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)]
63 ($($arg:tt)*) => ($crate::io::_print($crate::format_args!($($arg)*)));
66 /// Prints to the standard output, with a newline.
68 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
69 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`)).
71 /// Use the [`format!`] syntax to write data to the standard output.
72 /// See [`std::fmt`] for more information.
74 /// Use `println!` only for the primary output of your program. Use
75 /// [`eprintln!`] instead to print error and progress messages.
77 /// [`std::fmt`]: crate::fmt
81 /// Panics if writing to [`io::stdout`] fails.
83 /// [`io::stdout`]: crate::io::stdout
88 /// println!(); // prints just a newline
89 /// println!("hello there!");
90 /// println!("format {} arguments", "some");
93 #[stable(feature = "rust1", since = "1.0.0")]
94 #[cfg_attr(not(test), rustc_diagnostic_item = "println_macro")]
95 #[allow_internal_unstable(print_internals, format_args_nl)]
96 macro_rules! println {
97 () => ($crate::print!("\n"));
99 $crate::io::_print($crate::format_args_nl!($($arg)*));
103 /// Prints to the standard error.
105 /// Equivalent to the [`print!`] macro, except that output goes to
106 /// [`io::stderr`] instead of [`io::stdout`]. See [`print!`] for
109 /// Use `eprint!` only for error and progress messages. Use `print!`
110 /// instead for the primary output of your program.
112 /// [`io::stderr`]: crate::io::stderr
113 /// [`io::stdout`]: crate::io::stdout
117 /// Panics if writing to `io::stderr` fails.
122 /// eprint!("Error: Could not complete task");
125 #[stable(feature = "eprint", since = "1.19.0")]
126 #[cfg_attr(not(test), rustc_diagnostic_item = "eprint_macro")]
127 #[allow_internal_unstable(print_internals)]
128 macro_rules! eprint {
129 ($($arg:tt)*) => ($crate::io::_eprint($crate::format_args!($($arg)*)));
132 /// Prints to the standard error, with a newline.
134 /// Equivalent to the [`println!`] macro, except that output goes to
135 /// [`io::stderr`] instead of [`io::stdout`]. See [`println!`] for
138 /// Use `eprintln!` only for error and progress messages. Use `println!`
139 /// instead for the primary output of your program.
141 /// [`io::stderr`]: crate::io::stderr
142 /// [`io::stdout`]: crate::io::stdout
146 /// Panics if writing to `io::stderr` fails.
151 /// eprintln!("Error: Could not complete task");
154 #[stable(feature = "eprint", since = "1.19.0")]
155 #[cfg_attr(not(test), rustc_diagnostic_item = "eprintln_macro")]
156 #[allow_internal_unstable(print_internals, format_args_nl)]
157 macro_rules! eprintln {
158 () => ($crate::eprint!("\n"));
160 $crate::io::_eprint($crate::format_args_nl!($($arg)*));
164 /// Prints and returns the value of a given expression for quick and dirty
171 /// let b = dbg!(a * 2) + 1;
172 /// // ^-- prints: [src/main.rs:2] a * 2 = 4
173 /// assert_eq!(b, 5);
176 /// The macro works by using the `Debug` implementation of the type of
177 /// the given expression to print the value to [stderr] along with the
178 /// source location of the macro invocation as well as the source code
179 /// of the expression.
181 /// Invoking the macro on an expression moves and takes ownership of it
182 /// before returning the evaluated expression unchanged. If the type
183 /// of the expression does not implement `Copy` and you don't want
184 /// to give up ownership, you can instead borrow with `dbg!(&expr)`
185 /// for some expression `expr`.
187 /// The `dbg!` macro works exactly the same in release builds.
188 /// This is useful when debugging issues that only occur in release
189 /// builds or when debugging in release mode is significantly faster.
191 /// Note that the macro is intended as a debugging tool and therefore you
192 /// should avoid having uses of it in version control for long periods
193 /// (other than in tests and similar).
194 /// Debug output from production code is better done with other facilities
195 /// such as the [`debug!`] macro from the [`log`] crate.
199 /// The exact output printed by this macro should not be relied upon
200 /// and is subject to future changes.
204 /// Panics if writing to `io::stderr` fails.
206 /// # Further examples
208 /// With a method call:
211 /// fn foo(n: usize) {
212 /// if let Some(_) = dbg!(n.checked_sub(4)) {
220 /// This prints to [stderr]:
223 /// [src/main.rs:4] n.checked_sub(4) = None
226 /// Naive factorial implementation:
229 /// fn factorial(n: u32) -> u32 {
230 /// if dbg!(n <= 1) {
233 /// dbg!(n * factorial(n - 1))
237 /// dbg!(factorial(4));
240 /// This prints to [stderr]:
243 /// [src/main.rs:3] n <= 1 = false
244 /// [src/main.rs:3] n <= 1 = false
245 /// [src/main.rs:3] n <= 1 = false
246 /// [src/main.rs:3] n <= 1 = true
247 /// [src/main.rs:4] 1 = 1
248 /// [src/main.rs:5] n * factorial(n - 1) = 2
249 /// [src/main.rs:5] n * factorial(n - 1) = 6
250 /// [src/main.rs:5] n * factorial(n - 1) = 24
251 /// [src/main.rs:11] factorial(4) = 24
254 /// The `dbg!(..)` macro moves the input:
257 /// /// A wrapper around `usize` which importantly is not Copyable.
259 /// struct NoCopy(usize);
261 /// let a = NoCopy(42);
262 /// let _ = dbg!(a); // <-- `a` is moved here.
263 /// let _ = dbg!(a); // <-- `a` is moved again; error!
266 /// You can also use `dbg!()` without a value to just print the
267 /// file and line whenever it's reached.
269 /// Finally, if you want to `dbg!(..)` multiple values, it will treat them as
270 /// a tuple (and return it, too):
273 /// assert_eq!(dbg!(1usize, 2u32), (1, 2));
276 /// However, a single argument with a trailing comma will still not be treated
277 /// as a tuple, following the convention of ignoring trailing commas in macro
278 /// invocations. You can use a 1-tuple directly if you need one:
281 /// assert_eq!(1, dbg!(1u32,)); // trailing comma ignored
282 /// assert_eq!((1,), dbg!((1u32,))); // 1-tuple
285 /// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
286 /// [`debug!`]: https://docs.rs/log/*/log/macro.debug.html
287 /// [`log`]: https://crates.io/crates/log
289 #[cfg_attr(not(test), rustc_diagnostic_item = "dbg_macro")]
290 #[stable(feature = "dbg_macro", since = "1.32.0")]
292 // NOTE: We cannot use `concat!` to make a static string as a format argument
293 // of `eprintln!` because `file!` could contain a `{` or
294 // `$val` expression could be a block (`{ .. }`), in which case the `eprintln!`
295 // will be malformed.
297 $crate::eprintln!("[{}:{}]", $crate::file!(), $crate::line!())
299 ($val:expr $(,)?) => {
300 // Use of `match` here is intentional because it affects the lifetimes
301 // of temporaries - https://stackoverflow.com/a/48732525/1063961
304 $crate::eprintln!("[{}:{}] {} = {:#?}",
305 $crate::file!(), $crate::line!(), $crate::stringify!($val), &tmp);
310 ($($val:expr),+ $(,)?) => {
311 ($($crate::dbg!($val)),+,)
316 macro_rules! assert_approx_eq {
317 ($a:expr, $b:expr) => {{
318 let (a, b) = (&$a, &$b);
319 assert!((*a - *b).abs() < 1.0e-6, "{} is not approximately equal to {}", *a, *b);