either_impl.rs

#![feature(unboxed_closures, fn_traits)]

pub enum Either<L, R> {
    Left(L),
    Right(R),
}

impl<L: FnOnce() -> i32, R: FnOnce() -> i32> FnOnce<()> for Either<L, R> {
    type Output = i32;

    extern "rust-call" fn call_once(self, _args: ()) -> i32 {
        match self {
            Either::Left(l) => l(),
            Either::Right(r) => r()
        }
    }
}

impl<L: FnMut() -> i32, R: FnMut() -> i32> FnMut<()> for Either<L, R> {
    extern "rust-call" fn call_mut(&mut self, _args: ()) -> i32 {
        match self {
            Either::Left(l) => l(),
            Either::Right(r) => r()
        }
    }
}

impl<L: Fn() -> i32, R: Fn() -> i32> Fn<()> for Either<L, R> {
    extern "rust-call" fn call(&self, _args: ()) -> i32 {
        match self {
            Either::Left(l) => l(),
            Either::Right(r) => r()
        }
    }    
}

pub extern fn foobarbaz(b: bool) -> Either<impl Fn() -> i32, impl Fn() -> i32> {
    if b {
        Either::Left(|| 10)
    } else {
        Either::Right(|| 20)
    }

}

fn main() {
    println!("{}", foobarbaz(true)());
    println!("{}", foobarbaz(false)());
}