macros.rs.html -- source
Description: Source to the Rust file `src/libstd/macros.rs`.
Keywords: rust, rustlang, rust-lang

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    //! 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 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<Any>` 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.
    ///
    /// See also the macro [`compile_error!`], for raising errors during compilation.
    ///
    /// [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
    /// [`compile_error!`]: ../std/macro.compile_error.html
    /// [book]: ../book/ch09-00-error-handling.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!()))
        });
        ($msg:expr,) => ({
            panic!($msg)
        });
        ($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.eprintln.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")]
    #[allow_internal_unstable]
    macro_rules! println {
        () => (print!("\n"));
        ($($arg:tt)*) => ({
            $crate::io::_print(format_args_nl!($($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")]
    #[allow_internal_unstable]
    macro_rules! eprintln {
        () => (eprint!("\n"));
        ($($arg:tt)*) => ({
            $crate::io::_eprint(format_args_nl!($($arg)*));
        })
    }

    /// A macro for quick and dirty debugging with which you can inspect
    /// the value of a given expression. An example:
    ///
    /// ```rust
    /// let a = 2;
    /// let b = dbg!(a * 2) + 1;
    /// //      ^-- prints: [src/main.rs:2] a * 2 = 4
    /// assert_eq!(b, 5);
    /// ```
    ///
    /// The macro works by using the `Debug` implementation of the type of
    /// the given expression to print the value to [stderr] along with the
    /// source location of the macro invocation as well as the source code
    /// of the expression.
    ///
    /// Invoking the macro on an expression moves and takes ownership of it
    /// before returning the evaluated expression unchanged. If the type
    /// of the expression does not implement `Copy` and you don't want
    /// to give up ownership, you can instead borrow with `dbg!(&expr)`
    /// for some expression `expr`.
    ///
    /// Note that the macro is intended as a debugging tool and therefore you
    /// should avoid having uses of it in version control for longer periods.
    /// Use cases involving debug output that should be added to version control
    /// may be better served by macros such as `debug!` from the `log` crate.
    ///
    /// # Stability
    ///
    /// The exact output printed by this macro should not be relied upon
    /// and is subject to future changes.
    ///
    /// # Panics
    ///
    /// Panics if writing to `io::stderr` fails.
    ///
    /// # Further examples
    ///
    /// With a method call:
    ///
    /// ```rust
    /// fn foo(n: usize) {
    ///     if let Some(_) = dbg!(n.checked_sub(4)) {
    ///         // ...
    ///     }
    /// }
    ///
    /// foo(3)
    /// ```
    ///
    /// This prints to [stderr]:
    ///
    /// ```text,ignore
    /// [src/main.rs:4] n.checked_sub(4) = None
    /// ```
    ///
    /// Naive factorial implementation:
    ///
    /// ```rust
    /// fn factorial(n: u32) -> u32 {
    ///     if dbg!(n <= 1) {
    ///         dbg!(1)
    ///     } else {
    ///         dbg!(n * factorial(n - 1))
    ///     }
    /// }
    ///
    /// dbg!(factorial(4));
    /// ```
    ///
    /// This prints to [stderr]:
    ///
    /// ```text,ignore
    /// [src/main.rs:3] n <= 1 = false
    /// [src/main.rs:3] n <= 1 = false
    /// [src/main.rs:3] n <= 1 = false
    /// [src/main.rs:3] n <= 1 = true
    /// [src/main.rs:4] 1 = 1
    /// [src/main.rs:5] n * factorial(n - 1) = 2
    /// [src/main.rs:5] n * factorial(n - 1) = 6
    /// [src/main.rs:5] n * factorial(n - 1) = 24
    /// [src/main.rs:11] factorial(4) = 24
    /// ```
    ///
    /// The `dbg!(..)` macro moves the input:
    ///
    /// ```compile_fail
    /// /// A wrapper around `usize` which importantly is not Copyable.
    /// #[derive(Debug)]
    /// struct NoCopy(usize);
    ///
    /// let a = NoCopy(42);
    /// let _ = dbg!(a); // <-- `a` is moved here.
    /// let _ = dbg!(a); // <-- `a` is moved again; error!
    /// ```
    ///
    /// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
    #[macro_export]
    #[stable(feature = "dbg_macro", since = "1.32.0")]
    macro_rules! dbg {
        ($val:expr) => {
            // Use of `match` here is intentional because it affects the lifetimes
            // of temporaries - https://stackoverflow.com/a/48732525/1063961
            match $val {
                tmp => {
                    eprintln!("[{}:{}] {} = {:#?}",
                        file!(), line!(), stringify!($val), &tmp);
                    tmp
                }
            }
        }
    }

    /// A macro to await on an async call.
    #[macro_export]
    #[unstable(feature = "await_macro", issue = "50547")]
    #[allow_internal_unstable]
    #[allow_internal_unsafe]
    macro_rules! await {
        ($e:expr) => { {
            let mut pinned = $e;
            loop {
                if let $crate::task::Poll::Ready(x) =
                    $crate::future::poll_with_tls_waker(unsafe {
                        $crate::pin::Pin::new_unchecked(&mut pinned)
                    })
                {
                    break x;
                }
                // FIXME(cramertj) prior to stabilizing await, we have to ensure that this
                // can't be used to create a generator on stable via `|| await!()`.
                yield
            }
        } }
    }

    /// 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")]
    #[rustc_deprecated(since = "1.32.0",
                       reason = "channel selection will be removed in a future release")]
    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(rustdoc)]
    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. It's the compiler-level form of [`panic!`],
        /// which emits an error at *runtime*, rather than during compilation.
        ///
        /// # Examples
        ///
        /// Two such examples are macros and `#[cfg]` environments.
        ///
        /// Emit better compiler error if a macro is passed invalid values. Without the final branch,
        /// the compiler would still emit an error, but the error's message would not mention the two
        /// valid values.
        ///
        /// ```compile_fail
        /// macro_rules! give_me_foo_or_bar {
        ///     (foo) => {};
        ///     (bar) => {};
        ///     ($x:ident) => {
        ///         compile_error!("This macro only accepts `foo` or `bar`");
        ///     }
        /// }
        ///
        /// give_me_foo_or_bar!(neither);
        /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
        /// ```
        ///
        /// Emit compiler error if one of a number of features isn't available.
        ///
        /// ```compile_fail
        /// #[cfg(not(any(feature = "foo", feature = "bar")))]
        /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.")
        /// ```
        ///
        /// [`panic!`]: ../std/macro.panic.html
        #[stable(feature = "compile_error_macro", since = "1.20.0")]
        #[rustc_doc_only_macro]
        macro_rules! compile_error {
            ($msg:expr) => ({ /* compiler built-in */ });
            ($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.
        ///
        /// You can use the [`fmt::Arguments`] value that `format_args!` returns
        /// in `Debug` and `Display` contexts as seen below. The example also shows
        /// that `Debug` and `Display` format to the same thing: the interpolated
        /// format string in `format_args!`.
        ///
        /// ```rust
        /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
        /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
        /// assert_eq!("1 foo 2", display);
        /// assert_eq!(display, debug);
        /// ```
        ///
        /// 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")]
        #[rustc_doc_only_macro]
        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 $PATH variable at the time of compiling was: {}", path);
        /// ```
        ///
        /// You can customize the error message by passing a string as the second
        /// parameter:
        ///
        /// ```compile_fail
        /// let doc: &'static str = env!("documentation", "what's that?!");
        /// ```
        ///
        /// If the `documentation` environment variable is not defined, you'll get
        /// the following error:
        ///
        /// ```text
        /// error: what's that?!
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_doc_only_macro]
        macro_rules! env {
            ($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<T>`][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")]
        #[rustc_doc_only_macro]
        macro_rules! option_env {
            ($name:expr) => ({ /* compiler built-in */ });
            ($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")]
        #[rustc_doc_only_macro]
        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")]
        #[rustc_doc_only_macro]
        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 necessarily* the line of the `line!` invocation 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")]
        #[rustc_doc_only_macro]
        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 necessarily* the line of the `column!` invocation 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")]
        #[rustc_doc_only_macro]
        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")]
        #[rustc_doc_only_macro]
        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")]
        #[rustc_doc_only_macro]
        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")]
        #[rustc_doc_only_macro]
        macro_rules! include_str {
            ($file:expr) => ({ /* compiler built-in */ });
            ($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")]
        #[rustc_doc_only_macro]
        macro_rules! include_bytes {
            ($file:expr) => ({ /* compiler built-in */ });
            ($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")]
        #[rustc_doc_only_macro]
        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.
        ///
        /// # Examples
        ///
        /// ```
        /// let my_directory = if cfg!(windows) {
        ///     "windows-specific-directory"
        /// } else {
        ///     "unix-directory"
        /// };
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_doc_only_macro]
        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::<String>()
        /// ```
        ///
        /// 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")]
        #[rustc_doc_only_macro]
        macro_rules! include {
            ($file:expr) => ({ /* compiler built-in */ });
            ($file:expr,) => ({ /* compiler built-in */ });
        }

        /// Ensure that a boolean expression is `true` at runtime.
        ///
        /// This will invoke the [`panic!`] macro if the provided expression cannot be
        /// evaluated to `true` at runtime.
        ///
        /// # Uses
        ///
        /// Assertions are always checked in both debug and release builds, and cannot
        /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
        /// release builds by default.
        ///
        /// Unsafe code relies on `assert!` to enforce run-time invariants that, if
        /// violated could lead to unsafety.
        ///
        /// Other use-cases of `assert!` include testing and enforcing run-time
        /// invariants in safe code (whose violation cannot result in unsafety).
        ///
        /// # Custom Messages
        ///
        /// This macro has a second form, where a custom panic message can
        /// be provided with or without arguments for formatting.  See [`std::fmt`]
        /// for syntax for this form.
        ///
        /// [`panic!`]: macro.panic.html
        /// [`debug_assert!`]: macro.debug_assert.html
        /// [`std::fmt`]: ../std/fmt/index.html
        ///
        /// # Examples
        ///
        /// ```
        /// // the panic message for these assertions is the stringified value of the
        /// // expression given.
        /// assert!(true);
        ///
        /// fn some_computation() -> bool { true } // a very simple function
        ///
        /// assert!(some_computation());
        ///
        /// // assert with a custom message
        /// let x = true;
        /// assert!(x, "x wasn't true!");
        ///
        /// let a = 3; let b = 27;
        /// assert!(a + b == 30, "a = {}, b = {}", a, b);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_doc_only_macro]
        macro_rules! assert {
            ($cond:expr) => ({ /* compiler built-in */ });
            ($cond:expr,) => ({ /* compiler built-in */ });
            ($cond:expr, $($arg:tt)+) => ({ /* 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)*
        }
    }

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