1 //! Standard library macros
3 //! This module contains a set of macros which are exported from the standard
4 //! library. Each macro is available for use when linking against the standard
7 #[doc = include_str!("../../core/src/macros/panic.md")]
9 #[rustc_builtin_macro(std_panic)]
10 #[stable(feature = "rust1", since = "1.0.0")]
11 #[allow_internal_unstable(edition_panic)]
12 #[cfg_attr(not(test), rustc_diagnostic_item = "std_panic_macro")]
14 // Expands to either `$crate::panic::panic_2015` or `$crate::panic::panic_2021`
15 // depending on the edition of the caller.
17 /* compiler built-in */
21 /// Prints to the standard output.
23 /// Equivalent to the [`println!`] macro except that a newline is not printed at
24 /// the end of the message.
26 /// Note that stdout is frequently line-buffered by default so it may be
27 /// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted
30 /// Use `print!` only for the primary output of your program. Use
31 /// [`eprint!`] instead to print error and progress messages.
33 /// [flush]: crate::io::Write::flush
37 /// Panics if writing to `io::stdout()` fails.
42 /// use std::io::{self, Write};
52 /// io::stdout().flush().unwrap();
54 /// print!("this string has a newline, why not choose println! instead?\n");
56 /// io::stdout().flush().unwrap();
59 #[stable(feature = "rust1", since = "1.0.0")]
60 #[cfg_attr(not(test), rustc_diagnostic_item = "print_macro")]
61 #[allow_internal_unstable(print_internals)]
64 $crate::io::_print($crate::format_args!($($arg)*))
68 /// Prints to the standard output, with a newline.
70 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
71 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`)).
73 /// Use the [`format!`] syntax to write data to the standard output.
74 /// See [`std::fmt`] for more information.
76 /// Use `println!` only for the primary output of your program. Use
77 /// [`eprintln!`] instead to print error and progress messages.
79 /// [`std::fmt`]: crate::fmt
83 /// Panics if writing to [`io::stdout`] fails.
85 /// [`io::stdout`]: crate::io::stdout
90 /// println!(); // prints just a newline
91 /// println!("hello there!");
92 /// println!("format {} arguments", "some");
95 #[stable(feature = "rust1", since = "1.0.0")]
96 #[cfg_attr(not(test), rustc_diagnostic_item = "println_macro")]
97 #[allow_internal_unstable(print_internals, format_args_nl)]
98 macro_rules! println {
103 $crate::io::_print($crate::format_args_nl!($($arg)*))
107 /// Prints to the standard error.
109 /// Equivalent to the [`print!`] macro, except that output goes to
110 /// [`io::stderr`] instead of [`io::stdout`]. See [`print!`] for
113 /// Use `eprint!` only for error and progress messages. Use `print!`
114 /// instead for the primary output of your program.
116 /// [`io::stderr`]: crate::io::stderr
117 /// [`io::stdout`]: crate::io::stdout
121 /// Panics if writing to `io::stderr` fails.
126 /// eprint!("Error: Could not complete task");
129 #[stable(feature = "eprint", since = "1.19.0")]
130 #[cfg_attr(not(test), rustc_diagnostic_item = "eprint_macro")]
131 #[allow_internal_unstable(print_internals)]
132 macro_rules! eprint {
134 $crate::io::_eprint($crate::format_args!($($arg)*))
138 /// Prints to the standard error, with a newline.
140 /// Equivalent to the [`println!`] macro, except that output goes to
141 /// [`io::stderr`] instead of [`io::stdout`]. See [`println!`] for
144 /// Use `eprintln!` only for error and progress messages. Use `println!`
145 /// instead for the primary output of your program.
147 /// [`io::stderr`]: crate::io::stderr
148 /// [`io::stdout`]: crate::io::stdout
152 /// Panics if writing to `io::stderr` fails.
157 /// eprintln!("Error: Could not complete task");
160 #[stable(feature = "eprint", since = "1.19.0")]
161 #[cfg_attr(not(test), rustc_diagnostic_item = "eprintln_macro")]
162 #[allow_internal_unstable(print_internals, format_args_nl)]
163 macro_rules! eprintln {
165 $crate::eprint!("\n")
168 $crate::io::_eprint($crate::format_args_nl!($($arg)*))
172 /// Prints and returns the value of a given expression for quick and dirty
179 /// let b = dbg!(a * 2) + 1;
180 /// // ^-- prints: [src/main.rs:2] a * 2 = 4
181 /// assert_eq!(b, 5);
184 /// The macro works by using the `Debug` implementation of the type of
185 /// the given expression to print the value to [stderr] along with the
186 /// source location of the macro invocation as well as the source code
187 /// of the expression.
189 /// Invoking the macro on an expression moves and takes ownership of it
190 /// before returning the evaluated expression unchanged. If the type
191 /// of the expression does not implement `Copy` and you don't want
192 /// to give up ownership, you can instead borrow with `dbg!(&expr)`
193 /// for some expression `expr`.
195 /// The `dbg!` macro works exactly the same in release builds.
196 /// This is useful when debugging issues that only occur in release
197 /// builds or when debugging in release mode is significantly faster.
199 /// Note that the macro is intended as a debugging tool and therefore you
200 /// should avoid having uses of it in version control for long periods
201 /// (other than in tests and similar).
202 /// Debug output from production code is better done with other facilities
203 /// such as the [`debug!`] macro from the [`log`] crate.
207 /// The exact output printed by this macro should not be relied upon
208 /// and is subject to future changes.
212 /// Panics if writing to `io::stderr` fails.
214 /// # Further examples
216 /// With a method call:
219 /// fn foo(n: usize) {
220 /// if let Some(_) = dbg!(n.checked_sub(4)) {
228 /// This prints to [stderr]:
231 /// [src/main.rs:4] n.checked_sub(4) = None
234 /// Naive factorial implementation:
237 /// fn factorial(n: u32) -> u32 {
238 /// if dbg!(n <= 1) {
241 /// dbg!(n * factorial(n - 1))
245 /// dbg!(factorial(4));
248 /// This prints to [stderr]:
251 /// [src/main.rs:3] n <= 1 = false
252 /// [src/main.rs:3] n <= 1 = false
253 /// [src/main.rs:3] n <= 1 = false
254 /// [src/main.rs:3] n <= 1 = true
255 /// [src/main.rs:4] 1 = 1
256 /// [src/main.rs:5] n * factorial(n - 1) = 2
257 /// [src/main.rs:5] n * factorial(n - 1) = 6
258 /// [src/main.rs:5] n * factorial(n - 1) = 24
259 /// [src/main.rs:11] factorial(4) = 24
262 /// The `dbg!(..)` macro moves the input:
265 /// /// A wrapper around `usize` which importantly is not Copyable.
267 /// struct NoCopy(usize);
269 /// let a = NoCopy(42);
270 /// let _ = dbg!(a); // <-- `a` is moved here.
271 /// let _ = dbg!(a); // <-- `a` is moved again; error!
274 /// You can also use `dbg!()` without a value to just print the
275 /// file and line whenever it's reached.
277 /// Finally, if you want to `dbg!(..)` multiple values, it will treat them as
278 /// a tuple (and return it, too):
281 /// assert_eq!(dbg!(1usize, 2u32), (1, 2));
284 /// However, a single argument with a trailing comma will still not be treated
285 /// as a tuple, following the convention of ignoring trailing commas in macro
286 /// invocations. You can use a 1-tuple directly if you need one:
289 /// assert_eq!(1, dbg!(1u32,)); // trailing comma ignored
290 /// assert_eq!((1,), dbg!((1u32,))); // 1-tuple
293 /// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
294 /// [`debug!`]: https://docs.rs/log/*/log/macro.debug.html
295 /// [`log`]: https://crates.io/crates/log
297 #[cfg_attr(not(test), rustc_diagnostic_item = "dbg_macro")]
298 #[stable(feature = "dbg_macro", since = "1.32.0")]
300 // NOTE: We cannot use `concat!` to make a static string as a format argument
301 // of `eprintln!` because `file!` could contain a `{` or
302 // `$val` expression could be a block (`{ .. }`), in which case the `eprintln!`
303 // will be malformed.
305 $crate::eprintln!("[{}:{}]", $crate::file!(), $crate::line!())
307 ($val:expr $(,)?) => {
308 // Use of `match` here is intentional because it affects the lifetimes
309 // of temporaries - https://stackoverflow.com/a/48732525/1063961
312 $crate::eprintln!("[{}:{}] {} = {:#?}",
313 $crate::file!(), $crate::line!(), $crate::stringify!($val), &tmp);
318 ($($val:expr),+ $(,)?) => {
319 ($($crate::dbg!($val)),+,)
324 macro_rules! assert_approx_eq {
325 ($a:expr, $b:expr) => {{
326 let (a, b) = (&$a, &$b);
327 assert!((*a - *b).abs() < 1.0e-6, "{} is not approximately equal to {}", *a, *b);