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 /// The `print!` macro will lock the standard output on each call. If you call
31 /// `print!` within a hot loop, this behavior may be the bottleneck of the loop.
32 /// To avoid this, lock stdout with [`io::stdout().lock()`][lock]:
34 /// use std::io::{stdout, Write};
36 /// let mut lock = stdout().lock();
37 /// write!(lock, "hello world").unwrap();
40 /// Use `print!` only for the primary output of your program. Use
41 /// [`eprint!`] instead to print error and progress messages.
43 /// [flush]: crate::io::Write::flush
44 /// [`println!`]: crate::println
45 /// [`eprint!`]: crate::eprint
46 /// [lock]: crate::io::Stdout
50 /// Panics if writing to `io::stdout()` fails.
52 /// Writing to non-blocking stdout can cause an error, which will lead
53 /// this macro to panic.
58 /// use std::io::{self, Write};
68 /// io::stdout().flush().unwrap();
70 /// print!("this string has a newline, why not choose println! instead?\n");
72 /// io::stdout().flush().unwrap();
75 #[stable(feature = "rust1", since = "1.0.0")]
76 #[cfg_attr(not(test), rustc_diagnostic_item = "print_macro")]
77 #[allow_internal_unstable(print_internals)]
80 $crate::io::_print($crate::format_args!($($arg)*));
84 /// Prints to the standard output, with a newline.
86 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
87 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`)).
89 /// This macro uses the same syntax as [`format!`], but writes to the standard output instead.
90 /// See [`std::fmt`] for more information.
92 /// The `println!` macro will lock the standard output on each call. If you call
93 /// `println!` within a hot loop, this behavior may be the bottleneck of the loop.
94 /// To avoid this, lock stdout with [`io::stdout().lock()`][lock]:
96 /// use std::io::{stdout, Write};
98 /// let mut lock = stdout().lock();
99 /// writeln!(lock, "hello world").unwrap();
102 /// Use `println!` only for the primary output of your program. Use
103 /// [`eprintln!`] instead to print error and progress messages.
105 /// [`std::fmt`]: crate::fmt
106 /// [`eprintln!`]: crate::eprintln
107 /// [lock]: crate::io::Stdout
111 /// Panics if writing to [`io::stdout`] fails.
113 /// Writing to non-blocking stdout can cause an error, which will lead
114 /// this macro to panic.
116 /// [`io::stdout`]: crate::io::stdout
121 /// println!(); // prints just a newline
122 /// println!("hello there!");
123 /// println!("format {} arguments", "some");
124 /// let local_variable = "some";
125 /// println!("format {local_variable} arguments");
128 #[stable(feature = "rust1", since = "1.0.0")]
129 #[cfg_attr(not(test), rustc_diagnostic_item = "println_macro")]
130 #[allow_internal_unstable(print_internals, format_args_nl)]
131 macro_rules! println {
136 $crate::io::_print($crate::format_args_nl!($($arg)*));
140 /// Prints to the standard error.
142 /// Equivalent to the [`print!`] macro, except that output goes to
143 /// [`io::stderr`] instead of [`io::stdout`]. See [`print!`] for
146 /// Use `eprint!` only for error and progress messages. Use `print!`
147 /// instead for the primary output of your program.
149 /// [`io::stderr`]: crate::io::stderr
150 /// [`io::stdout`]: crate::io::stdout
154 /// Panics if writing to `io::stderr` fails.
156 /// Writing to non-blocking stdout can cause an error, which will lead
157 /// this macro to panic.
162 /// eprint!("Error: Could not complete task");
165 #[stable(feature = "eprint", since = "1.19.0")]
166 #[cfg_attr(not(test), rustc_diagnostic_item = "eprint_macro")]
167 #[allow_internal_unstable(print_internals)]
168 macro_rules! eprint {
170 $crate::io::_eprint($crate::format_args!($($arg)*));
174 /// Prints to the standard error, with a newline.
176 /// Equivalent to the [`println!`] macro, except that output goes to
177 /// [`io::stderr`] instead of [`io::stdout`]. See [`println!`] for
180 /// Use `eprintln!` only for error and progress messages. Use `println!`
181 /// instead for the primary output of your program.
183 /// [`io::stderr`]: crate::io::stderr
184 /// [`io::stdout`]: crate::io::stdout
185 /// [`println!`]: crate::println
189 /// Panics if writing to `io::stderr` fails.
191 /// Writing to non-blocking stdout can cause an error, which will lead
192 /// this macro to panic.
197 /// eprintln!("Error: Could not complete task");
200 #[stable(feature = "eprint", since = "1.19.0")]
201 #[cfg_attr(not(test), rustc_diagnostic_item = "eprintln_macro")]
202 #[allow_internal_unstable(print_internals, format_args_nl)]
203 macro_rules! eprintln {
205 $crate::eprint!("\n")
208 $crate::io::_eprint($crate::format_args_nl!($($arg)*));
212 /// Prints and returns the value of a given expression for quick and dirty
219 /// let b = dbg!(a * 2) + 1;
220 /// // ^-- prints: [src/main.rs:2] a * 2 = 4
221 /// assert_eq!(b, 5);
224 /// The macro works by using the `Debug` implementation of the type of
225 /// the given expression to print the value to [stderr] along with the
226 /// source location of the macro invocation as well as the source code
227 /// of the expression.
229 /// Invoking the macro on an expression moves and takes ownership of it
230 /// before returning the evaluated expression unchanged. If the type
231 /// of the expression does not implement `Copy` and you don't want
232 /// to give up ownership, you can instead borrow with `dbg!(&expr)`
233 /// for some expression `expr`.
235 /// The `dbg!` macro works exactly the same in release builds.
236 /// This is useful when debugging issues that only occur in release
237 /// builds or when debugging in release mode is significantly faster.
239 /// Note that the macro is intended as a debugging tool and therefore you
240 /// should avoid having uses of it in version control for long periods
241 /// (other than in tests and similar).
242 /// Debug output from production code is better done with other facilities
243 /// such as the [`debug!`] macro from the [`log`] crate.
247 /// The exact output printed by this macro should not be relied upon
248 /// and is subject to future changes.
252 /// Panics if writing to `io::stderr` fails.
254 /// # Further examples
256 /// With a method call:
259 /// fn foo(n: usize) {
260 /// if let Some(_) = dbg!(n.checked_sub(4)) {
268 /// This prints to [stderr]:
271 /// [src/main.rs:4] n.checked_sub(4) = None
274 /// Naive factorial implementation:
277 /// fn factorial(n: u32) -> u32 {
278 /// if dbg!(n <= 1) {
281 /// dbg!(n * factorial(n - 1))
285 /// dbg!(factorial(4));
288 /// This prints to [stderr]:
291 /// [src/main.rs:3] n <= 1 = false
292 /// [src/main.rs:3] n <= 1 = false
293 /// [src/main.rs:3] n <= 1 = false
294 /// [src/main.rs:3] n <= 1 = true
295 /// [src/main.rs:4] 1 = 1
296 /// [src/main.rs:5] n * factorial(n - 1) = 2
297 /// [src/main.rs:5] n * factorial(n - 1) = 6
298 /// [src/main.rs:5] n * factorial(n - 1) = 24
299 /// [src/main.rs:11] factorial(4) = 24
302 /// The `dbg!(..)` macro moves the input:
305 /// /// A wrapper around `usize` which importantly is not Copyable.
307 /// struct NoCopy(usize);
309 /// let a = NoCopy(42);
310 /// let _ = dbg!(a); // <-- `a` is moved here.
311 /// let _ = dbg!(a); // <-- `a` is moved again; error!
314 /// You can also use `dbg!()` without a value to just print the
315 /// file and line whenever it's reached.
317 /// Finally, if you want to `dbg!(..)` multiple values, it will treat them as
318 /// a tuple (and return it, too):
321 /// assert_eq!(dbg!(1usize, 2u32), (1, 2));
324 /// However, a single argument with a trailing comma will still not be treated
325 /// as a tuple, following the convention of ignoring trailing commas in macro
326 /// invocations. You can use a 1-tuple directly if you need one:
329 /// assert_eq!(1, dbg!(1u32,)); // trailing comma ignored
330 /// assert_eq!((1,), dbg!((1u32,))); // 1-tuple
333 /// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
334 /// [`debug!`]: https://docs.rs/log/*/log/macro.debug.html
335 /// [`log`]: https://crates.io/crates/log
337 #[cfg_attr(not(test), rustc_diagnostic_item = "dbg_macro")]
338 #[stable(feature = "dbg_macro", since = "1.32.0")]
340 // NOTE: We cannot use `concat!` to make a static string as a format argument
341 // of `eprintln!` because `file!` could contain a `{` or
342 // `$val` expression could be a block (`{ .. }`), in which case the `eprintln!`
343 // will be malformed.
345 $crate::eprintln!("[{}:{}]", $crate::file!(), $crate::line!())
347 ($val:expr $(,)?) => {
348 // Use of `match` here is intentional because it affects the lifetimes
349 // of temporaries - https://stackoverflow.com/a/48732525/1063961
352 $crate::eprintln!("[{}:{}] {} = {:#?}",
353 $crate::file!(), $crate::line!(), $crate::stringify!($val), &tmp);
358 ($($val:expr),+ $(,)?) => {
359 ($($crate::dbg!($val)),+,)
364 macro_rules! assert_approx_eq {
365 ($a:expr, $b:expr) => {{
366 let (a, b) = (&$a, &$b);
367 assert!((*a - *b).abs() < 1.0e-6, "{} is not approximately equal to {}", *a, *b);