src/libstd/macros.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Standard library macros
  12 //!
  13 //! This modules contains a set of macros which are exported from the standard
  14 //! library. Each macro is available for use when linking against the standard
  15 //! library.
  16
  17 /// The entry point for panic of Rust threads.
  18 ///
  19 /// This macro is used to inject panic into a Rust thread, causing the thread to
  20 /// unwind and panic entirely. Each thread's panic can be reaped as the
  21 /// `Box<Any>` type, and the single-argument form of the `panic!` macro will be
  22 /// the value which is transmitted.
  23 ///
  24 /// The multi-argument form of this macro panics with a string and has the
  25 /// `format!` syntax for building a string.
  26 ///
  27 /// # Examples
  28 ///
  29 /// ```should_panic
  30 /// # #![allow(unreachable_code)]
  31 /// panic!();
  32 /// panic!("this is a terrible mistake!");
  33 /// panic!(4); // panic with the value of 4 to be collected elsewhere
  34 /// panic!("this is a {} {message}", "fancy", message = "message");
  35 /// ```
  36 #[macro_export]
  37 #[stable(feature = "rust1", since = "1.0.0")]
  38 #[allow_internal_unstable]
  39 /// The entry point for panic of Rust threads.
  40 ///
  41 /// This macro is used to inject panic into a Rust thread, causing the thread to
  42 /// unwind and panic entirely. Each thread's panic can be reaped as the
  43 /// `Box<Any>` type, and the single-argument form of the `panic!` macro will be
  44 /// the value which is transmitted.
  45 ///
  46 /// The multi-argument form of this macro panics with a string and has the
  47 /// `format!` syntax for building a string.
  48 ///
  49 /// # Examples
  50 ///
  51 /// ```should_panic
  52 /// # #![allow(unreachable_code)]
  53 /// panic!();
  54 /// panic!("this is a terrible mistake!");
  55 /// panic!(4); // panic with the value of 4 to be collected elsewhere
  56 /// panic!("this is a {} {message}", "fancy", message = "message");
  57 /// ```
  58 #[macro_export]
  59 #[stable(feature = "rust1", since = "1.0.0")]
  60 #[allow_internal_unstable]
  61 macro_rules! panic {
  62     () => ({
  63         panic!("explicit panic")
  64     });
  65     ($msg:expr) => ({
  66         $crate::rt::begin_unwind($msg, {
  67             // static requires less code at runtime, more constant data
  68             static _FILE_LINE: (&'static str, u32) = (file!(), line!());
  69             &_FILE_LINE
  70         })
  71     });
  72     ($fmt:expr, $($arg:tt)+) => ({
  73         $crate::rt::begin_unwind_fmt(format_args!($fmt, $($arg)+), {
  74             // The leading _'s are to avoid dead code warnings if this is
  75             // used inside a dead function. Just `#[allow(dead_code)]` is
  76             // insufficient, since the user may have
  77             // `#[forbid(dead_code)]` and which cannot be overridden.
  78             static _FILE_LINE: (&'static str, u32) = (file!(), line!());
  79             &_FILE_LINE
  80         })
  81     });
  82 }
  83
  84 /// Macro for printing to the standard output.
  85 ///
  86 /// Equivalent to the `println!` macro except that a newline is not printed at
  87 /// the end of the message.
  88 ///
  89 /// Note that stdout is frequently line-buffered by default so it may be
  90 /// necessary to use `io::stdout().flush()` to ensure the output is emitted
  91 /// immediately.
  92 #[macro_export]
  93 #[stable(feature = "rust1", since = "1.0.0")]
  94 #[allow_internal_unstable]
  95 macro_rules! print {
  96     ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
  97 }
  98
  99 /// Macro for printing to the standard output.
 100 ///
 101 /// Use the `format!` syntax to write data to the standard output.
 102 /// See `std::fmt` for more information.
 103 ///
 104 /// # Examples
 105 ///
 106 /// ```
 107 /// println!("hello there!");
 108 /// println!("format {} arguments", "some");
 109 /// ```
 110 #[macro_export]
 111 #[stable(feature = "rust1", since = "1.0.0")]
 112 macro_rules! println {
 113     ($fmt:expr) => (print!(concat!($fmt, "\n")));
 114     ($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*));
 115 }
 116
 117 /// Helper macro for unwrapping `Result` values while returning early with an
 118 /// error if the value of the expression is `Err`. Can only be used in
 119 /// functions that return `Result` because of the early return of `Err` that
 120 /// it provides.
 121 ///
 122 /// # Examples
 123 ///
 124 /// ```
 125 /// use std::io;
 126 /// use std::fs::File;
 127 /// use std::io::prelude::*;
 128 ///
 129 /// fn write_to_file_using_try() -> Result<(), io::Error> {
 130 ///     let mut file = try!(File::create("my_best_friends.txt"));
 131 ///     try!(file.write_all(b"This is a list of my best friends."));
 132 ///     println!("I wrote to the file");
 133 ///     Ok(())
 134 /// }
 135 /// // This is equivalent to:
 136 /// fn write_to_file_using_match() -> Result<(), io::Error> {
 137 ///     let mut file = try!(File::create("my_best_friends.txt"));
 138 ///     match file.write_all(b"This is a list of my best friends.") {
 139 ///         Ok(_) => (),
 140 ///         Err(e) => return Err(e),
 141 ///     }
 142 ///     println!("I wrote to the file");
 143 ///     Ok(())
 144 /// }
 145 /// ```
 146 #[macro_export]
 147 #[stable(feature = "rust1", since = "1.0.0")]
 148 macro_rules! try {
 149     ($expr:expr) => (match $expr {
 150         $crate::result::Result::Ok(val) => val,
 151         $crate::result::Result::Err(err) => {
 152             return $crate::result::Result::Err($crate::convert::From::from(err))
 153         }
 154     })
 155 }
 156
 157 /// A macro to select an event from a number of receivers.
 158 ///
 159 /// This macro is used to wait for the first event to occur on a number of
 160 /// receivers. It places no restrictions on the types of receivers given to
 161 /// this macro, this can be viewed as a heterogeneous select.
 162 ///
 163 /// # Examples
 164 ///
 165 /// ```
 166 /// # #![feature(mpsc_select)]
 167 /// use std::thread;
 168 /// use std::sync::mpsc;
 169 ///
 170 /// // two placeholder functions for now
 171 /// fn long_running_thread() {}
 172 /// fn calculate_the_answer() -> u32 { 42 }
 173 ///
 174 /// let (tx1, rx1) = mpsc::channel();
 175 /// let (tx2, rx2) = mpsc::channel();
 176 ///
 177 /// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); });
 178 /// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); });
 179 ///
 180 /// select! {
 181 ///     _ = rx1.recv() => println!("the long running thread finished first"),
 182 ///     answer = rx2.recv() => {
 183 ///         println!("the answer was: {}", answer.unwrap());
 184 ///     }
 185 /// }
 186 /// # drop(rx1.recv());
 187 /// # drop(rx2.recv());
 188 /// ```
 189 ///
 190 /// For more information about select, see the `std::sync::mpsc::Select` structure.
 191 #[macro_export]
 192 #[unstable(feature = "mpsc_select")]
 193 macro_rules! select {
 194     (
 195         $($name:pat = $rx:ident.$meth:ident() => $code:expr),+
 196     ) => ({
 197         use $crate::sync::mpsc::Select;
 198         let sel = Select::new();
 199         $( let mut $rx = sel.handle(&$rx); )+
 200         unsafe {
 201             $( $rx.add(); )+
 202         }
 203         let ret = sel.wait();
 204         $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+
 205         { unreachable!() }
 206     })
 207 }
 208
 209 // When testing the standard library, we link to the liblog crate to get the
 210 // logging macros. In doing so, the liblog crate was linked against the real
 211 // version of libstd, and uses a different std::fmt module than the test crate
 212 // uses. To get around this difference, we redefine the log!() macro here to be
 213 // just a dumb version of what it should be.
 214 #[cfg(test)]
 215 macro_rules! log {
 216     ($lvl:expr, $($args:tt)*) => (
 217         if log_enabled!($lvl) { println!($($args)*) }
 218     )
 219 }
 220
 221 /// Built-in macros to the compiler itself.
 222 ///
 223 /// These macros do not have any corresponding definition with a `macro_rules!`
 224 /// macro, but are documented here. Their implementations can be found hardcoded
 225 /// into libsyntax itself.
 226 #[cfg(dox)]
 227 pub mod builtin {
 228     /// The core macro for formatted string creation & output.
 229     ///
 230     /// This macro produces a value of type `fmt::Arguments`. This value can be
 231     /// passed to the functions in `std::fmt` for performing useful functions.
 232     /// All other formatting macros (`format!`, `write!`, `println!`, etc) are
 233     /// proxied through this one.
 234     ///
 235     /// For more information, see the documentation in `std::fmt`.
 236     ///
 237     /// # Examples
 238     ///
 239     /// ```
 240     /// use std::fmt;
 241     ///
 242     /// let s = fmt::format(format_args!("hello {}", "world"));
 243     /// assert_eq!(s, format!("hello {}", "world"));
 244     ///
 245     /// ```
 246     #[macro_export]
 247     macro_rules! format_args { ($fmt:expr, $($args:tt)*) => ({
 248         /* compiler built-in */
 249     }) }
 250
 251     /// Inspect an environment variable at compile time.
 252     ///
 253     /// This macro will expand to the value of the named environment variable at
 254     /// compile time, yielding an expression of type `&'static str`.
 255     ///
 256     /// If the environment variable is not defined, then a compilation error
 257     /// will be emitted.  To not emit a compile error, use the `option_env!`
 258     /// macro instead.
 259     ///
 260     /// # Examples
 261     ///
 262     /// ```
 263     /// let path: &'static str = env!("PATH");
 264     /// println!("the $PATH variable at the time of compiling was: {}", path);
 265     /// ```
 266     #[macro_export]
 267     macro_rules! env { ($name:expr) => ({ /* compiler built-in */ }) }
 268
 269     /// Optionally inspect an environment variable at compile time.
 270     ///
 271     /// If the named environment variable is present at compile time, this will
 272     /// expand into an expression of type `Option<&'static str>` whose value is
 273     /// `Some` of the value of the environment variable. If the environment
 274     /// variable is not present, then this will expand to `None`.
 275     ///
 276     /// A compile time error is never emitted when using this macro regardless
 277     /// of whether the environment variable is present or not.
 278     ///
 279     /// # Examples
 280     ///
 281     /// ```
 282     /// let key: Option<&'static str> = option_env!("SECRET_KEY");
 283     /// println!("the secret key might be: {:?}", key);
 284     /// ```
 285     #[macro_export]
 286     macro_rules! option_env { ($name:expr) => ({ /* compiler built-in */ }) }
 287
 288     /// Concatenate identifiers into one identifier.
 289     ///
 290     /// This macro takes any number of comma-separated identifiers, and
 291     /// concatenates them all into one, yielding an expression which is a new
 292     /// identifier. Note that hygiene makes it such that this macro cannot
 293     /// capture local variables, and macros are only allowed in item,
 294     /// statement or expression position, meaning this macro may be difficult to
 295     /// use in some situations.
 296     ///
 297     /// # Examples
 298     ///
 299     /// ```
 300     /// #![feature(concat_idents)]
 301     ///
 302     /// # fn main() {
 303     /// fn foobar() -> u32 { 23 }
 304     ///
 305     /// let f = concat_idents!(foo, bar);
 306     /// println!("{}", f());
 307     /// # }
 308     /// ```
 309     #[macro_export]
 310     macro_rules! concat_idents {
 311         ($($e:ident),*) => ({ /* compiler built-in */ })
 312     }
 313
 314     /// Concatenates literals into a static string slice.
 315     ///
 316     /// This macro takes any number of comma-separated literals, yielding an
 317     /// expression of type `&'static str` which represents all of the literals
 318     /// concatenated left-to-right.
 319     ///
 320     /// Integer and floating point literals are stringified in order to be
 321     /// concatenated.
 322     ///
 323     /// # Examples
 324     ///
 325     /// ```
 326     /// let s = concat!("test", 10, 'b', true);
 327     /// assert_eq!(s, "test10btrue");
 328     /// ```
 329     #[macro_export]
 330     macro_rules! concat { ($($e:expr),*) => ({ /* compiler built-in */ }) }
 331
 332     /// A macro which expands to the line number on which it was invoked.
 333     ///
 334     /// The expanded expression has type `u32`, and the returned line is not
 335     /// the invocation of the `line!()` macro itself, but rather the first macro
 336     /// invocation leading up to the invocation of the `line!()` macro.
 337     ///
 338     /// # Examples
 339     ///
 340     /// ```
 341     /// let current_line = line!();
 342     /// println!("defined on line: {}", current_line);
 343     /// ```
 344     #[macro_export]
 345     macro_rules! line { () => ({ /* compiler built-in */ }) }
 346
 347     /// A macro which expands to the column number on which it was invoked.
 348     ///
 349     /// The expanded expression has type `u32`, and the returned column is not
 350     /// the invocation of the `column!()` macro itself, but rather the first macro
 351     /// invocation leading up to the invocation of the `column!()` macro.
 352     ///
 353     /// # Examples
 354     ///
 355     /// ```
 356     /// let current_col = column!();
 357     /// println!("defined on column: {}", current_col);
 358     /// ```
 359     #[macro_export]
 360     macro_rules! column { () => ({ /* compiler built-in */ }) }
 361
 362     /// A macro which expands to the file name from which it was invoked.
 363     ///
 364     /// The expanded expression has type `&'static str`, and the returned file
 365     /// is not the invocation of the `file!()` macro itself, but rather the
 366     /// first macro invocation leading up to the invocation of the `file!()`
 367     /// macro.
 368     ///
 369     /// # Examples
 370     ///
 371     /// ```
 372     /// let this_file = file!();
 373     /// println!("defined in file: {}", this_file);
 374     /// ```
 375     #[macro_export]
 376     macro_rules! file { () => ({ /* compiler built-in */ }) }
 377
 378     /// A macro which stringifies its argument.
 379     ///
 380     /// This macro will yield an expression of type `&'static str` which is the
 381     /// stringification of all the tokens passed to the macro. No restrictions
 382     /// are placed on the syntax of the macro invocation itself.
 383     ///
 384     /// # Examples
 385     ///
 386     /// ```
 387     /// let one_plus_one = stringify!(1 + 1);
 388     /// assert_eq!(one_plus_one, "1 + 1");
 389     /// ```
 390     #[macro_export]
 391     macro_rules! stringify { ($t:tt) => ({ /* compiler built-in */ }) }
 392
 393     /// Includes a utf8-encoded file as a string.
 394     ///
 395     /// This macro will yield an expression of type `&'static str` which is the
 396     /// contents of the filename specified. The file is located relative to the
 397     /// current file (similarly to how modules are found),
 398     ///
 399     /// # Examples
 400     ///
 401     /// ```rust,ignore
 402     /// let secret_key = include_str!("secret-key.ascii");
 403     /// ```
 404     #[macro_export]
 405     macro_rules! include_str { ($file:expr) => ({ /* compiler built-in */ }) }
 406
 407     /// Includes a file as a byte slice.
 408     ///
 409     /// This macro will yield an expression of type `&'static [u8]` which is
 410     /// the contents of the filename specified. The file is located relative to
 411     /// the current file (similarly to how modules are found),
 412     ///
 413     /// # Examples
 414     ///
 415     /// ```rust,ignore
 416     /// let secret_key = include_bytes!("secret-key.bin");
 417     /// ```
 418     #[macro_export]
 419     macro_rules! include_bytes { ($file:expr) => ({ /* compiler built-in */ }) }
 420
 421     /// Expands to a string that represents the current module path.
 422     ///
 423     /// The current module path can be thought of as the hierarchy of modules
 424     /// leading back up to the crate root. The first component of the path
 425     /// returned is the name of the crate currently being compiled.
 426     ///
 427     /// # Examples
 428     ///
 429     /// ```
 430     /// mod test {
 431     ///     pub fn foo() {
 432     ///         assert!(module_path!().ends_with("test"));
 433     ///     }
 434     /// }
 435     ///
 436     /// test::foo();
 437     /// ```
 438     #[macro_export]
 439     macro_rules! module_path { () => ({ /* compiler built-in */ }) }
 440
 441     /// Boolean evaluation of configuration flags.
 442     ///
 443     /// In addition to the `#[cfg]` attribute, this macro is provided to allow
 444     /// boolean expression evaluation of configuration flags. This frequently
 445     /// leads to less duplicated code.
 446     ///
 447     /// The syntax given to this macro is the same syntax as the `cfg`
 448     /// attribute.
 449     ///
 450     /// # Examples
 451     ///
 452     /// ```
 453     /// let my_directory = if cfg!(windows) {
 454     ///     "windows-specific-directory"
 455     /// } else {
 456     ///     "unix-directory"
 457     /// };
 458     /// ```
 459     #[macro_export]
 460     macro_rules! cfg { ($cfg:tt) => ({ /* compiler built-in */ }) }
 461
 462     /// Parse the current given file as an expression.
 463     ///
 464     /// This is generally a bad idea, because it's going to behave unhygienically.
 465     ///
 466     /// # Examples
 467     ///
 468     /// ```ignore
 469     /// fn foo() {
 470     ///     include!("/path/to/a/file")
 471     /// }
 472     /// ```
 473     #[macro_export]
 474     macro_rules! include { ($cfg:tt) => ({ /* compiler built-in */ }) }
 475 }