exponential compile times for simple function compositions

[aliemjay]

Each additional statement let x = Apply(x, drop); surprisingly increases the compilation time by a factor of two:

struct Apply<X, F: Fn(X)>(X, F);

pub fn main() {
    let x = &();
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let x = Apply(x, drop);
    let _ = x;
}

The reason behind this exponential blowup is that the function drop has an "early-bound" generic parameter (drop::<_>), making the types of x grow exponentially with each level of nesting type X2 = Apply<X1, drop::<X1>>;

This should be fixed by removing the distinction between early- and late-bound function generics.