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.


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

fn main() {}