librustc: Check built-in trait bounds on implementations when direct

[pcwalton]

method calls are involved.

This breaks code like:

impl<T:Copy> Foo for T { ... }

fn take_param<T:Foo>(foo: &T) { ... }

fn main() {
    let x = box 3i; // note no `Copy` bound
    take_param(&x);
}

Change this code to not contain a type error. For example:

impl<T:Copy> Foo for T { ... }

fn take_param<T:Foo>(foo: &T) { ... }

fn main() {
    let x = 3i; // satisfies `Copy` bound
    take_param(&x);
}

Closes #15860.

[breaking-change]

r? @alexcrichton

[alexcrichton]

This looks fine to me, but I'm not personally familiar enough with vtable-related code to render a verdict. Perhaps @nikomatsakis, @nick29581, or @pnkfelix could take a look?

[pcwalton]

r? @pnkfelix

[pcwalton]

Heh, we were relying on this in rustc.