Attribute syntax extensions don't expand inside blocks

[huonw]

The following should work:

fn main() {
     #[deriving(Eq)]
     struct A;

     A == A
}

synext-test.rs:6:4: 6:10 error: binary operation == cannot be applied to type `main::A`
synext-test.rs:6     A == A
                         ^~~~~~
error: aborting due to previous error

The syntax extension is not expanded (this isn't specific to deriving, e.g. using #[auto_encode] would give a compile error if it was expanded, but it passes silently).

(I'm fairly sure this used to work, but I'm not certain.)

[bblum]

nominating well-covered milestone

[huonw]

Dup of #4913.