Associated type projections don't count for local types in trait impls

[alexcrichton]

I ran into a case with generated code today where I believe the compiler is giving a false error trying to verify a trait implementation.

Given a crate foo defined as:

// foo.rs
pub trait Foo {}

and then a consumer crate bar:

// bar.rs
struct A;

trait B {
    type C;
}

impl B for A {
    type C = MyType;
}

struct MyType;

impl foo::Foo for <A as B>::C {}

when compiled this yields:

$ rustc +nightly foo.rs --crate-type lib
$ rustc +nightly bar.rs --crate-type lib --extern foo=libfoo.rlib
error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
  --> bar.rs:13:1
   |
13 | impl foo::Foo for <A as B>::C {}
   | ^^^^^^^^^^^^^^^^^^-----------
   | |                 |
   | |                 `<A as B>::C` is not defined in the current crate
   | impl doesn't use only types from inside the current crate
   |
   = note: define and implement a trait or new type instead

error: aborting due to previous error

For more information about this error, try `rustc --explain E0117`.

I don't believe that the error here is accurate since A, B, C, and MyType are all local to the crate and the compiler would ideally follow the projection to conclude that this trait implementation is valid.

[jyn514]

This works ok if you rewrite it to impl inner::Foo for MyType {}, so this is probably a bug somewhere in coherence checking.