// Copyright 2014 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 or the MIT license // , at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! Standard library macros //! //! This modules contains a set of macros which are exported from the standard //! library. Each macro is available for use when linking against the standard //! library. /// The entry point for panic of Rust threads. /// /// This allows a program to to terminate immediately and provide feedback /// to the caller of the program. `panic!` should be used when a program reaches /// an unrecoverable problem. /// /// This macro is the perfect way to assert conditions in example code and in /// tests. `panic!` is closely tied with the `unwrap` method of both [`Option`] /// and [`Result`][runwrap] enums. Both implementations call `panic!` when they are set /// to None or Err variants. /// /// This macro is used to inject panic into a Rust thread, causing the thread to /// panic entirely. Each thread's panic can be reaped as the `Box` type, /// and the single-argument form of the `panic!` macro will be the value which /// is transmitted. /// /// [`Result`] enum is often a better solution for recovering from errors than /// using the `panic!` macro. This macro should be used to avoid proceeding using /// incorrect values, such as from external sources. Detailed information about /// error handling is found in the [book]. /// /// The multi-argument form of this macro panics with a string and has the /// [`format!`] syntax for building a string. /// /// [runwrap]: ../std/result/enum.Result.html#method.unwrap /// [`Option`]: ../std/option/enum.Option.html#method.unwrap /// [`Result`]: ../std/result/enum.Result.html /// [`format!`]: ../std/macro.format.html /// [book]: ../book/second-edition/ch09-01-unrecoverable-errors-with-panic.html /// /// # Current implementation /// /// If the main thread panics it will terminate all your threads and end your /// program with code `101`. /// /// # Examples /// /// ```should_panic /// # #![allow(unreachable_code)] /// panic!(); /// panic!("this is a terrible mistake!"); /// panic!(4); // panic with the value of 4 to be collected elsewhere /// panic!("this is a {} {message}", "fancy", message = "message"); /// ``` #[macro_export] #[stable(feature = "rust1", since = "1.0.0")] #[allow_internal_unstable] macro_rules! panic { () => ({ panic!("explicit panic") }); ($msg:expr) => ({ $crate::rt::begin_panic($msg, &(file!(), line!(), __rust_unstable_column!())) }); ($fmt:expr, $($arg:tt)+) => ({ $crate::rt::begin_panic_fmt(&format_args!($fmt, $($arg)+), &(file!(), line!(), __rust_unstable_column!())) }); } /// Macro for printing to the standard output. /// /// Equivalent to the [`println!`] macro except that a newline is not printed at /// the end of the message. /// /// Note that stdout is frequently line-buffered by default so it may be /// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted /// immediately. /// /// Use `print!` only for the primary output of your program. Use /// [`eprint!`] instead to print error and progress messages. /// /// [`println!`]: ../std/macro.println.html /// [flush]: ../std/io/trait.Write.html#tymethod.flush /// [`eprint!`]: ../std/macro.eprint.html /// /// # Panics /// /// Panics if writing to `io::stdout()` fails. /// /// # Examples /// /// ``` /// use std::io::{self, Write}; /// /// print!("this "); /// print!("will "); /// print!("be "); /// print!("on "); /// print!("the "); /// print!("same "); /// print!("line "); /// /// io::stdout().flush().unwrap(); /// /// print!("this string has a newline, why not choose println! instead?\n"); /// /// io::stdout().flush().unwrap(); /// ``` #[macro_export] #[stable(feature = "rust1", since = "1.0.0")] #[allow_internal_unstable] macro_rules! print { ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*))); } /// Macro for printing to the standard output, with a newline. /// /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone /// (no additional CARRIAGE RETURN (`\r`/`U+000D`). /// /// Use the [`format!`] syntax to write data to the standard output. /// See [`std::fmt`] for more information. /// /// Use `println!` only for the primary output of your program. Use /// [`eprintln!`] instead to print error and progress messages. /// /// [`format!`]: ../std/macro.format.html /// [`std::fmt`]: ../std/fmt/index.html /// [`eprintln!`]: ../std/macro.eprint.html /// # Panics /// /// Panics if writing to `io::stdout` fails. /// /// # Examples /// /// ``` /// println!(); // prints just a newline /// println!("hello there!"); /// println!("format {} arguments", "some"); /// ``` #[macro_export] #[stable(feature = "rust1", since = "1.0.0")] macro_rules! println { () => (print!("\n")); ($fmt:expr) => (print!(concat!($fmt, "\n"))); ($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*)); } /// Macro for printing to the standard error. /// /// Equivalent to the [`print!`] macro, except that output goes to /// [`io::stderr`] instead of `io::stdout`. See [`print!`] for /// example usage. /// /// Use `eprint!` only for error and progress messages. Use `print!` /// instead for the primary output of your program. /// /// [`io::stderr`]: ../std/io/struct.Stderr.html /// [`print!`]: ../std/macro.print.html /// /// # Panics /// /// Panics if writing to `io::stderr` fails. /// /// # Examples /// /// ``` /// eprint!("Error: Could not complete task"); /// ``` #[macro_export] #[stable(feature = "eprint", since = "1.19.0")] #[allow_internal_unstable] macro_rules! eprint { ($($arg:tt)*) => ($crate::io::_eprint(format_args!($($arg)*))); } /// Macro for printing to the standard error, with a newline. /// /// Equivalent to the [`println!`] macro, except that output goes to /// [`io::stderr`] instead of `io::stdout`. See [`println!`] for /// example usage. /// /// Use `eprintln!` only for error and progress messages. Use `println!` /// instead for the primary output of your program. /// /// [`io::stderr`]: ../std/io/struct.Stderr.html /// [`println!`]: ../std/macro.println.html /// /// # Panics /// /// Panics if writing to `io::stderr` fails. /// /// # Examples /// /// ``` /// eprintln!("Error: Could not complete task"); /// ``` #[macro_export] #[stable(feature = "eprint", since = "1.19.0")] macro_rules! eprintln { () => (eprint!("\n")); ($fmt:expr) => (eprint!(concat!($fmt, "\n"))); ($fmt:expr, $($arg:tt)*) => (eprint!(concat!($fmt, "\n"), $($arg)*)); } /// A macro to select an event from a number of receivers. /// /// This macro is used to wait for the first event to occur on a number of /// receivers. It places no restrictions on the types of receivers given to /// this macro, this can be viewed as a heterogeneous select. /// /// # Examples /// /// ``` /// #![feature(mpsc_select)] /// /// use std::thread; /// use std::sync::mpsc; /// /// // two placeholder functions for now /// fn long_running_thread() {} /// fn calculate_the_answer() -> u32 { 42 } /// /// let (tx1, rx1) = mpsc::channel(); /// let (tx2, rx2) = mpsc::channel(); /// /// thread::spawn(move|| { long_running_thread(); tx1.send(()).unwrap(); }); /// thread::spawn(move|| { tx2.send(calculate_the_answer()).unwrap(); }); /// /// select! { /// _ = rx1.recv() => println!("the long running thread finished first"), /// answer = rx2.recv() => { /// println!("the answer was: {}", answer.unwrap()); /// } /// } /// # drop(rx1.recv()); /// # drop(rx2.recv()); /// ``` /// /// For more information about select, see the `std::sync::mpsc::Select` structure. #[macro_export] #[unstable(feature = "mpsc_select", issue = "27800")] macro_rules! select { ( $($name:pat = $rx:ident.$meth:ident() => $code:expr),+ ) => ({ use $crate::sync::mpsc::Select; let sel = Select::new(); $( let mut $rx = sel.handle(&$rx); )+ unsafe { $( $rx.add(); )+ } let ret = sel.wait(); $( if ret == $rx.id() { let $name = $rx.$meth(); $code } else )+ { unreachable!() } }) } #[cfg(test)] macro_rules! assert_approx_eq { ($a:expr, $b:expr) => ({ let (a, b) = (&$a, &$b); assert!((*a - *b).abs() < 1.0e-6, "{} is not approximately equal to {}", *a, *b); }) } /// Built-in macros to the compiler itself. /// /// These macros do not have any corresponding definition with a `macro_rules!` /// macro, but are documented here. Their implementations can be found hardcoded /// into libsyntax itself. #[cfg(dox)] pub mod builtin { /// Unconditionally causes compilation to fail with the given error message when encountered. /// /// This macro should be used when a crate uses a conditional compilation strategy to provide /// better error messages for erroneous conditions. /// /// # Examples /// /// Two such examples are macros and `#[cfg]` environments. /// /// Emit better compiler error if a macro is passed invalid values. /// /// ```compile_fail /// macro_rules! give_me_foo_or_bar { /// (foo) => {}; /// (bar) => {}; /// ($x:ident) => { /// compile_error!("This macro only accepts `foo` or `bar`"); /// } /// } /// /// give_me_foo_or_bar!(neither); /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`" /// ``` /// /// Emit compiler error if one of a number of features isn't available. /// /// ```compile_fail /// #[cfg(not(any(feature = "foo", feature = "bar")))] /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.") /// ``` #[stable(feature = "compile_error_macro", since = "1.20.0")] #[macro_export] macro_rules! compile_error { ($msg:expr) => ({ /* compiler built-in */ }) } /// The core macro for formatted string creation & output. /// /// This macro functions by taking a formatting string literal containing /// `{}` for each additional argument passed. `format_args!` prepares the /// additional parameters to ensure the output can be interpreted as a string /// and canonicalizes the arguments into a single type. Any value that implements /// the [`Display`] trait can be passed to `format_args!`, as can any /// [`Debug`] implementation be passed to a `{:?}` within the formatting string. /// /// This macro produces a value of type [`fmt::Arguments`]. This value can be /// passed to the macros within [`std::fmt`] for performing useful redirection. /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are /// proxied through this one. `format_args!`, unlike its derived macros, avoids /// heap allocations. /// /// For more information, see the documentation in [`std::fmt`]. /// /// [`Display`]: ../std/fmt/trait.Display.html /// [`Debug`]: ../std/fmt/trait.Debug.html /// [`fmt::Arguments`]: ../std/fmt/struct.Arguments.html /// [`std::fmt`]: ../std/fmt/index.html /// [`format!`]: ../std/macro.format.html /// [`write!`]: ../std/macro.write.html /// [`println!`]: ../std/macro.println.html /// /// # Examples /// /// ``` /// use std::fmt; /// /// let s = fmt::format(format_args!("hello {}", "world")); /// assert_eq!(s, format!("hello {}", "world")); /// /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! format_args { ($fmt:expr) => ({ /* compiler built-in */ }); ($fmt:expr, $($args:tt)*) => ({ /* compiler built-in */ }); } /// Inspect an environment variable at compile time. /// /// This macro will expand to the value of the named environment variable at /// compile time, yielding an expression of type `&'static str`. /// /// If the environment variable is not defined, then a compilation error /// will be emitted. To not emit a compile error, use the [`option_env!`] /// macro instead. /// /// [`option_env!`]: ../std/macro.option_env.html /// /// # Examples /// /// ``` /// let path: &'static str = env!("PATH"); /// println!("the $PATH variable at the time of compiling was: {}", path); /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! env { ($name:expr) => ({ /* compiler built-in */ }); ($name:expr,) => ({ /* compiler built-in */ }); } /// Optionally inspect an environment variable at compile time. /// /// If the named environment variable is present at compile time, this will /// expand into an expression of type `Option<&'static str>` whose value is /// `Some` of the value of the environment variable. If the environment /// variable is not present, then this will expand to `None`. See /// [`Option`][option] for more information on this type. /// /// A compile time error is never emitted when using this macro regardless /// of whether the environment variable is present or not. /// /// [option]: ../std/option/enum.Option.html /// /// # Examples /// /// ``` /// let key: Option<&'static str> = option_env!("SECRET_KEY"); /// println!("the secret key might be: {:?}", key); /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! option_env { ($name:expr) => ({ /* compiler built-in */ }) } /// Concatenate identifiers into one identifier. /// /// This macro takes any number of comma-separated identifiers, and /// concatenates them all into one, yielding an expression which is a new /// identifier. Note that hygiene makes it such that this macro cannot /// capture local variables. Also, as a general rule, macros are only /// allowed in item, statement or expression position. That means while /// you may use this macro for referring to existing variables, functions or /// modules etc, you cannot define a new one with it. /// /// # Examples /// /// ``` /// #![feature(concat_idents)] /// /// # fn main() { /// fn foobar() -> u32 { 23 } /// /// let f = concat_idents!(foo, bar); /// println!("{}", f()); /// /// // fn concat_idents!(new, fun, name) { } // not usable in this way! /// # } /// ``` #[unstable(feature = "concat_idents_macro", issue = "29599")] #[macro_export] macro_rules! concat_idents { ($($e:ident),*) => ({ /* compiler built-in */ }); ($($e:ident,)*) => ({ /* compiler built-in */ }); } /// Concatenates literals into a static string slice. /// /// This macro takes any number of comma-separated literals, yielding an /// expression of type `&'static str` which represents all of the literals /// concatenated left-to-right. /// /// Integer and floating point literals are stringified in order to be /// concatenated. /// /// # Examples /// /// ``` /// let s = concat!("test", 10, 'b', true); /// assert_eq!(s, "test10btrue"); /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! concat { ($($e:expr),*) => ({ /* compiler built-in */ }); ($($e:expr,)*) => ({ /* compiler built-in */ }); } /// A macro which expands to the line number on which it was invoked. /// /// With [`column!`] and [`file!`], these macros provide debugging information for /// developers about the location within the source. /// /// The expanded expression has type `u32` and is 1-based, so the first line /// in each file evaluates to 1, the second to 2, etc. This is consistent /// with error messages by common compilers or popular editors. /// The returned line is not the invocation of the `line!` macro itself, /// but rather the first macro invocation leading up to the invocation /// of the `line!` macro. /// /// [`column!`]: macro.column.html /// [`file!`]: macro.file.html /// /// # Examples /// /// ``` /// let current_line = line!(); /// println!("defined on line: {}", current_line); /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! line { () => ({ /* compiler built-in */ }) } /// A macro which expands to the column number on which it was invoked. /// /// With [`line!`] and [`file!`], these macros provide debugging information for /// developers about the location within the source. /// /// The expanded expression has type `u32` and is 1-based, so the first column /// in each line evaluates to 1, the second to 2, etc. This is consistent /// with error messages by common compilers or popular editors. /// The returned column is not the invocation of the `column!` macro itself, /// but rather the first macro invocation leading up to the invocation /// of the `column!` macro. /// /// [`line!`]: macro.line.html /// [`file!`]: macro.file.html /// /// # Examples /// /// ``` /// let current_col = column!(); /// println!("defined on column: {}", current_col); /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! column { () => ({ /* compiler built-in */ }) } /// A macro which expands to the file name from which it was invoked. /// /// With [`line!`] and [`column!`], these macros provide debugging information for /// developers about the location within the source. /// /// /// The expanded expression has type `&'static str`, and the returned file /// is not the invocation of the `file!` macro itself, but rather the /// first macro invocation leading up to the invocation of the `file!` /// macro. /// /// [`line!`]: macro.line.html /// [`column!`]: macro.column.html /// /// # Examples /// /// ``` /// let this_file = file!(); /// println!("defined in file: {}", this_file); /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! file { () => ({ /* compiler built-in */ }) } /// A macro which stringifies its arguments. /// /// This macro will yield an expression of type `&'static str` which is the /// stringification of all the tokens passed to the macro. No restrictions /// are placed on the syntax of the macro invocation itself. /// /// Note that the expanded results of the input tokens may change in the /// future. You should be careful if you rely on the output. /// /// # Examples /// /// ``` /// let one_plus_one = stringify!(1 + 1); /// assert_eq!(one_plus_one, "1 + 1"); /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! stringify { ($($t:tt)*) => ({ /* compiler built-in */ }) } /// Includes a utf8-encoded file as a string. /// /// The file is located relative to the current file. (similarly to how /// modules are found) /// /// This macro will yield an expression of type `&'static str` which is the /// contents of the file. /// /// # Examples /// /// Assume there are two files in the same directory with the following /// contents: /// /// File 'spanish.in': /// /// ```text /// adiós /// ``` /// /// File 'main.rs': /// /// ```ignore (cannot-doctest-external-file-dependency) /// fn main() { /// let my_str = include_str!("spanish.in"); /// assert_eq!(my_str, "adiós\n"); /// print!("{}", my_str); /// } /// ``` /// /// Compiling 'main.rs' and running the resulting binary will print "adiós". #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! include_str { ($file:expr) => ({ /* compiler built-in */ }) } /// Includes a file as a reference to a byte array. /// /// The file is located relative to the current file. (similarly to how /// modules are found) /// /// This macro will yield an expression of type `&'static [u8; N]` which is /// the contents of the file. /// /// # Examples /// /// Assume there are two files in the same directory with the following /// contents: /// /// File 'spanish.in': /// /// ```text /// adiós /// ``` /// /// File 'main.rs': /// /// ```ignore (cannot-doctest-external-file-dependency) /// fn main() { /// let bytes = include_bytes!("spanish.in"); /// assert_eq!(bytes, b"adi\xc3\xb3s\n"); /// print!("{}", String::from_utf8_lossy(bytes)); /// } /// ``` /// /// Compiling 'main.rs' and running the resulting binary will print "adiós". #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! include_bytes { ($file:expr) => ({ /* compiler built-in */ }) } /// Expands to a string that represents the current module path. /// /// The current module path can be thought of as the hierarchy of modules /// leading back up to the crate root. The first component of the path /// returned is the name of the crate currently being compiled. /// /// # Examples /// /// ``` /// mod test { /// pub fn foo() { /// assert!(module_path!().ends_with("test")); /// } /// } /// /// test::foo(); /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! module_path { () => ({ /* compiler built-in */ }) } /// Boolean evaluation of configuration flags, at compile-time. /// /// In addition to the `#[cfg]` attribute, this macro is provided to allow /// boolean expression evaluation of configuration flags. This frequently /// leads to less duplicated code. /// /// The syntax given to this macro is the same syntax as [the `cfg` /// attribute](../book/first-edition/conditional-compilation.html). /// /// # Examples /// /// ``` /// let my_directory = if cfg!(windows) { /// "windows-specific-directory" /// } else { /// "unix-directory" /// }; /// ``` #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! cfg { ($($cfg:tt)*) => ({ /* compiler built-in */ }) } /// Parse a file as an expression or an item according to the context. /// /// The file is located relative to the current file (similarly to how /// modules are found). /// /// Using this macro is often a bad idea, because if the file is /// parsed as an expression, it is going to be placed in the /// surrounding code unhygienically. This could result in variables /// or functions being different from what the file expected if /// there are variables or functions that have the same name in /// the current file. /// /// # Examples /// /// Assume there are two files in the same directory with the following /// contents: /// /// File 'monkeys.in': /// /// ```ignore (only-for-syntax-highlight) /// ['🙈', '🙊', '🙉'] /// .iter() /// .cycle() /// .take(6) /// .collect::() /// ``` /// /// File 'main.rs': /// /// ```ignore (cannot-doctest-external-file-dependency) /// fn main() { /// let my_string = include!("monkeys.in"); /// assert_eq!("🙈🙊🙉🙈🙊🙉", my_string); /// println!("{}", my_string); /// } /// ``` /// /// Compiling 'main.rs' and running the resulting binary will print /// "🙈🙊🙉🙈🙊🙉". #[stable(feature = "rust1", since = "1.0.0")] #[macro_export] macro_rules! include { ($file:expr) => ({ /* compiler built-in */ }) } } /// A macro for defining #[cfg] if-else statements. /// /// This is similar to the `if/elif` C preprocessor macro by allowing definition /// of a cascade of `#[cfg]` cases, emitting the implementation which matches /// first. /// /// This allows you to conveniently provide a long list #[cfg]'d blocks of code /// without having to rewrite each clause multiple times. macro_rules! cfg_if { ($( if #[cfg($($meta:meta),*)] { $($it:item)* } ) else * else { $($it2:item)* }) => { __cfg_if_items! { () ; $( ( ($($meta),*) ($($it)*) ), )* ( () ($($it2)*) ), } } } macro_rules! __cfg_if_items { (($($not:meta,)*) ; ) => {}; (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => { __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* } __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* } } } macro_rules! __cfg_if_apply { ($m:meta, $($it:item)*) => { $(#[$m] $it)* } }