unboxed_closures - The Rust Unstable Book

# `unboxed_closures`

The tracking issue for this feature is #29625

See Also: `fn_traits`

The `unboxed_closures` feature allows you to write functions using the `"rust-call"` ABI, required for implementing the `Fn*` family of traits. `"rust-call"` functions must have exactly one (non self) argument, a tuple representing the argument list.

    #![feature(unboxed_closures)]

    extern "rust-call" fn add_args(args: (u32, u32)) -> u32 {
        args.0 + args.1
    }

    fn main() {}