Better error message for matching against not-imported enum variant

[edwardw]

If only the enumeration type itself is imported but not its variants, a match expression reports unreachable pattern instead of unrecognised type:

pub mod inner {
  pub enum E {
    E1,
    E2
  }
}

pub mod foo {
  use inner::E; // change this to use inner::{E, E1, E2} then all will be fine

  pub fn bar(e: E) -> int {
    match e {
      E1 => 0,
      E2 => 1    // error: unreachable pattern
    }
  }
}

pub fn main() {}

Is this something intentional? Or should we give it a more sensible error message?

[pnkfelix]

This is a duplicate of #4612, #4639, #7807, #10402.

The above were closed either as dupes of each other or with a statement saying that the work remaining was captured by a more general lint for our capitalization convention. I am inclined to disagree with the latter: a very specific error message can and should be applied here, IMO.

Anyway, the suggested lints are in metabug #3070 and more specifically #10304 and #7526 (a related issue).

[pnkfelix]

Also PR #12300 will probably help with this (its basically the aforementioned capitalization lint). Though I continue to think a more specific error check is warranted here.

(In particular, automatic FFI binding generators will in general introduce names that do not fit our capitalization conventions. My opinion is that we should not attempt to force binding generators to follow our conventions in all cases. In a case like this, we can provide a better error message, e.g. via the strategy outlined in #4639.)

[pnkfelix]

closing as duplicate of the issues mentioned in earlier comments.