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    // 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 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, 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<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.
    ///
    /// [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 errornous 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<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")]
        #[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::<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")]
        #[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)*
        }
    }

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