src/libstd/macros.rs

   1 //! Standard library macros
   2 //!
   3 //! This modules 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 /// Panics the current thread.
   8 ///
   9 /// This allows a program to terminate immediately and provide feedback
  10 /// to the caller of the program. `panic!` should be used when a program reaches
  11 /// an unrecoverable state.
  12 ///
  13 /// This macro is the perfect way to assert conditions in example code and in
  14 /// tests. `panic!` is closely tied with the `unwrap` method of both [`Option`]
  15 /// and [`Result`][runwrap] enums. Both implementations call `panic!` when they are set
  16 /// to None or Err variants.
  17 ///
  18 /// This macro is used to inject panic into a Rust thread, causing the thread to
  19 /// panic entirely. Each thread's panic can be reaped as the `Box<Any>` type,
  20 /// and the single-argument form of the `panic!` macro will be the value which
  21 /// is transmitted.
  22 ///
  23 /// [`Result`] enum is often a better solution for recovering from errors than
  24 /// using the `panic!` macro. This macro should be used to avoid proceeding using
  25 /// incorrect values, such as from external sources. Detailed information about
  26 /// error handling is found in the [book].
  27 ///
  28 /// The multi-argument form of this macro panics with a string and has the
  29 /// [`format!`] syntax for building a string.
  30 ///
  31 /// See also the macro [`compile_error!`], for raising errors during compilation.
  32 ///
  33 /// [runwrap]: ../std/result/enum.Result.html#method.unwrap
  34 /// [`Option`]: ../std/option/enum.Option.html#method.unwrap
  35 /// [`Result`]: ../std/result/enum.Result.html
  36 /// [`format!`]: ../std/macro.format.html
  37 /// [`compile_error!`]: ../std/macro.compile_error.html
  38 /// [book]: ../book/ch09-00-error-handling.html
  39 ///
  40 /// # Current implementation
  41 ///
  42 /// If the main thread panics it will terminate all your threads and end your
  43 /// program with code `101`.
  44 ///
  45 /// # Examples
  46 ///
  47 /// ```should_panic
  48 /// # #![allow(unreachable_code)]
  49 /// panic!();
  50 /// panic!("this is a terrible mistake!");
  51 /// panic!(4); // panic with the value of 4 to be collected elsewhere
  52 /// panic!("this is a {} {message}", "fancy", message = "message");
  53 /// ```
  54 #[macro_export]
  55 #[stable(feature = "rust1", since = "1.0.0")]
  56 #[allow_internal_unstable(libstd_sys_internals)]
  57 macro_rules! panic {
  58     () => ({
  59         $crate::panic!("explicit panic")
  60     });
  61     ($msg:expr) => ({
  62         $crate::rt::begin_panic($msg, &($crate::file!(), $crate::line!(), $crate::column!()))
  63     });
  64     ($msg:expr,) => ({
  65         $crate::panic!($msg)
  66     });
  67     ($fmt:expr, $($arg:tt)+) => ({
  68         $crate::rt::begin_panic_fmt(&$crate::format_args!($fmt, $($arg)+),
  69                                     &($crate::file!(), $crate::line!(), $crate::column!()))
  70     });
  71 }
  72
  73 /// Prints to the standard output.
  74 ///
  75 /// Equivalent to the [`println!`] macro except that a newline is not printed at
  76 /// the end of the message.
  77 ///
  78 /// Note that stdout is frequently line-buffered by default so it may be
  79 /// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted
  80 /// immediately.
  81 ///
  82 /// Use `print!` only for the primary output of your program. Use
  83 /// [`eprint!`] instead to print error and progress messages.
  84 ///
  85 /// [`println!`]: ../std/macro.println.html
  86 /// [flush]: ../std/io/trait.Write.html#tymethod.flush
  87 /// [`eprint!`]: ../std/macro.eprint.html
  88 ///
  89 /// # Panics
  90 ///
  91 /// Panics if writing to `io::stdout()` fails.
  92 ///
  93 /// # Examples
  94 ///
  95 /// ```
  96 /// use std::io::{self, Write};
  97 ///
  98 /// print!("this ");
  99 /// print!("will ");
 100 /// print!("be ");
 101 /// print!("on ");
 102 /// print!("the ");
 103 /// print!("same ");
 104 /// print!("line ");
 105 ///
 106 /// io::stdout().flush().unwrap();
 107 ///
 108 /// print!("this string has a newline, why not choose println! instead?\n");
 109 ///
 110 /// io::stdout().flush().unwrap();
 111 /// ```
 112 #[macro_export]
 113 #[stable(feature = "rust1", since = "1.0.0")]
 114 #[allow_internal_unstable(print_internals)]
 115 macro_rules! print {
 116     ($($arg:tt)*) => ($crate::io::_print($crate::format_args!($($arg)*)));
 117 }
 118
 119 /// Prints to the standard output, with a newline.
 120 ///
 121 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
 122 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`)).
 123 ///
 124 /// Use the [`format!`] syntax to write data to the standard output.
 125 /// See [`std::fmt`] for more information.
 126 ///
 127 /// Use `println!` only for the primary output of your program. Use
 128 /// [`eprintln!`] instead to print error and progress messages.
 129 ///
 130 /// [`format!`]: ../std/macro.format.html
 131 /// [`std::fmt`]: ../std/fmt/index.html
 132 /// [`eprintln!`]: ../std/macro.eprintln.html
 133 /// # Panics
 134 ///
 135 /// Panics if writing to `io::stdout` fails.
 136 ///
 137 /// # Examples
 138 ///
 139 /// ```
 140 /// println!(); // prints just a newline
 141 /// println!("hello there!");
 142 /// println!("format {} arguments", "some");
 143 /// ```
 144 #[macro_export]
 145 #[stable(feature = "rust1", since = "1.0.0")]
 146 #[allow_internal_unstable(print_internals, format_args_nl)]
 147 macro_rules! println {
 148     () => ($crate::print!("\n"));
 149     ($($arg:tt)*) => ({
 150         $crate::io::_print($crate::format_args_nl!($($arg)*));
 151     })
 152 }
 153
 154 /// Prints to the standard error.
 155 ///
 156 /// Equivalent to the [`print!`] macro, except that output goes to
 157 /// [`io::stderr`] instead of `io::stdout`. See [`print!`] for
 158 /// example usage.
 159 ///
 160 /// Use `eprint!` only for error and progress messages. Use `print!`
 161 /// instead for the primary output of your program.
 162 ///
 163 /// [`io::stderr`]: ../std/io/struct.Stderr.html
 164 /// [`print!`]: ../std/macro.print.html
 165 ///
 166 /// # Panics
 167 ///
 168 /// Panics if writing to `io::stderr` fails.
 169 ///
 170 /// # Examples
 171 ///
 172 /// ```
 173 /// eprint!("Error: Could not complete task");
 174 /// ```
 175 #[macro_export]
 176 #[stable(feature = "eprint", since = "1.19.0")]
 177 #[allow_internal_unstable(print_internals)]
 178 macro_rules! eprint {
 179     ($($arg:tt)*) => ($crate::io::_eprint($crate::format_args!($($arg)*)));
 180 }
 181
 182 /// Prints to the standard error, with a newline.
 183 ///
 184 /// Equivalent to the [`println!`] macro, except that output goes to
 185 /// [`io::stderr`] instead of `io::stdout`. See [`println!`] for
 186 /// example usage.
 187 ///
 188 /// Use `eprintln!` only for error and progress messages. Use `println!`
 189 /// instead for the primary output of your program.
 190 ///
 191 /// [`io::stderr`]: ../std/io/struct.Stderr.html
 192 /// [`println!`]: ../std/macro.println.html
 193 ///
 194 /// # Panics
 195 ///
 196 /// Panics if writing to `io::stderr` fails.
 197 ///
 198 /// # Examples
 199 ///
 200 /// ```
 201 /// eprintln!("Error: Could not complete task");
 202 /// ```
 203 #[macro_export]
 204 #[stable(feature = "eprint", since = "1.19.0")]
 205 #[allow_internal_unstable(print_internals, format_args_nl)]
 206 macro_rules! eprintln {
 207     () => ($crate::eprint!("\n"));
 208     ($($arg:tt)*) => ({
 209         $crate::io::_eprint($crate::format_args_nl!($($arg)*));
 210     })
 211 }
 212
 213 /// Prints and returns the value of a given expression for quick and dirty
 214 /// debugging.
 215 ///
 216 /// An example:
 217 ///
 218 /// ```rust
 219 /// let a = 2;
 220 /// let b = dbg!(a * 2) + 1;
 221 /// //      ^-- prints: [src/main.rs:2] a * 2 = 4
 222 /// assert_eq!(b, 5);
 223 /// ```
 224 ///
 225 /// The macro works by using the `Debug` implementation of the type of
 226 /// the given expression to print the value to [stderr] along with the
 227 /// source location of the macro invocation as well as the source code
 228 /// of the expression.
 229 ///
 230 /// Invoking the macro on an expression moves and takes ownership of it
 231 /// before returning the evaluated expression unchanged. If the type
 232 /// of the expression does not implement `Copy` and you don't want
 233 /// to give up ownership, you can instead borrow with `dbg!(&expr)`
 234 /// for some expression `expr`.
 235 ///
 236 /// The `dbg!` macro works exactly the same in release builds.
 237 /// This is useful when debugging issues that only occur in release
 238 /// builds or when debugging in release mode is significantly faster.
 239 ///
 240 /// Note that the macro is intended as a debugging tool and therefore you
 241 /// should avoid having uses of it in version control for longer periods.
 242 /// Use cases involving debug output that should be added to version control
 243 /// are better served by macros such as [`debug!`] from the [`log`] crate.
 244 ///
 245 /// # Stability
 246 ///
 247 /// The exact output printed by this macro should not be relied upon
 248 /// and is subject to future changes.
 249 ///
 250 /// # Panics
 251 ///
 252 /// Panics if writing to `io::stderr` fails.
 253 ///
 254 /// # Further examples
 255 ///
 256 /// With a method call:
 257 ///
 258 /// ```rust
 259 /// fn foo(n: usize) {
 260 ///     if let Some(_) = dbg!(n.checked_sub(4)) {
 261 ///         // ...
 262 ///     }
 263 /// }
 264 ///
 265 /// foo(3)
 266 /// ```
 267 ///
 268 /// This prints to [stderr]:
 269 ///
 270 /// ```text,ignore
 271 /// [src/main.rs:4] n.checked_sub(4) = None
 272 /// ```
 273 ///
 274 /// Naive factorial implementation:
 275 ///
 276 /// ```rust
 277 /// fn factorial(n: u32) -> u32 {
 278 ///     if dbg!(n <= 1) {
 279 ///         dbg!(1)
 280 ///     } else {
 281 ///         dbg!(n * factorial(n - 1))
 282 ///     }
 283 /// }
 284 ///
 285 /// dbg!(factorial(4));
 286 /// ```
 287 ///
 288 /// This prints to [stderr]:
 289 ///
 290 /// ```text,ignore
 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
 300 /// ```
 301 ///
 302 /// The `dbg!(..)` macro moves the input:
 303 ///
 304 /// ```compile_fail
 305 /// /// A wrapper around `usize` which importantly is not Copyable.
 306 /// #[derive(Debug)]
 307 /// struct NoCopy(usize);
 308 ///
 309 /// let a = NoCopy(42);
 310 /// let _ = dbg!(a); // <-- `a` is moved here.
 311 /// let _ = dbg!(a); // <-- `a` is moved again; error!
 312 /// ```
 313 ///
 314 /// You can also use `dbg!()` without a value to just print the
 315 /// file and line whenever it's reached.
 316 ///
 317 /// Finally, if you want to `dbg!(..)` multiple values, it will treat them as
 318 /// a tuple (and return it, too):
 319 ///
 320 /// ```
 321 /// assert_eq!(dbg!(1usize, 2u32), (1, 2));
 322 /// ```
 323 ///
 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:
 327 ///
 328 /// ```
 329 /// assert_eq!(1, dbg!(1u32,)); // trailing comma ignored
 330 /// assert_eq!((1,), dbg!((1u32,))); // 1-tuple
 331 /// ```
 332 ///
 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
 336 #[macro_export]
 337 #[stable(feature = "dbg_macro", since = "1.32.0")]
 338 macro_rules! dbg {
 339     () => {
 340         $crate::eprintln!("[{}:{}]", $crate::file!(), $crate::line!());
 341     };
 342     ($val:expr) => {
 343         // Use of `match` here is intentional because it affects the lifetimes
 344         // of temporaries - https://stackoverflow.com/a/48732525/1063961
 345         match $val {
 346             tmp => {
 347                 $crate::eprintln!("[{}:{}] {} = {:#?}",
 348                     $crate::file!(), $crate::line!(), $crate::stringify!($val), &tmp);
 349                 tmp
 350             }
 351         }
 352     };
 353     // Trailing comma with single argument is ignored
 354     ($val:expr,) => { $crate::dbg!($val) };
 355     ($($val:expr),+ $(,)?) => {
 356         ($($crate::dbg!($val)),+,)
 357     };
 358 }
 359
 360 #[cfg(test)]
 361 macro_rules! assert_approx_eq {
 362     ($a:expr, $b:expr) => ({
 363         let (a, b) = (&$a, &$b);
 364         assert!((*a - *b).abs() < 1.0e-6,
 365                 "{} is not approximately equal to {}", *a, *b);
 366     })
 367 }