Borrowck doesn't propagate mutability to supercomponents correctly

[nikomatsakis]

Borrowck doesn't always propagate mutability correctly to base components.

Here is an example:

struct Foo {
    x: uint
}

struct Bar {
    foo: Foo
}

fn main() {
    let mut b = Bar { foo: Foo { x: 3 } };
    let p = &b;
    let q = &mut b.foo.x;
    let r = &p.foo.x;
    io::println(fmt!("*r = %u", *r));
    *q += 1;
    io::println(fmt!("*r = %u", *r));
}

Here, r has type &uint and thus *r should always yield the same result. The problem is that when creating the loan for &mut we propagate &const as the required type for base loans. This causes us to miss the conflict.

I have a fix for this already prepared.