// This macro is used by the panic macros for the `const_panic` feature.
885 /// This macro will be removed once `format_args` is allowed in const contexts.
886 #[unstable(feature = "const_format_args", issue = "none")]
887 #[allow_internal_unstable(fmt_internals, const_fmt_arguments_new)]
888 #[rustc_builtin_macro]
890 macro_rules! const_format_args {
891 ($fmt:expr) => {{ /* compiler built-in */ }};
892 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
895 /// Same as [`format_args`], but adds a newline in the end.
897 feature = "format_args_nl",
899 reason = "`format_args_nl` is only for internal \
900 language use and is subject to change"
902 #[allow_internal_unstable(fmt_internals)]
903 #[rustc_builtin_macro]
905 macro_rules! format_args_nl {
906 ($fmt:expr) => {{ /* compiler built-in */ }};
907 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
910 /// Inspects an environment variable at compile time.
912 /// This macro will expand to the value of the named environment variable at
913 /// compile time, yielding an expression of type `&'static str`. Use
914 /// [`std::env::var`] instead if you want to read the value at runtime.
916 /// [`std::env::var`]: ../std/env/fn.var.html
918 /// If the environment variable is not defined, then a compilation error
919 /// will be emitted. To not emit a compile error, use the [`option_env!`]
925 /// let path: &'static str = env!("PATH");
926 /// println!("the $PATH variable at the time of compiling was: {path}");
929 /// You can customize the error message by passing a string as the second
933 /// let doc: &'static str = env!("documentation", "what's that?!");
936 /// If the `documentation` environment variable is not defined, you'll get
937 /// the following error:
940 /// error: what's that?!
942 #[stable(feature = "rust1", since = "1.0.0")]
943 #[rustc_builtin_macro]
946 ($name:expr $(,)?) => {{ /* compiler built-in */ }};
947 ($name:expr, $error_msg:expr $(,)?) => {{ /* compiler built-in */ }};
950 /// Optionally inspects an environment variable at compile time.
952 /// If the named environment variable is present at compile time, this will
953 /// expand into an expression of type `Option<&'static str>` whose value is
954 /// `Some` of the value of the environment variable. If the environment
955 /// variable is not present, then this will expand to `None`. See
956 /// [`Option<T>`][Option] for more information on this type. Use
957 /// [`std::env::var`] instead if you want to read the value at runtime.
959 /// [`std::env::var`]: ../std/env/fn.var.html
961 /// A compile time error is never emitted when using this macro regardless
962 /// of whether the environment variable is present or not.
967 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
968 /// println!("the secret key might be: {key:?}");
970 #[stable(feature = "rust1", since = "1.0.0")]
971 #[rustc_builtin_macro]
973 macro_rules! option_env {
974 ($name:expr $(,)?) => {{ /* compiler built-in */ }};
977 /// Concatenates identifiers into one identifier.
979 /// This macro takes any number of comma-separated identifiers, and
980 /// concatenates them all into one, yielding an expression which is a new
981 /// identifier. Note that hygiene makes it such that this macro cannot
982 /// capture local variables. Also, as a general rule, macros are only
983 /// allowed in item, statement or expression position. That means while
984 /// you may use this macro for referring to existing variables, functions or
985 /// modules etc, you cannot define a new one with it.
990 /// #![feature(concat_idents)]
993 /// fn foobar() -> u32 { 23 }
995 /// let f = concat_idents!(foo, bar);
996 /// println!("{}", f());
998 /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
1002 feature = "concat_idents",
1004 reason = "`concat_idents` is not stable enough for use and is subject to change"
1006 #[rustc_builtin_macro]
1008 macro_rules! concat_idents {
1009 ($($e:ident),+ $(,)?) => {{ /* compiler built-in */ }};
1012 /// Concatenates literals into a byte slice.
1014 /// This macro takes any number of comma-separated literals, and concatenates them all into
1015 /// one, yielding an expression of type `&[u8; _]`, which represents all of the literals
1016 /// concatenated left-to-right. The literals passed can be any combination of:
1018 /// - byte literals (`b'r'`)
1019 /// - byte strings (`b"Rust"`)
1020 /// - arrays of bytes/numbers (`[b'A', 66, b'C']`)
1025 /// #![feature(concat_bytes)]
1028 /// let s: &[u8; 6] = concat_bytes!(b'A', b"BC", [68, b'E', 70]);
1029 /// assert_eq!(s, b"ABCDEF");
1032 #[unstable(feature = "concat_bytes", issue = "87555")]
1033 #[rustc_builtin_macro]
1035 macro_rules! concat_bytes {
1036 ($($e:literal),+ $(,)?) => {{ /* compiler built-in */ }};
1039 /// Concatenates literals into a static string slice.
1041 /// This macro takes any number of comma-separated literals, yielding an
1042 /// expression of type `&'static str` which represents all of the literals
1043 /// concatenated left-to-right.
1045 /// Integer and floating point literals are stringified in order to be
1051 /// let s = concat!("test", 10, 'b', true);
1052 /// assert_eq!(s, "test10btrue");
1054 #[stable(feature = "rust1", since = "1.0.0")]
1055 #[rustc_builtin_macro]
1057 macro_rules! concat {
1058 ($($e:expr),* $(,)?) => {{ /* compiler built-in */ }};
1061 /// Expands to the line number on which it was invoked.
1063 /// With [`column!`] and [`file!`], these macros provide debugging information for
1064 /// developers about the location within the source.
1066 /// The expanded expression has type `u32` and is 1-based, so the first line
1067 /// in each file evaluates to 1, the second to 2, etc. This is consistent
1068 /// with error messages by common compilers or popular editors.
1069 /// The returned line is *not necessarily* the line of the `line!` invocation itself,
1070 /// but rather the first macro invocation leading up to the invocation
1071 /// of the `line!` macro.
1076 /// let current_line = line!();
1077 /// println!("defined on line: {current_line}");
1079 #[stable(feature = "rust1", since = "1.0.0")]
1080 #[rustc_builtin_macro]
1084 /* compiler built-in */
1088 /// Expands to the column number at which it was invoked.
1090 /// With [`line!`] and [`file!`], these macros provide debugging information for
1091 /// developers about the location within the source.
1093 /// The expanded expression has type `u32` and is 1-based, so the first column
1094 /// in each line evaluates to 1, the second to 2, etc. This is consistent
1095 /// with error messages by common compilers or popular editors.
1096 /// The returned column is *not necessarily* the line of the `column!` invocation itself,
1097 /// but rather the first macro invocation leading up to the invocation
1098 /// of the `column!` macro.
1103 /// let current_col = column!();
1104 /// println!("defined on column: {current_col}");
1107 /// `column!` counts Unicode code points, not bytes or graphemes. As a result, the first two
1108 /// invocations return the same value, but the third does not.
1111 /// let a = ("foobar", column!()).1;
1112 /// let b = ("人之初性本善", column!()).1;
1113 /// let c = ("f̅o̅o̅b̅a̅r̅", column!()).1; // Uses combining overline (U+0305)
1115 /// assert_eq!(a, b);
1116 /// assert_ne!(b, c);
1118 #[stable(feature = "rust1", since = "1.0.0")]
1119 #[rustc_builtin_macro]
1121 macro_rules! column {
1123 /* compiler built-in */
1127 /// Expands to the file name in which it was invoked.
1129 /// With [`line!`] and [`column!`], these macros provide debugging information for
1130 /// developers about the location within the source.
1132 /// The expanded expression has type `&'static str`, and the returned file
1133 /// is not the invocation of the `file!` macro itself, but rather the
1134 /// first macro invocation leading up to the invocation of the `file!`
1140 /// let this_file = file!();
1141 /// println!("defined in file: {this_file}");
1143 #[stable(feature = "rust1", since = "1.0.0")]
1144 #[rustc_builtin_macro]
1148 /* compiler built-in */
1152 /// Stringifies its arguments.
1154 /// This macro will yield an expression of type `&'static str` which is the
1155 /// stringification of all the tokens passed to the macro. No restrictions
1156 /// are placed on the syntax of the macro invocation itself.
1158 /// Note that the expanded results of the input tokens may change in the
1159 /// future. You should be careful if you rely on the output.
1164 /// let one_plus_one = stringify!(1 + 1);
1165 /// assert_eq!(one_plus_one, "1 + 1");
1167 #[stable(feature = "rust1", since = "1.0.0")]
1168 #[rustc_builtin_macro]
1170 macro_rules! stringify {
1172 /* compiler built-in */
1176 /// Includes a UTF-8 encoded file as a string.
1178 /// The file is located relative to the current file (similarly to how
1179 /// modules are found). The provided path is interpreted in a platform-specific
1180 /// way at compile time. So, for instance, an invocation with a Windows path
1181 /// containing backslashes `\` would not compile correctly on Unix.
1183 /// This macro will yield an expression of type `&'static str` which is the
1184 /// contents of the file.
1188 /// Assume there are two files in the same directory with the following
1191 /// File 'spanish.in':
1199 /// ```ignore (cannot-doctest-external-file-dependency)
1201 /// let my_str = include_str!("spanish.in");
1202 /// assert_eq!(my_str, "adiós\n");
1203 /// print!("{my_str}");
1207 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
1208 #[stable(feature = "rust1", since = "1.0.0")]
1209 #[rustc_builtin_macro]
1211 #[cfg_attr(not(test), rustc_diagnostic_item = "include_str_macro")]
1212 macro_rules! include_str {
1213 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1216 /// Includes a file as a reference to a byte array.
1218 /// The file is located relative to the current file (similarly to how
1219 /// modules are found). The provided path is interpreted in a platform-specific
1220 /// way at compile time. So, for instance, an invocation with a Windows path
1221 /// containing backslashes `\` would not compile correctly on Unix.
1223 /// This macro will yield an expression of type `&'static [u8; N]` which is
1224 /// the contents of the file.
1228 /// Assume there are two files in the same directory with the following
1231 /// File 'spanish.in':
1239 /// ```ignore (cannot-doctest-external-file-dependency)
1241 /// let bytes = include_bytes!("spanish.in");
1242 /// assert_eq!(bytes, b"adi\xc3\xb3s\n");
1243 /// print!("{}", String::from_utf8_lossy(bytes));
1247 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
1248 #[stable(feature = "rust1", since = "1.0.0")]
1249 #[rustc_builtin_macro]
1251 #[cfg_attr(not(test), rustc_diagnostic_item = "include_bytes_macro")]
1252 macro_rules! include_bytes {
1253 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1256 /// Expands to a string that represents the current module path.
1258 /// The current module path can be thought of as the hierarchy of modules
1259 /// leading back up to the crate root. The first component of the path
1260 /// returned is the name of the crate currently being compiled.
1267 /// assert!(module_path!().ends_with("test"));
1273 #[stable(feature = "rust1", since = "1.0.0")]
1274 #[rustc_builtin_macro]
1276 macro_rules! module_path {
1278 /* compiler built-in */
1282 /// Evaluates boolean combinations of configuration flags at compile-time.
1284 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
1285 /// boolean expression evaluation of configuration flags. This frequently
1286 /// leads to less duplicated code.
1288 /// The syntax given to this macro is the same syntax as the [`cfg`]
1291 /// `cfg!`, unlike `#[cfg]`, does not remove any code and only evaluates to true or false. For
1292 /// example, all blocks in an if/else expression need to be valid when `cfg!` is used for
1293 /// the condition, regardless of what `cfg!` is evaluating.
1295 /// [`cfg`]: ../reference/conditional-compilation.html#the-cfg-attribute
1300 /// let my_directory = if cfg!(windows) {
1301 /// "windows-specific-directory"
1303 /// "unix-directory"
1306 #[stable(feature = "rust1", since = "1.0.0")]
1307 #[rustc_builtin_macro]
1311 /* compiler built-in */
1315 /// Parses a file as an expression or an item according to the context.
1317 /// The file is located relative to the current file (similarly to how
1318 /// modules are found). The provided path is interpreted in a platform-specific
1319 /// way at compile time. So, for instance, an invocation with a Windows path
1320 /// containing backslashes `\` would not compile correctly on Unix.
1322 /// Using this macro is often a bad idea, because if the file is
1323 /// parsed as an expression, it is going to be placed in the
1324 /// surrounding code unhygienically. This could result in variables
1325 /// or functions being different from what the file expected if
1326 /// there are variables or functions that have the same name in
1327 /// the current file.
1331 /// Assume there are two files in the same directory with the following
1334 /// File 'monkeys.in':
1336 /// ```ignore (only-for-syntax-highlight)
1341 /// .collect::<String>()
1346 /// ```ignore (cannot-doctest-external-file-dependency)
1348 /// let my_string = include!("monkeys.in");
1349 /// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
1350 /// println!("{my_string}");
1354 /// Compiling 'main.rs' and running the resulting binary will print
1356 #[stable(feature = "rust1", since = "1.0.0")]
1357 #[rustc_builtin_macro]
1359 macro_rules! include {
1360 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1363 /// Asserts that a boolean expression is `true` at runtime.
1365 /// This will invoke the [`panic!`] macro if the provided expression cannot be
1366 /// evaluated to `true` at runtime.
1370 /// Assertions are always checked in both debug and release builds, and cannot
1371 /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
1372 /// release builds by default.
1374 /// Unsafe code may rely on `assert!` to enforce run-time invariants that, if
1375 /// violated could lead to unsafety.
1377 /// Other use-cases of `assert!` include testing and enforcing run-time
1378 /// invariants in safe code (whose violation cannot result in unsafety).
1380 /// # Custom Messages
1382 /// This macro has a second form, where a custom panic message can
1383 /// be provided with or without arguments for formatting. See [`std::fmt`]
1384 /// for syntax for this form. Expressions used as format arguments will only
1385 /// be evaluated if the assertion fails.
1387 /// [`std::fmt`]: ../std/fmt/index.html
1392 /// // the panic message for these assertions is the stringified value of the
1393 /// // expression given.
1396 /// fn some_computation() -> bool { true } // a very simple function
1398 /// assert!(some_computation());
1400 /// // assert with a custom message
1402 /// assert!(x, "x wasn't true!");
1404 /// let a = 3; let b = 27;
1405 /// assert!(a + b == 30, "a = {}, b = {}", a, b);
1407 #[stable(feature = "rust1", since = "1.0.0")]
1408 #[rustc_builtin_macro]
1410 #[rustc_diagnostic_item = "assert_macro"]
1411 #[allow_internal_unstable(core_panic, edition_panic)]
1412 macro_rules! assert {
1413 ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
1414 ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};
1417 /// Prints passed tokens into the standard output.
1419 feature = "log_syntax",
1421 reason = "`log_syntax!` is not stable enough for use and is subject to change"
1423 #[rustc_builtin_macro]
1425 macro_rules! log_syntax {
1427 /* compiler built-in */
1431 /// Enables or disables tracing functionality used for debugging other macros.
1433 feature = "trace_macros",
1435 reason = "`trace_macros` is not stable enough for use and is subject to change"
1437 #[rustc_builtin_macro]
1439 macro_rules! trace_macros {
1440 (true) => {{ /* compiler built-in */ }};
1441 (false) => {{ /* compiler built-in */ }};
1444 /// Attribute macro used to apply derive macros.
1446 /// See [the reference] for more info.
1448 /// [the reference]: ../../../reference/attributes/derive.html
1449 #[stable(feature = "rust1", since = "1.0.0")]
1450 #[rustc_builtin_macro]
1451 pub macro derive($item:item) {
1452 /* compiler built-in */
1455 /// Attribute macro used to apply derive macros for implementing traits
1456 /// in a const context.
1458 /// See [the reference] for more info.
1460 /// [the reference]: ../../../reference/attributes/derive.html
1461 #[unstable(feature = "derive_const", issue = "none")]
1462 #[rustc_builtin_macro]
1463 #[cfg(not(bootstrap))]
1464 pub macro derive_const($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 #[cfg(not(bootstrap))]
1519 #[unstable(feature = "alloc_error_handler", issue = "51540")]
1520 #[allow_internal_unstable(rustc_attrs)]
1521 #[rustc_builtin_macro]
1522 pub macro alloc_error_handler($item:item) {
1523 /* compiler built-in */
1526 /// Keeps the item it's applied to if the passed path is accessible, and removes it otherwise.
1528 feature = "cfg_accessible",
1530 reason = "`cfg_accessible` is not fully implemented"
1532 #[rustc_builtin_macro]
1533 pub macro cfg_accessible($item:item) {
1534 /* compiler built-in */
1537 /// Expands all `#[cfg]` and `#[cfg_attr]` attributes in the code fragment it's applied to.
1539 feature = "cfg_eval",
1541 reason = "`cfg_eval` is a recently implemented feature"
1543 #[rustc_builtin_macro]
1544 pub macro cfg_eval($($tt:tt)*) {
1545 /* compiler built-in */
1548 /// Unstable implementation detail of the `rustc` compiler, do not use.
1549 #[rustc_builtin_macro]
1550 #[stable(feature = "rust1", since = "1.0.0")]
1551 #[allow_internal_unstable(core_intrinsics, libstd_sys_internals, rt)]
1552 #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
1553 #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
1554 pub macro RustcDecodable($item:item) {
1555 /* compiler built-in */
1558 /// Unstable implementation detail of the `rustc` compiler, do not use.
1559 #[rustc_builtin_macro]
1560 #[stable(feature = "rust1", since = "1.0.0")]
1561 #[allow_internal_unstable(core_intrinsics, rt)]
1562 #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
1563 #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
1564 pub macro RustcEncodable($item:item) {
1565 /* compiler built-in */