src/lib.rs

//! 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))]

// ANDROID: Use std to allow building as a dylib.
#[cfg(android_dylib)]
extern crate std;

/// 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!();
                }
            }
        }
    }
}