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    //! A macro for defining `#[cfg]` if-else statements.
    //!
    //! The macro provided by this crate, `cfg_if`, 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.
    //!
    //! # Example
    //!
    //! ```
    //! cfg_if::cfg_if! {
    //!     if #[cfg(unix)] {
    //!         fn foo() { /* unix specific functionality */ }
    //!     } else if #[cfg(target_pointer_width = "32")] {
    //!         fn foo() { /* non-unix, 32-bit functionality */ }
    //!     } else {
    //!         fn foo() { /* fallback implementation */ }
    //!     }
    //! }
    //!
    //! # fn main() {}
    //! ```

    #![no_std]
    #![doc(html_root_url = "https://docs.rs/cfg-if")]
    #![deny(missing_docs)]
    #![cfg_attr(test, deny(warnings))]

    /// The main macro provided by this crate. See crate documentation for more
    /// information.
    #[macro_export]
    macro_rules! cfg_if {
        // match if/else chains with a final `else`
        ($(
            if #[cfg($($meta:meta),*)] { $($tokens:tt)* }
        ) else * else {
            $($tokens2:tt)*
        }) => {
            $crate::cfg_if! {
                @__items
                () ;
                $( ( ($($meta),*) ($($tokens)*) ), )*
                ( () ($($tokens2)*) ),
            }
        };

        // match if/else chains lacking a final `else`
        (
            if #[cfg($($i_met:meta),*)] { $($i_tokens:tt)* }
            $(
                else if #[cfg($($e_met:meta),*)] { $($e_tokens:tt)* }
            )*
        ) => {
            $crate::cfg_if! {
                @__items
                () ;
                ( ($($i_met),*) ($($i_tokens)*) ),
                $( ( ($($e_met),*) ($($e_tokens)*) ), )*
                ( () () ),
            }
        };

        // Internal and recursive macro to emit all the items
        //
        // Collects all the negated cfgs in a list at the beginning and after the
        // semicolon is all the remaining items
        (@__items ($($not:meta,)*) ; ) => {};
        (@__items ($($not:meta,)*) ; ( ($($m:meta),*) ($($tokens:tt)*) ), $($rest:tt)*) => {
            // Emit all items within one block, applying an appropriate #[cfg]. The
            // #[cfg] will require all `$m` matchers specified and must also negate
            // all previous matchers.
            #[cfg(all($($m,)* not(any($($not),*))))] $crate::cfg_if! { @__identity $($tokens)* }

            // Recurse to emit all other items in `$rest`, and when we do so add all
            // our `$m` matchers to the list of `$not` matchers as future emissions
            // will have to negate everything we just matched as well.
            $crate::cfg_if! { @__items ($($not,)* $($m,)*) ; $($rest)* }
        };

        // Internal macro to make __apply work out right for different match types,
        // because of how macros matching/expand stuff.
        (@__identity $($tokens:tt)*) => {
            $($tokens)*
        };
    }

    #[cfg(test)]
    mod tests {
        cfg_if! {
            if #[cfg(test)] {
                use core::option::Option as Option2;
                fn works1() -> Option2<u32> { Some(1) }
            } else {
                fn works1() -> Option<u32> { None }
            }
        }

        cfg_if! {
            if #[cfg(foo)] {
                fn works2() -> bool { false }
            } else if #[cfg(test)] {
                fn works2() -> bool { true }
            } else {
                fn works2() -> bool { false }
            }
        }

        cfg_if! {
            if #[cfg(foo)] {
                fn works3() -> bool { false }
            } else {
                fn works3() -> bool { true }
            }
        }

        cfg_if! {
            if #[cfg(test)] {
                use core::option::Option as Option3;
                fn works4() -> Option3<u32> { Some(1) }
            }
        }

        cfg_if! {
            if #[cfg(foo)] {
                fn works5() -> bool { false }
            } else if #[cfg(test)] {
                fn works5() -> bool { true }
            }
        }

        #[test]
        fn it_works() {
            assert!(works1().is_some());
            assert!(works2());
            assert!(works3());
            assert!(works4().is_some());
            assert!(works5());
        }

        #[test]
        #[allow(clippy::assertions_on_constants)]
        fn test_usage_within_a_function() {
            cfg_if! {if #[cfg(debug_assertions)] {
                // we want to put more than one thing here to make sure that they
                // all get configured properly.
                assert!(cfg!(debug_assertions));
                assert_eq!(4, 2+2);
            } else {
                assert!(works1().is_some());
                assert_eq!(10, 5+5);
            }}
        }

        trait Trait {
            fn blah(&self);
        }

        #[allow(dead_code)]
        struct Struct;

        impl Trait for Struct {
            cfg_if! {
                if #[cfg(feature = "blah")] {
                    fn blah(&self) {
                        unimplemented!();
                    }
                } else {
                    fn blah(&self) {
                        unimplemented!();
                    }
                }
            }
        }
    }