trait FnLt<'a> {
    fn apply(self, input: &'a u8) -> &'a u8;
}

// Struct impl
struct Foo;
impl<'a> FnLt<'a> for Foo {
    fn apply(self, input: &'a u8) -> &'a u8 {
        input
    }
}

// Closure impl
impl<'a, T> FnLt<'a> for T
where
    T: FnOnce(&'a u8) -> &'a u8,
{
    fn apply(self, input: &'a u8) -> &'a u8 {
        (self)(input)
    }
}

fn take_fn_lt(_: impl for<'a> FnLt<'a>) {}

fn fix<F: FnOnce(&u8) -> &u8>(x: F) -> F {
    x
}

fn main() {
    take_fn_lt(Foo); // Works

    fn foo(x: &u8) -> &u8 { x }
    take_fn_lt(foo); // Works

    //take_fn_lt(|x: &u8| -> &u8 { x }); // Doesn't work
    take_fn_lt(fix(|x: &u8| -> &u8 { x })); // Doesn work
}