src/test/ui/unboxed-closures/unboxed-closures-manual-impl.rs

// run-pass
#![feature(unboxed_closures, fn_traits)]

struct S;

impl FnMut<(i32,)> for S {
    extern "rust-call" fn call_mut(&mut self, (x,): (i32,)) -> i32 {
        x * x
    }
}

impl FnOnce<(i32,)> for S {
    type Output = i32;

    extern "rust-call" fn call_once(mut self, args: (i32,)) -> i32 { self.call_mut(args) }
}

fn call_it<F:FnMut(i32)->i32>(mut f: F, x: i32) -> i32 {
    f(x) + 3
}

fn call_box(f: &mut dyn FnMut(i32) -> i32, x: i32) -> i32 {
    f(x) + 3
}

fn main() {
    let x = call_it(S, 1);
    let y = call_box(&mut S, 1);
    assert_eq!(x, 4);
    assert_eq!(y, 4);
}