trait MyFnMut: FnMut() {}
impl<T: FnMut()> MyFnMut for T {}

trait MyFn: Fn() {}
impl<T: Fn()> MyFn for T {}

fn constrain_mut<F: MyFnMut>(f: F) -> F { f }
fn constrain<F: MyFn>(f: F) -> F { f }

fn main() {
    constrain_mut(constrain(|| {}));
    constrain(constrain_mut(|| {}));
}