ICE: cannot relate bound region if using `Self`

[alexcrichton]

This code generates an ICE:

pub trait Foo {
    fn method1() {}
    fn method2();
}

struct Slice<'a, T: 'a>(&'a [T]);

impl <'a, T: 'a> Foo for Slice<'a, T> {
    fn method2() {
        <Self as Foo>::method1();
    }
}

wut.rs:10:9: 10:32 error: internal compiler error: cannot relate bound region: '_#0r <= ReEarlyBound(33, TypeSpace, 0, 'a)
wut.rs:10         <Self as Foo>::method1();
                  ^~~~~~~~~~~~~~~~~~~~~~~
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace

Interestingly enough, if Self as Foo is changed to Slice<'a, T> as Foo then it compiles just fine!

[sfackler]

First seen in rust-postgres. Appears to be a regression from some time in the last couple of days.

[sfackler]

cc @nikomatsakis