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 #[cfg_attr(bootstrap, rustc_builtin_macro = "std_panic")]
10 #[cfg_attr(not(bootstrap), rustc_builtin_macro(std_panic))]
11 #[stable(feature = "rust1", since = "1.0.0")]
12 #[allow_internal_unstable(edition_panic)]
13 #[cfg_attr(not(test), rustc_diagnostic_item = "std_panic_macro")]
15 // Expands to either `$crate::panic::panic_2015` or `$crate::panic::panic_2021`
16 // depending on the edition of the caller.
18 /* compiler built-in */
22 /// Prints to the standard output.
24 /// Equivalent to the [`println!`] macro except that a newline is not printed at
25 /// the end of the message.
27 /// Note that stdout is frequently line-buffered by default so it may be
28 /// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted
31 /// Use `print!` only for the primary output of your program. Use
32 /// [`eprint!`] instead to print error and progress messages.
34 /// [flush]: crate::io::Write::flush
38 /// Panics if writing to `io::stdout()` fails.
43 /// use std::io::{self, Write};
53 /// io::stdout().flush().unwrap();
55 /// print!("this string has a newline, why not choose println! instead?\n");
57 /// io::stdout().flush().unwrap();
60 #[stable(feature = "rust1", since = "1.0.0")]
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 #[allow_internal_unstable(print_internals, format_args_nl)]
95 macro_rules! println {
96 () => ($crate::print!("\n"));
98 $crate::io::_print($crate::format_args_nl!($($arg)*));
102 /// Prints to the standard error.
104 /// Equivalent to the [`print!`] macro, except that output goes to
105 /// [`io::stderr`] instead of [`io::stdout`]. See [`print!`] for
108 /// Use `eprint!` only for error and progress messages. Use `print!`
109 /// instead for the primary output of your program.
111 /// [`io::stderr`]: crate::io::stderr
112 /// [`io::stdout`]: crate::io::stdout
116 /// Panics if writing to `io::stderr` fails.
121 /// eprint!("Error: Could not complete task");
124 #[stable(feature = "eprint", since = "1.19.0")]
125 #[allow_internal_unstable(print_internals)]
126 macro_rules! eprint {
127 ($($arg:tt)*) => ($crate::io::_eprint($crate::format_args!($($arg)*)));
130 /// Prints to the standard error, with a newline.
132 /// Equivalent to the [`println!`] macro, except that output goes to
133 /// [`io::stderr`] instead of [`io::stdout`]. See [`println!`] for
136 /// Use `eprintln!` only for error and progress messages. Use `println!`
137 /// instead for the primary output of your program.
139 /// [`io::stderr`]: crate::io::stderr
140 /// [`io::stdout`]: crate::io::stdout
144 /// Panics if writing to `io::stderr` fails.
149 /// eprintln!("Error: Could not complete task");
152 #[stable(feature = "eprint", since = "1.19.0")]
153 #[allow_internal_unstable(print_internals, format_args_nl)]
154 macro_rules! eprintln {
155 () => ($crate::eprint!("\n"));
157 $crate::io::_eprint($crate::format_args_nl!($($arg)*));
161 /// Prints and returns the value of a given expression for quick and dirty
168 /// let b = dbg!(a * 2) + 1;
169 /// // ^-- prints: [src/main.rs:2] a * 2 = 4
170 /// assert_eq!(b, 5);
173 /// The macro works by using the `Debug` implementation of the type of
174 /// the given expression to print the value to [stderr] along with the
175 /// source location of the macro invocation as well as the source code
176 /// of the expression.
178 /// Invoking the macro on an expression moves and takes ownership of it
179 /// before returning the evaluated expression unchanged. If the type
180 /// of the expression does not implement `Copy` and you don't want
181 /// to give up ownership, you can instead borrow with `dbg!(&expr)`
182 /// for some expression `expr`.
184 /// The `dbg!` macro works exactly the same in release builds.
185 /// This is useful when debugging issues that only occur in release
186 /// builds or when debugging in release mode is significantly faster.
188 /// Note that the macro is intended as a debugging tool and therefore you
189 /// should avoid having uses of it in version control for long periods
190 /// (other than in tests and similar).
191 /// Debug output from production code is better done with other facilities
192 /// such as the [`debug!`] macro from the [`log`] crate.
196 /// The exact output printed by this macro should not be relied upon
197 /// and is subject to future changes.
201 /// Panics if writing to `io::stderr` fails.
203 /// # Further examples
205 /// With a method call:
208 /// fn foo(n: usize) {
209 /// if let Some(_) = dbg!(n.checked_sub(4)) {
217 /// This prints to [stderr]:
220 /// [src/main.rs:4] n.checked_sub(4) = None
223 /// Naive factorial implementation:
226 /// fn factorial(n: u32) -> u32 {
227 /// if dbg!(n <= 1) {
230 /// dbg!(n * factorial(n - 1))
234 /// dbg!(factorial(4));
237 /// This prints to [stderr]:
240 /// [src/main.rs:3] n <= 1 = false
241 /// [src/main.rs:3] n <= 1 = false
242 /// [src/main.rs:3] n <= 1 = false
243 /// [src/main.rs:3] n <= 1 = true
244 /// [src/main.rs:4] 1 = 1
245 /// [src/main.rs:5] n * factorial(n - 1) = 2
246 /// [src/main.rs:5] n * factorial(n - 1) = 6
247 /// [src/main.rs:5] n * factorial(n - 1) = 24
248 /// [src/main.rs:11] factorial(4) = 24
251 /// The `dbg!(..)` macro moves the input:
254 /// /// A wrapper around `usize` which importantly is not Copyable.
256 /// struct NoCopy(usize);
258 /// let a = NoCopy(42);
259 /// let _ = dbg!(a); // <-- `a` is moved here.
260 /// let _ = dbg!(a); // <-- `a` is moved again; error!
263 /// You can also use `dbg!()` without a value to just print the
264 /// file and line whenever it's reached.
266 /// Finally, if you want to `dbg!(..)` multiple values, it will treat them as
267 /// a tuple (and return it, too):
270 /// assert_eq!(dbg!(1usize, 2u32), (1, 2));
273 /// However, a single argument with a trailing comma will still not be treated
274 /// as a tuple, following the convention of ignoring trailing commas in macro
275 /// invocations. You can use a 1-tuple directly if you need one:
278 /// assert_eq!(1, dbg!(1u32,)); // trailing comma ignored
279 /// assert_eq!((1,), dbg!((1u32,))); // 1-tuple
282 /// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
283 /// [`debug!`]: https://docs.rs/log/*/log/macro.debug.html
284 /// [`log`]: https://crates.io/crates/log
286 #[stable(feature = "dbg_macro", since = "1.32.0")]
288 // NOTE: We cannot use `concat!` to make a static string as a format argument
289 // of `eprintln!` because `file!` could contain a `{` or
290 // `$val` expression could be a block (`{ .. }`), in which case the `eprintln!`
291 // will be malformed.
293 $crate::eprintln!("[{}:{}]", $crate::file!(), $crate::line!());
295 ($val:expr $(,)?) => {
296 // Use of `match` here is intentional because it affects the lifetimes
297 // of temporaries - https://stackoverflow.com/a/48732525/1063961
300 $crate::eprintln!("[{}:{}] {} = {:#?}",
301 $crate::file!(), $crate::line!(), $crate::stringify!($val), &tmp);
306 ($($val:expr),+ $(,)?) => {
307 ($($crate::dbg!($val)),+,)
312 macro_rules! assert_approx_eq {
313 ($a:expr, $b:expr) => {{
314 let (a, b) = (&$a, &$b);
315 assert!((*a - *b).abs() < 1.0e-6, "{} is not approximately equal to {}", *a, *b);