Absolute paths in enum variant names cause "a variety of fun errors"

[brendanzab]

enum ABC {
    A, B, C
}

mod test {
    fn test(abc: ::ABC) {
        match abc {
            ::A => {}
            ::B => {}
            ::C => {}
        }
    }
}

fn main() {}

$ rustc enum-abc.rs
enum-abc.rs:6:17: 6:22 error: use of undeclared type name `ABC`
enum-abc.rs:6     fn test(abc: ::ABC) {
                               ^~~~~
enum-abc.rs:8:12: 8:15 error: unresolved name: `A`.
enum-abc.rs:8             ::A => {}
                          ^~~
enum-abc.rs:9:12: 9:15 error: unresolved name: `B`.
enum-abc.rs:9             ::B => {}
                          ^~~
enum-abc.rs:10:12: 10:15 error: unresolved name: `C`.
enum-abc.rs:10             ::C => {}
                           ^~~
error: aborting due to 4 previous errors

---

pub enum ABC {
    A, B, C
}

mod test {
    fn test(abc: ::ABC) {
        match abc {
            ::A => {}
            ::B => {}
            ::C => {}
        }
    }
}

fn main() {}

$ rustc enum-abc.rs
error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug
note: try running with RUST_LOG=rustc=1,::rt::backtrace to get further details and report the results to github.com/mozilla/rust/issues

---

enum ABC {
    A(()), B, C
}

mod test {
    fn test(abc: ::ABC) {
        match abc {
            ::A(_) => {}
            ::B    => {}
            ::C    => {}
        }
    }
}

fn main() {}

$ rustc enum-abc.rs
enum-abc.rs:8:16: 8:17 error: unexpected token: `_`
enum-abc.rs:8             ::A(_) => {}
                              ^

[nikomatsakis]

(Note: the title really only applies to the final case)

[brendanzab]

@nikomatsakis title changed :)

[catamorphism]

I think this is "1, well-defined" and not "3, feature-complete". Re-nominating.

[brendanzab]

This should probably also be marked with an ICE tag.

[huonw]

This is the same as #6449, I guess one can be closed?

[graydon]

continued in #6449

[pnkfelix]

linking to #6143 for unified tracking of resolve