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 was added to replace `try!` and should be used instead.
354 /// Furthermore, `try` is a reserved word in Rust 2018, so if you must use
355 /// it, you will need to use the [raw-identifier syntax][ris]: `r#try`.
357 /// [ris]: https://doc.rust-lang.org/nightly/rust-by-example/compatibility/raw_identifiers.html
359 /// `try!` matches the given [`Result`]. In case of the `Ok` variant, the
360 /// expression has the value of the wrapped value.
362 /// In case of the `Err` variant, it retrieves the inner error. `try!` then
363 /// performs conversion using `From`. This provides automatic conversion
364 /// between specialized errors and more general ones. The resulting
365 /// error is then immediately returned.
367 /// Because of the early return, `try!` can only be used in functions that
368 /// return [`Result`].
374 /// use std::fs::File;
375 /// use std::io::prelude::*;
381 /// impl From<io::Error> for MyError {
382 /// fn from(e: io::Error) -> MyError {
383 /// MyError::FileWriteError
387 /// // The preferred method of quick returning Errors
388 /// fn write_to_file_question() -> Result<(), MyError> {
389 /// let mut file = File::create("my_best_friends.txt")?;
390 /// file.write_all(b"This is a list of my best friends.")?;
394 /// // The previous method of quick returning Errors
395 /// fn write_to_file_using_try() -> Result<(), MyError> {
396 /// let mut file = r#try!(File::create("my_best_friends.txt"));
397 /// r#try!(file.write_all(b"This is a list of my best friends."));
401 /// // This is equivalent to:
402 /// fn write_to_file_using_match() -> Result<(), MyError> {
403 /// let mut file = r#try!(File::create("my_best_friends.txt"));
404 /// match file.write_all(b"This is a list of my best friends.") {
406 /// Err(e) => return Err(From::from(e)),
412 #[stable(feature = "rust1", since = "1.0.0")]
413 #[deprecated(since = "1.39.0", note = "use the `?` operator instead")]
416 ($expr:expr $(,)?) => {
418 $crate::result::Result::Ok(val) => val,
419 $crate::result::Result::Err(err) => {
420 return $crate::result::Result::Err($crate::convert::From::from(err));
426 /// Writes formatted data into a buffer.
428 /// This macro accepts a 'writer', a format string, and a list of arguments. Arguments will be
429 /// formatted according to the specified format string and the result will be passed to the writer.
430 /// The writer may be any value with a `write_fmt` method; generally this comes from an
431 /// implementation of either the [`fmt::Write`] or the [`io::Write`] trait. The macro
432 /// returns whatever the `write_fmt` method returns; commonly a [`fmt::Result`], or an
435 /// See [`std::fmt`] for more information on the format string syntax.
437 /// [`std::fmt`]: ../std/fmt/index.html
438 /// [`fmt::Write`]: crate::fmt::Write
439 /// [`io::Write`]: ../std/io/trait.Write.html
440 /// [`fmt::Result`]: crate::fmt::Result
441 /// [`io::Result`]: ../std/io/type.Result.html
446 /// use std::io::Write;
448 /// fn main() -> std::io::Result<()> {
449 /// let mut w = Vec::new();
450 /// write!(&mut w, "test")?;
451 /// write!(&mut w, "formatted {}", "arguments")?;
453 /// assert_eq!(w, b"testformatted arguments");
458 /// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
459 /// implementing either, as objects do not typically implement both. However, the module must
460 /// import the traits qualified so their names do not conflict:
463 /// use std::fmt::Write as FmtWrite;
464 /// use std::io::Write as IoWrite;
466 /// fn main() -> Result<(), Box<dyn std::error::Error>> {
467 /// let mut s = String::new();
468 /// let mut v = Vec::new();
470 /// write!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
471 /// write!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
472 /// assert_eq!(v, b"s = \"abc 123\"");
477 /// Note: This macro can be used in `no_std` setups as well.
478 /// In a `no_std` setup you are responsible for the implementation details of the components.
481 /// # extern crate core;
482 /// use core::fmt::Write;
486 /// impl Write for Example {
487 /// fn write_str(&mut self, _s: &str) -> core::fmt::Result {
488 /// unimplemented!();
492 /// let mut m = Example{};
493 /// write!(&mut m, "Hello World").expect("Not written");
496 #[stable(feature = "rust1", since = "1.0.0")]
497 #[cfg_attr(not(test), rustc_diagnostic_item = "write_macro")]
499 ($dst:expr, $($arg:tt)*) => {{
500 let result = $dst.write_fmt($crate::format_args!($($arg)*));
505 /// Write formatted data into a buffer, with a newline appended.
507 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
508 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
510 /// For more information, see [`write!`]. For information on the format string syntax, see
513 /// [`std::fmt`]: ../std/fmt/index.html
518 /// use std::io::{Write, Result};
520 /// fn main() -> Result<()> {
521 /// let mut w = Vec::new();
522 /// writeln!(&mut w)?;
523 /// writeln!(&mut w, "test")?;
524 /// writeln!(&mut w, "formatted {}", "arguments")?;
526 /// assert_eq!(&w[..], "\ntest\nformatted arguments\n".as_bytes());
531 /// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
532 /// implementing either, as objects do not typically implement both. However, the module must
533 /// import the traits qualified so their names do not conflict:
536 /// use std::fmt::Write as FmtWrite;
537 /// use std::io::Write as IoWrite;
539 /// fn main() -> Result<(), Box<dyn std::error::Error>> {
540 /// let mut s = String::new();
541 /// let mut v = Vec::new();
543 /// writeln!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
544 /// writeln!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
545 /// assert_eq!(v, b"s = \"abc 123\\n\"\n");
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 let result = $dst.write_fmt($crate::format_args_nl!($($arg)*));
563 /// Indicates unreachable code.
565 /// This is useful any time that the compiler can't determine that some code is unreachable. For
568 /// * Match arms with guard conditions.
569 /// * Loops that dynamically terminate.
570 /// * Iterators that dynamically terminate.
572 /// If the determination that the code is unreachable proves incorrect, the
573 /// program immediately terminates with a [`panic!`].
575 /// The unsafe counterpart of this macro is the [`unreachable_unchecked`] function, which
576 /// will cause undefined behavior if the code is reached.
578 /// [`unreachable_unchecked`]: crate::hint::unreachable_unchecked
582 /// This will always [`panic!`] because `unreachable!` is just a shorthand for `panic!` with a
583 /// fixed, specific message.
585 /// Like `panic!`, this macro has a second form for displaying custom values.
592 /// # #[allow(dead_code)]
593 /// fn foo(x: Option<i32>) {
595 /// Some(n) if n >= 0 => println!("Some(Non-negative)"),
596 /// Some(n) if n < 0 => println!("Some(Negative)"),
597 /// Some(_) => unreachable!(), // compile error if commented out
598 /// None => println!("None")
606 /// # #[allow(dead_code)]
607 /// fn divide_by_three(x: u32) -> u32 { // one of the poorest implementations of x/3
609 /// if 3*i < i { panic!("u32 overflow"); }
610 /// if x < 3*i { return i-1; }
612 /// unreachable!("The loop should always return");
616 #[rustc_builtin_macro(unreachable)]
617 #[allow_internal_unstable(edition_panic)]
618 #[stable(feature = "rust1", since = "1.0.0")]
619 #[cfg_attr(not(test), rustc_diagnostic_item = "unreachable_macro")]
620 macro_rules! unreachable {
621 // Expands to either `$crate::panic::unreachable_2015` or `$crate::panic::unreachable_2021`
622 // depending on the edition of the caller.
624 /* compiler built-in */
628 /// Indicates unimplemented code by panicking with a message of "not implemented".
630 /// This allows your code to type-check, which is useful if you are prototyping or
631 /// implementing a trait that requires multiple methods which you don't plan to use all of.
633 /// The difference between `unimplemented!` and [`todo!`] is that while `todo!`
634 /// conveys an intent of implementing the functionality later and the message is "not yet
635 /// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
636 /// Also some IDEs will mark `todo!`s.
640 /// This will always [`panic!`] because `unimplemented!` is just a shorthand for `panic!` with a
641 /// fixed, specific message.
643 /// Like `panic!`, this macro has a second form for displaying custom values.
645 /// [`todo!`]: crate::todo
649 /// Say we have a trait `Foo`:
653 /// fn bar(&self) -> u8;
655 /// fn qux(&self) -> Result<u64, ()>;
659 /// We want to implement `Foo` for 'MyStruct', but for some reason it only makes sense
660 /// to implement the `bar()` function. `baz()` and `qux()` will still need to be defined
661 /// in our implementation of `Foo`, but we can use `unimplemented!` in their definitions
662 /// to allow our code to compile.
664 /// We still want to have our program stop running if the unimplemented methods are
669 /// # fn bar(&self) -> u8;
671 /// # fn qux(&self) -> Result<u64, ()>;
675 /// impl Foo for MyStruct {
676 /// fn bar(&self) -> u8 {
681 /// // It makes no sense to `baz` a `MyStruct`, so we have no logic here
683 /// // This will display "thread 'main' panicked at 'not implemented'".
684 /// unimplemented!();
687 /// fn qux(&self) -> Result<u64, ()> {
688 /// // We have some logic here,
689 /// // We can add a message to unimplemented! to display our omission.
690 /// // This will display:
691 /// // "thread 'main' panicked at 'not implemented: MyStruct isn't quxable'".
692 /// unimplemented!("MyStruct isn't quxable");
697 /// let s = MyStruct;
702 #[stable(feature = "rust1", since = "1.0.0")]
703 #[cfg_attr(not(test), rustc_diagnostic_item = "unimplemented_macro")]
704 #[allow_internal_unstable(core_panic)]
705 macro_rules! unimplemented {
707 $crate::panicking::panic("not implemented")
710 $crate::panic!("not implemented: {}", $crate::format_args!($($arg)+))
714 /// Indicates unfinished code.
716 /// This can be useful if you are prototyping and are just looking to have your
719 /// The difference between [`unimplemented!`] and `todo!` is that while `todo!` conveys
720 /// an intent of implementing the functionality later and the message is "not yet
721 /// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
722 /// Also some IDEs will mark `todo!`s.
726 /// This will always [`panic!`].
730 /// Here's an example of some in-progress code. We have a trait `Foo`:
739 /// We want to implement `Foo` on one of our types, but we also want to work on
740 /// just `bar()` first. In order for our code to compile, we need to implement
741 /// `baz()`, so we can use `todo!`:
750 /// impl Foo for MyStruct {
752 /// // implementation goes here
756 /// // let's not worry about implementing baz() for now
762 /// let s = MyStruct;
765 /// // we aren't even using baz(), so this is fine.
769 #[stable(feature = "todo_macro", since = "1.40.0")]
770 #[cfg_attr(not(test), rustc_diagnostic_item = "todo_macro")]
771 #[allow_internal_unstable(core_panic)]
774 $crate::panicking::panic("not yet implemented")
777 $crate::panic!("not yet implemented: {}", $crate::format_args!($($arg)+))
781 /// Definitions of built-in macros.
783 /// Most of the macro properties (stability, visibility, etc.) are taken from the source code here,
784 /// with exception of expansion functions transforming macro inputs into outputs,
785 /// those functions are provided by the compiler.
786 pub(crate) mod builtin {
788 /// Causes compilation to fail with the given error message when encountered.
790 /// This macro should be used when a crate uses a conditional compilation strategy to provide
791 /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
792 /// but emits an error during *compilation* rather than at *runtime*.
796 /// Two such examples are macros and `#[cfg]` environments.
798 /// Emit a better compiler error if a macro is passed invalid values. Without the final branch,
799 /// the compiler would still emit an error, but the error's message would not mention the two
803 /// macro_rules! give_me_foo_or_bar {
807 /// compile_error!("This macro only accepts `foo` or `bar`");
811 /// give_me_foo_or_bar!(neither);
812 /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
815 /// Emit a compiler error if one of a number of features isn't available.
818 /// #[cfg(not(any(feature = "foo", feature = "bar")))]
819 /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.");
821 #[stable(feature = "compile_error_macro", since = "1.20.0")]
822 #[rustc_builtin_macro]
824 #[cfg_attr(not(test), rustc_diagnostic_item = "compile_error_macro")]
825 macro_rules! compile_error {
826 ($msg:expr $(,)?) => {{ /* compiler built-in */ }};
829 /// Constructs parameters for the other string-formatting macros.
831 /// This macro functions by taking a formatting string literal containing
832 /// `{}` for each additional argument passed. `format_args!` prepares the
833 /// additional parameters to ensure the output can be interpreted as a string
834 /// and canonicalizes the arguments into a single type. Any value that implements
835 /// the [`Display`] trait can be passed to `format_args!`, as can any
836 /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
838 /// This macro produces a value of type [`fmt::Arguments`]. This value can be
839 /// passed to the macros within [`std::fmt`] for performing useful redirection.
840 /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
841 /// proxied through this one. `format_args!`, unlike its derived macros, avoids
842 /// heap allocations.
844 /// You can use the [`fmt::Arguments`] value that `format_args!` returns
845 /// in `Debug` and `Display` contexts as seen below. The example also shows
846 /// that `Debug` and `Display` format to the same thing: the interpolated
847 /// format string in `format_args!`.
850 /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
851 /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
852 /// assert_eq!("1 foo 2", display);
853 /// assert_eq!(display, debug);
856 /// For more information, see the documentation in [`std::fmt`].
858 /// [`Display`]: crate::fmt::Display
859 /// [`Debug`]: crate::fmt::Debug
860 /// [`fmt::Arguments`]: crate::fmt::Arguments
861 /// [`std::fmt`]: ../std/fmt/index.html
862 /// [`format!`]: ../std/macro.format.html
863 /// [`println!`]: ../std/macro.println.html
870 /// let s = fmt::format(format_args!("hello {}", "world"));
871 /// assert_eq!(s, format!("hello {}", "world"));
873 #[stable(feature = "rust1", since = "1.0.0")]
874 #[cfg_attr(not(test), rustc_diagnostic_item = "format_args_macro")]
875 #[allow_internal_unsafe]
876 #[allow_internal_unstable(fmt_internals)]
877 #[rustc_builtin_macro]
879 macro_rules! format_args {
880 ($fmt:expr) => {{ /* compiler built-in */ }};
881 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
884 /// Same as [`format_args`], but can be used in some const contexts.
886 /// This macro is used by the panic macros for the `const_panic` feature.
888 /// This macro will be removed once `format_args` is allowed in const contexts.
889 #[unstable(feature = "const_format_args", issue = "none")]
890 #[allow_internal_unstable(fmt_internals, const_fmt_arguments_new)]
891 #[rustc_builtin_macro]
893 macro_rules! const_format_args {
894 ($fmt:expr) => {{ /* compiler built-in */ }};
895 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
898 /// Same as [`format_args`], but adds a newline in the end.
900 feature = "format_args_nl",
902 reason = "`format_args_nl` is only for internal \
903 language use and is subject to change"
905 #[allow_internal_unstable(fmt_internals)]
906 #[rustc_builtin_macro]
908 macro_rules! format_args_nl {
909 ($fmt:expr) => {{ /* compiler built-in */ }};
910 ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
913 /// Inspects an environment variable at compile time.
915 /// This macro will expand to the value of the named environment variable at
916 /// compile time, yielding an expression of type `&'static str`. Use
917 /// [`std::env::var`] instead if you want to read the value at runtime.
919 /// [`std::env::var`]: ../std/env/fn.var.html
921 /// If the environment variable is not defined, then a compilation error
922 /// will be emitted. To not emit a compile error, use the [`option_env!`]
928 /// let path: &'static str = env!("PATH");
929 /// println!("the $PATH variable at the time of compiling was: {path}");
932 /// You can customize the error message by passing a string as the second
936 /// let doc: &'static str = env!("documentation", "what's that?!");
939 /// If the `documentation` environment variable is not defined, you'll get
940 /// the following error:
943 /// error: what's that?!
945 #[stable(feature = "rust1", since = "1.0.0")]
946 #[rustc_builtin_macro]
948 #[cfg_attr(not(test), rustc_diagnostic_item = "env_macro")]
950 ($name:expr $(,)?) => {{ /* compiler built-in */ }};
951 ($name:expr, $error_msg:expr $(,)?) => {{ /* compiler built-in */ }};
954 /// Optionally inspects an environment variable at compile time.
956 /// If the named environment variable is present at compile time, this will
957 /// expand into an expression of type `Option<&'static str>` whose value is
958 /// `Some` of the value of the environment variable. If the environment
959 /// variable is not present, then this will expand to `None`. See
960 /// [`Option<T>`][Option] for more information on this type. Use
961 /// [`std::env::var`] instead if you want to read the value at runtime.
963 /// [`std::env::var`]: ../std/env/fn.var.html
965 /// A compile time error is never emitted when using this macro regardless
966 /// of whether the environment variable is present or not.
971 /// let key: Option<&'static str> = option_env!("SECRET_KEY");
972 /// println!("the secret key might be: {key:?}");
974 #[stable(feature = "rust1", since = "1.0.0")]
975 #[rustc_builtin_macro]
977 #[cfg_attr(not(test), rustc_diagnostic_item = "option_env_macro")]
978 macro_rules! option_env {
979 ($name:expr $(,)?) => {{ /* compiler built-in */ }};
982 /// Concatenates identifiers into one identifier.
984 /// This macro takes any number of comma-separated identifiers, and
985 /// concatenates them all into one, yielding an expression which is a new
986 /// identifier. Note that hygiene makes it such that this macro cannot
987 /// capture local variables. Also, as a general rule, macros are only
988 /// allowed in item, statement or expression position. That means while
989 /// you may use this macro for referring to existing variables, functions or
990 /// modules etc, you cannot define a new one with it.
995 /// #![feature(concat_idents)]
998 /// fn foobar() -> u32 { 23 }
1000 /// let f = concat_idents!(foo, bar);
1001 /// println!("{}", f());
1003 /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
1007 feature = "concat_idents",
1009 reason = "`concat_idents` is not stable enough for use and is subject to change"
1011 #[rustc_builtin_macro]
1013 macro_rules! concat_idents {
1014 ($($e:ident),+ $(,)?) => {{ /* compiler built-in */ }};
1017 /// Concatenates literals into a byte slice.
1019 /// This macro takes any number of comma-separated literals, and concatenates them all into
1020 /// one, yielding an expression of type `&[u8, _]`, which represents all of the literals
1021 /// concatenated left-to-right. The literals passed can be any combination of:
1023 /// - byte literals (`b'r'`)
1024 /// - byte strings (`b"Rust"`)
1025 /// - arrays of bytes/numbers (`[b'A', 66, b'C']`)
1030 /// #![feature(concat_bytes)]
1033 /// let s: &[u8; 6] = concat_bytes!(b'A', b"BC", [68, b'E', 70]);
1034 /// assert_eq!(s, b"ABCDEF");
1037 #[unstable(feature = "concat_bytes", issue = "87555")]
1038 #[rustc_builtin_macro]
1040 macro_rules! concat_bytes {
1041 ($($e:literal),+ $(,)?) => {{ /* compiler built-in */ }};
1044 /// Concatenates literals into a static string slice.
1046 /// This macro takes any number of comma-separated literals, yielding an
1047 /// expression of type `&'static str` which represents all of the literals
1048 /// concatenated left-to-right.
1050 /// Integer and floating point literals are stringified in order to be
1056 /// let s = concat!("test", 10, 'b', true);
1057 /// assert_eq!(s, "test10btrue");
1059 #[stable(feature = "rust1", since = "1.0.0")]
1060 #[rustc_builtin_macro]
1062 #[cfg_attr(not(test), rustc_diagnostic_item = "concat_macro")]
1063 macro_rules! concat {
1064 ($($e:expr),* $(,)?) => {{ /* compiler built-in */ }};
1067 /// Expands to the line number on which it was invoked.
1069 /// With [`column!`] and [`file!`], these macros provide debugging information for
1070 /// developers about the location within the source.
1072 /// The expanded expression has type `u32` and is 1-based, so the first line
1073 /// in each file evaluates to 1, the second to 2, etc. This is consistent
1074 /// with error messages by common compilers or popular editors.
1075 /// The returned line is *not necessarily* the line of the `line!` invocation itself,
1076 /// but rather the first macro invocation leading up to the invocation
1077 /// of the `line!` macro.
1082 /// let current_line = line!();
1083 /// println!("defined on line: {current_line}");
1085 #[stable(feature = "rust1", since = "1.0.0")]
1086 #[rustc_builtin_macro]
1088 #[cfg_attr(not(test), rustc_diagnostic_item = "line_macro")]
1091 /* compiler built-in */
1095 /// Expands to the column number at which it was invoked.
1097 /// With [`line!`] and [`file!`], these macros provide debugging information for
1098 /// developers about the location within the source.
1100 /// The expanded expression has type `u32` and is 1-based, so the first column
1101 /// in each line evaluates to 1, the second to 2, etc. This is consistent
1102 /// with error messages by common compilers or popular editors.
1103 /// The returned column is *not necessarily* the line of the `column!` invocation itself,
1104 /// but rather the first macro invocation leading up to the invocation
1105 /// of the `column!` macro.
1110 /// let current_col = column!();
1111 /// println!("defined on column: {current_col}");
1114 /// `column!` counts Unicode code points, not bytes or graphemes. As a result, the first two
1115 /// invocations return the same value, but the third does not.
1118 /// let a = ("foobar", column!()).1;
1119 /// let b = ("人之初性本善", column!()).1;
1120 /// let c = ("f̅o̅o̅b̅a̅r̅", column!()).1; // Uses combining overline (U+0305)
1122 /// assert_eq!(a, b);
1123 /// assert_ne!(b, c);
1125 #[stable(feature = "rust1", since = "1.0.0")]
1126 #[rustc_builtin_macro]
1128 #[cfg_attr(not(test), rustc_diagnostic_item = "column_macro")]
1129 macro_rules! column {
1131 /* compiler built-in */
1135 /// Expands to the file name in which it was invoked.
1137 /// With [`line!`] and [`column!`], these macros provide debugging information for
1138 /// developers about the location within the source.
1140 /// The expanded expression has type `&'static str`, and the returned file
1141 /// is not the invocation of the `file!` macro itself, but rather the
1142 /// first macro invocation leading up to the invocation of the `file!`
1148 /// let this_file = file!();
1149 /// println!("defined in file: {this_file}");
1151 #[stable(feature = "rust1", since = "1.0.0")]
1152 #[rustc_builtin_macro]
1154 #[cfg_attr(not(test), rustc_diagnostic_item = "file_macro")]
1157 /* compiler built-in */
1161 /// Stringifies its arguments.
1163 /// This macro will yield an expression of type `&'static str` which is the
1164 /// stringification of all the tokens passed to the macro. No restrictions
1165 /// are placed on the syntax of the macro invocation itself.
1167 /// Note that the expanded results of the input tokens may change in the
1168 /// future. You should be careful if you rely on the output.
1173 /// let one_plus_one = stringify!(1 + 1);
1174 /// assert_eq!(one_plus_one, "1 + 1");
1176 #[stable(feature = "rust1", since = "1.0.0")]
1177 #[rustc_builtin_macro]
1179 #[cfg_attr(not(test), rustc_diagnostic_item = "stringify_macro")]
1180 macro_rules! stringify {
1182 /* compiler built-in */
1186 /// Includes a UTF-8 encoded file as a string.
1188 /// The file is located relative to the current file (similarly to how
1189 /// modules are found). The provided path is interpreted in a platform-specific
1190 /// way at compile time. So, for instance, an invocation with a Windows path
1191 /// containing backslashes `\` would not compile correctly on Unix.
1193 /// This macro will yield an expression of type `&'static str` which is the
1194 /// contents of the file.
1198 /// Assume there are two files in the same directory with the following
1201 /// File 'spanish.in':
1209 /// ```ignore (cannot-doctest-external-file-dependency)
1211 /// let my_str = include_str!("spanish.in");
1212 /// assert_eq!(my_str, "adiós\n");
1213 /// print!("{my_str}");
1217 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
1218 #[stable(feature = "rust1", since = "1.0.0")]
1219 #[rustc_builtin_macro]
1221 #[cfg_attr(not(test), rustc_diagnostic_item = "include_str_macro")]
1222 macro_rules! include_str {
1223 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1226 /// Includes a file as a reference to a byte array.
1228 /// The file is located relative to the current file (similarly to how
1229 /// modules are found). The provided path is interpreted in a platform-specific
1230 /// way at compile time. So, for instance, an invocation with a Windows path
1231 /// containing backslashes `\` would not compile correctly on Unix.
1233 /// This macro will yield an expression of type `&'static [u8; N]` which is
1234 /// the contents of the file.
1238 /// Assume there are two files in the same directory with the following
1241 /// File 'spanish.in':
1249 /// ```ignore (cannot-doctest-external-file-dependency)
1251 /// let bytes = include_bytes!("spanish.in");
1252 /// assert_eq!(bytes, b"adi\xc3\xb3s\n");
1253 /// print!("{}", String::from_utf8_lossy(bytes));
1257 /// Compiling 'main.rs' and running the resulting binary will print "adiós".
1258 #[stable(feature = "rust1", since = "1.0.0")]
1259 #[rustc_builtin_macro]
1261 #[cfg_attr(not(test), rustc_diagnostic_item = "include_bytes_macro")]
1262 macro_rules! include_bytes {
1263 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1266 /// Expands to a string that represents the current module path.
1268 /// The current module path can be thought of as the hierarchy of modules
1269 /// leading back up to the crate root. The first component of the path
1270 /// returned is the name of the crate currently being compiled.
1277 /// assert!(module_path!().ends_with("test"));
1283 #[stable(feature = "rust1", since = "1.0.0")]
1284 #[rustc_builtin_macro]
1286 #[cfg_attr(not(test), rustc_diagnostic_item = "module_path_macro")]
1287 macro_rules! module_path {
1289 /* compiler built-in */
1293 /// Evaluates boolean combinations of configuration flags at compile-time.
1295 /// In addition to the `#[cfg]` attribute, this macro is provided to allow
1296 /// boolean expression evaluation of configuration flags. This frequently
1297 /// leads to less duplicated code.
1299 /// The syntax given to this macro is the same syntax as the [`cfg`]
1302 /// `cfg!`, unlike `#[cfg]`, does not remove any code and only evaluates to true or false. For
1303 /// example, all blocks in an if/else expression need to be valid when `cfg!` is used for
1304 /// the condition, regardless of what `cfg!` is evaluating.
1306 /// [`cfg`]: ../reference/conditional-compilation.html#the-cfg-attribute
1311 /// let my_directory = if cfg!(windows) {
1312 /// "windows-specific-directory"
1314 /// "unix-directory"
1317 #[stable(feature = "rust1", since = "1.0.0")]
1318 #[rustc_builtin_macro]
1320 #[cfg_attr(not(test), rustc_diagnostic_item = "cfg_macro")]
1323 /* compiler built-in */
1327 /// Parses a file as an expression or an item according to the context.
1329 /// The file is located relative to the current file (similarly to how
1330 /// modules are found). The provided path is interpreted in a platform-specific
1331 /// way at compile time. So, for instance, an invocation with a Windows path
1332 /// containing backslashes `\` would not compile correctly on Unix.
1334 /// Using this macro is often a bad idea, because if the file is
1335 /// parsed as an expression, it is going to be placed in the
1336 /// surrounding code unhygienically. This could result in variables
1337 /// or functions being different from what the file expected if
1338 /// there are variables or functions that have the same name in
1339 /// the current file.
1343 /// Assume there are two files in the same directory with the following
1346 /// File 'monkeys.in':
1348 /// ```ignore (only-for-syntax-highlight)
1353 /// .collect::<String>()
1358 /// ```ignore (cannot-doctest-external-file-dependency)
1360 /// let my_string = include!("monkeys.in");
1361 /// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
1362 /// println!("{my_string}");
1366 /// Compiling 'main.rs' and running the resulting binary will print
1368 #[stable(feature = "rust1", since = "1.0.0")]
1369 #[rustc_builtin_macro]
1371 #[cfg_attr(not(test), rustc_diagnostic_item = "include_macro")]
1372 macro_rules! include {
1373 ($file:expr $(,)?) => {{ /* compiler built-in */ }};
1376 /// Asserts that a boolean expression is `true` at runtime.
1378 /// This will invoke the [`panic!`] macro if the provided expression cannot be
1379 /// evaluated to `true` at runtime.
1383 /// Assertions are always checked in both debug and release builds, and cannot
1384 /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
1385 /// release builds by default.
1387 /// Unsafe code may rely on `assert!` to enforce run-time invariants that, if
1388 /// violated could lead to unsafety.
1390 /// Other use-cases of `assert!` include testing and enforcing run-time
1391 /// invariants in safe code (whose violation cannot result in unsafety).
1393 /// # Custom Messages
1395 /// This macro has a second form, where a custom panic message can
1396 /// be provided with or without arguments for formatting. See [`std::fmt`]
1397 /// for syntax for this form. Expressions used as format arguments will only
1398 /// be evaluated if the assertion fails.
1400 /// [`std::fmt`]: ../std/fmt/index.html
1405 /// // the panic message for these assertions is the stringified value of the
1406 /// // expression given.
1409 /// fn some_computation() -> bool { true } // a very simple function
1411 /// assert!(some_computation());
1413 /// // assert with a custom message
1415 /// assert!(x, "x wasn't true!");
1417 /// let a = 3; let b = 27;
1418 /// assert!(a + b == 30, "a = {}, b = {}", a, b);
1420 #[stable(feature = "rust1", since = "1.0.0")]
1421 #[rustc_builtin_macro]
1423 #[rustc_diagnostic_item = "assert_macro"]
1424 #[allow_internal_unstable(core_panic, edition_panic)]
1425 macro_rules! assert {
1426 ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
1427 ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};
1430 /// Prints passed tokens into the standard output.
1432 feature = "log_syntax",
1434 reason = "`log_syntax!` is not stable enough for use and is subject to change"
1436 #[rustc_builtin_macro]
1438 macro_rules! log_syntax {
1440 /* compiler built-in */
1444 /// Enables or disables tracing functionality used for debugging other macros.
1446 feature = "trace_macros",
1448 reason = "`trace_macros` is not stable enough for use and is subject to change"
1450 #[rustc_builtin_macro]
1452 macro_rules! trace_macros {
1453 (true) => {{ /* compiler built-in */ }};
1454 (false) => {{ /* compiler built-in */ }};
1457 /// Attribute macro used to apply derive macros.
1459 /// See [the reference] for more info.
1461 /// [the reference]: ../../../reference/attributes/derive.html
1462 #[stable(feature = "rust1", since = "1.0.0")]
1463 #[rustc_builtin_macro]
1464 pub macro derive($item:item) {
1465 /* compiler built-in */
1468 /// Attribute macro applied to a function to turn it into a unit test.
1470 /// See [the reference] for more info.
1472 /// [the reference]: ../../../reference/attributes/testing.html#the-test-attribute
1473 #[stable(feature = "rust1", since = "1.0.0")]
1474 #[allow_internal_unstable(test, rustc_attrs)]
1475 #[rustc_builtin_macro]
1476 pub macro test($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($item:item) {
1512 /* compiler built-in */
1515 /// Keeps the item it's applied to if the passed path is accessible, and removes it otherwise.
1517 feature = "cfg_accessible",
1519 reason = "`cfg_accessible` is not fully implemented"
1521 #[rustc_builtin_macro]
1522 pub macro cfg_accessible($item:item) {
1523 /* compiler built-in */
1526 /// Expands all `#[cfg]` and `#[cfg_attr]` attributes in the code fragment it's applied to.
1528 feature = "cfg_eval",
1530 reason = "`cfg_eval` is a recently implemented feature"
1532 #[rustc_builtin_macro]
1533 pub macro cfg_eval($($tt:tt)*) {
1534 /* compiler built-in */
1537 /// Unstable implementation detail of the `rustc` compiler, do not use.
1538 #[rustc_builtin_macro]
1539 #[stable(feature = "rust1", since = "1.0.0")]
1540 #[allow_internal_unstable(core_intrinsics, libstd_sys_internals, rt)]
1541 #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
1542 #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
1543 pub macro RustcDecodable($item:item) {
1544 /* compiler built-in */
1547 /// Unstable implementation detail of the `rustc` compiler, do not use.
1548 #[rustc_builtin_macro]
1549 #[stable(feature = "rust1", since = "1.0.0")]
1550 #[allow_internal_unstable(core_intrinsics, rt)]
1551 #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
1552 #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
1553 pub macro RustcEncodable($item:item) {
1554 /* compiler built-in */