Macro cfg_match

pub macro cfg_match {
    ({ $($tt:tt)* }) => { ... },
    (_ => { $($output:tt)* }) => { ... },
    ($cfg:meta => $output:tt $($($rest:tt)+)?) => { ... },
}

🔬This is a nightly-only experimental API. (cfg_match)

A macro for defining #[cfg] match-like statements.

It 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.

Trailing _ wildcard match arms are optional and they indicate a fallback branch when all previous declarations do not evaluate to true.

Example

#![feature(cfg_match)]

cfg_match! {
    unix => {
        fn foo() { /* unix specific functionality */ }
    }
    target_pointer_width = "32" => {
        fn foo() { /* non-unix, 32-bit functionality */ }
    }
    _ => {
        fn foo() { /* fallback implementation */ }
    }
}

If desired, it is possible to return expressions through the use of surrounding braces:

#![feature(cfg_match)]

let _some_string = cfg_match! {{
    unix => { "With great power comes great electricity bills" }
    _ => { "Behind every successful diet is an unwatched pizza" }
}};