library/std/src/macros.rs

   1 //! Standard library macros
   2 //!
   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
   5 //! library.
   6
   7 #[doc = include_str!("../../core/src/macros/panic.md")]
   8 #[macro_export]
   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")]
  13 macro_rules! panic {
  14     // Expands to either `$crate::panic::panic_2015` or `$crate::panic::panic_2021`
  15     // depending on the edition of the caller.
  16     ($($arg:tt)*) => {
  17         /* compiler built-in */
  18     };
  19 }
  20
  21 /// Prints to the standard output.
  22 ///
  23 /// Equivalent to the [`println!`] macro except that a newline is not printed at
  24 /// the end of the message.
  25 ///
  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
  28 /// immediately.
  29 ///
  30 /// Use `print!` only for the primary output of your program. Use
  31 /// [`eprint!`] instead to print error and progress messages.
  32 ///
  33 /// [flush]: crate::io::Write::flush
  34 ///
  35 /// # Panics
  36 ///
  37 /// Panics if writing to `io::stdout()` fails.
  38 ///
  39 /// # Examples
  40 ///
  41 /// ```
  42 /// use std::io::{self, Write};
  43 ///
  44 /// print!("this ");
  45 /// print!("will ");
  46 /// print!("be ");
  47 /// print!("on ");
  48 /// print!("the ");
  49 /// print!("same ");
  50 /// print!("line ");
  51 ///
  52 /// io::stdout().flush().unwrap();
  53 ///
  54 /// print!("this string has a newline, why not choose println! instead?\n");
  55 ///
  56 /// io::stdout().flush().unwrap();
  57 /// ```
  58 #[macro_export]
  59 #[stable(feature = "rust1", since = "1.0.0")]
  60 #[allow_internal_unstable(print_internals)]
  61 macro_rules! print {
  62     ($($arg:tt)*) => ($crate::io::_print($crate::format_args!($($arg)*)));
  63 }
  64
  65 /// Prints to the standard output, with a newline.
  66 ///
  67 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
  68 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`)).
  69 ///
  70 /// Use the [`format!`] syntax to write data to the standard output.
  71 /// See [`std::fmt`] for more information.
  72 ///
  73 /// Use `println!` only for the primary output of your program. Use
  74 /// [`eprintln!`] instead to print error and progress messages.
  75 ///
  76 /// [`std::fmt`]: crate::fmt
  77 ///
  78 /// # Panics
  79 ///
  80 /// Panics if writing to [`io::stdout`] fails.
  81 ///
  82 /// [`io::stdout`]: crate::io::stdout
  83 ///
  84 /// # Examples
  85 ///
  86 /// ```
  87 /// println!(); // prints just a newline
  88 /// println!("hello there!");
  89 /// println!("format {} arguments", "some");
  90 /// ```
  91 #[macro_export]
  92 #[stable(feature = "rust1", since = "1.0.0")]
  93 #[allow_internal_unstable(print_internals, format_args_nl)]
  94 macro_rules! println {
  95     () => ($crate::print!("\n"));
  96     ($($arg:tt)*) => ({
  97         $crate::io::_print($crate::format_args_nl!($($arg)*));
  98     })
  99 }
 100
 101 /// Prints to the standard error.
 102 ///
 103 /// Equivalent to the [`print!`] macro, except that output goes to
 104 /// [`io::stderr`] instead of [`io::stdout`]. See [`print!`] for
 105 /// example usage.
 106 ///
 107 /// Use `eprint!` only for error and progress messages. Use `print!`
 108 /// instead for the primary output of your program.
 109 ///
 110 /// [`io::stderr`]: crate::io::stderr
 111 /// [`io::stdout`]: crate::io::stdout
 112 ///
 113 /// # Panics
 114 ///
 115 /// Panics if writing to `io::stderr` fails.
 116 ///
 117 /// # Examples
 118 ///
 119 /// ```
 120 /// eprint!("Error: Could not complete task");
 121 /// ```
 122 #[macro_export]
 123 #[stable(feature = "eprint", since = "1.19.0")]
 124 #[allow_internal_unstable(print_internals)]
 125 macro_rules! eprint {
 126     ($($arg:tt)*) => ($crate::io::_eprint($crate::format_args!($($arg)*)));
 127 }
 128
 129 /// Prints to the standard error, with a newline.
 130 ///
 131 /// Equivalent to the [`println!`] macro, except that output goes to
 132 /// [`io::stderr`] instead of [`io::stdout`]. See [`println!`] for
 133 /// example usage.
 134 ///
 135 /// Use `eprintln!` only for error and progress messages. Use `println!`
 136 /// instead for the primary output of your program.
 137 ///
 138 /// [`io::stderr`]: crate::io::stderr
 139 /// [`io::stdout`]: crate::io::stdout
 140 ///
 141 /// # Panics
 142 ///
 143 /// Panics if writing to `io::stderr` fails.
 144 ///
 145 /// # Examples
 146 ///
 147 /// ```
 148 /// eprintln!("Error: Could not complete task");
 149 /// ```
 150 #[macro_export]
 151 #[stable(feature = "eprint", since = "1.19.0")]
 152 #[allow_internal_unstable(print_internals, format_args_nl)]
 153 macro_rules! eprintln {
 154     () => ($crate::eprint!("\n"));
 155     ($($arg:tt)*) => ({
 156         $crate::io::_eprint($crate::format_args_nl!($($arg)*));
 157     })
 158 }
 159
 160 /// Prints and returns the value of a given expression for quick and dirty
 161 /// debugging.
 162 ///
 163 /// An example:
 164 ///
 165 /// ```rust
 166 /// let a = 2;
 167 /// let b = dbg!(a * 2) + 1;
 168 /// //      ^-- prints: [src/main.rs:2] a * 2 = 4
 169 /// assert_eq!(b, 5);
 170 /// ```
 171 ///
 172 /// The macro works by using the `Debug` implementation of the type of
 173 /// the given expression to print the value to [stderr] along with the
 174 /// source location of the macro invocation as well as the source code
 175 /// of the expression.
 176 ///
 177 /// Invoking the macro on an expression moves and takes ownership of it
 178 /// before returning the evaluated expression unchanged. If the type
 179 /// of the expression does not implement `Copy` and you don't want
 180 /// to give up ownership, you can instead borrow with `dbg!(&expr)`
 181 /// for some expression `expr`.
 182 ///
 183 /// The `dbg!` macro works exactly the same in release builds.
 184 /// This is useful when debugging issues that only occur in release
 185 /// builds or when debugging in release mode is significantly faster.
 186 ///
 187 /// Note that the macro is intended as a debugging tool and therefore you
 188 /// should avoid having uses of it in version control for long periods.
 189 /// Use cases involving debug output that should be added to version control
 190 /// are better served by macros such as [`debug!`] from the [`log`] crate.
 191 ///
 192 /// # Stability
 193 ///
 194 /// The exact output printed by this macro should not be relied upon
 195 /// and is subject to future changes.
 196 ///
 197 /// # Panics
 198 ///
 199 /// Panics if writing to `io::stderr` fails.
 200 ///
 201 /// # Further examples
 202 ///
 203 /// With a method call:
 204 ///
 205 /// ```rust
 206 /// fn foo(n: usize) {
 207 ///     if let Some(_) = dbg!(n.checked_sub(4)) {
 208 ///         // ...
 209 ///     }
 210 /// }
 211 ///
 212 /// foo(3)
 213 /// ```
 214 ///
 215 /// This prints to [stderr]:
 216 ///
 217 /// ```text,ignore
 218 /// [src/main.rs:4] n.checked_sub(4) = None
 219 /// ```
 220 ///
 221 /// Naive factorial implementation:
 222 ///
 223 /// ```rust
 224 /// fn factorial(n: u32) -> u32 {
 225 ///     if dbg!(n <= 1) {
 226 ///         dbg!(1)
 227 ///     } else {
 228 ///         dbg!(n * factorial(n - 1))
 229 ///     }
 230 /// }
 231 ///
 232 /// dbg!(factorial(4));
 233 /// ```
 234 ///
 235 /// This prints to [stderr]:
 236 ///
 237 /// ```text,ignore
 238 /// [src/main.rs:3] n <= 1 = false
 239 /// [src/main.rs:3] n <= 1 = false
 240 /// [src/main.rs:3] n <= 1 = false
 241 /// [src/main.rs:3] n <= 1 = true
 242 /// [src/main.rs:4] 1 = 1
 243 /// [src/main.rs:5] n * factorial(n - 1) = 2
 244 /// [src/main.rs:5] n * factorial(n - 1) = 6
 245 /// [src/main.rs:5] n * factorial(n - 1) = 24
 246 /// [src/main.rs:11] factorial(4) = 24
 247 /// ```
 248 ///
 249 /// The `dbg!(..)` macro moves the input:
 250 ///
 251 /// ```compile_fail
 252 /// /// A wrapper around `usize` which importantly is not Copyable.
 253 /// #[derive(Debug)]
 254 /// struct NoCopy(usize);
 255 ///
 256 /// let a = NoCopy(42);
 257 /// let _ = dbg!(a); // <-- `a` is moved here.
 258 /// let _ = dbg!(a); // <-- `a` is moved again; error!
 259 /// ```
 260 ///
 261 /// You can also use `dbg!()` without a value to just print the
 262 /// file and line whenever it's reached.
 263 ///
 264 /// Finally, if you want to `dbg!(..)` multiple values, it will treat them as
 265 /// a tuple (and return it, too):
 266 ///
 267 /// ```
 268 /// assert_eq!(dbg!(1usize, 2u32), (1, 2));
 269 /// ```
 270 ///
 271 /// However, a single argument with a trailing comma will still not be treated
 272 /// as a tuple, following the convention of ignoring trailing commas in macro
 273 /// invocations. You can use a 1-tuple directly if you need one:
 274 ///
 275 /// ```
 276 /// assert_eq!(1, dbg!(1u32,)); // trailing comma ignored
 277 /// assert_eq!((1,), dbg!((1u32,))); // 1-tuple
 278 /// ```
 279 ///
 280 /// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
 281 /// [`debug!`]: https://docs.rs/log/*/log/macro.debug.html
 282 /// [`log`]: https://crates.io/crates/log
 283 #[macro_export]
 284 #[stable(feature = "dbg_macro", since = "1.32.0")]
 285 macro_rules! dbg {
 286     // NOTE: We cannot use `concat!` to make a static string as a format argument
 287     // of `eprintln!` because `file!` could contain a `{` or
 288     // `$val` expression could be a block (`{ .. }`), in which case the `eprintln!`
 289     // will be malformed.
 290     () => {
 291         $crate::eprintln!("[{}:{}]", $crate::file!(), $crate::line!());
 292     };
 293     ($val:expr $(,)?) => {
 294         // Use of `match` here is intentional because it affects the lifetimes
 295         // of temporaries - https://stackoverflow.com/a/48732525/1063961
 296         match $val {
 297             tmp => {
 298                 $crate::eprintln!("[{}:{}] {} = {:#?}",
 299                     $crate::file!(), $crate::line!(), $crate::stringify!($val), &tmp);
 300                 tmp
 301             }
 302         }
 303     };
 304     ($($val:expr),+ $(,)?) => {
 305         ($($crate::dbg!($val)),+,)
 306     };
 307 }
 308
 309 #[cfg(test)]
 310 macro_rules! assert_approx_eq {
 311     ($a:expr, $b:expr) => {{
 312         let (a, b) = (&$a, &$b);
 313         assert!((*a - *b).abs() < 1.0e-6, "{} is not approximately equal to {}", *a, *b);
 314     }};
 315 }