Correct procedure to retire compile diagnostic codes

[huonw]

Currently the rustc bootstrap has a persistent warning: diagnostic code E0123 never used warning. The E0123 code was introduced and then removed, but it was apparently not retired correctly.

Do we have a procedure for retirement? (just delete the declaration line?)

cc @jakub-

[steveklabnik]

In general, I would like to see error codes be immutable, so that they aren't re-used for a different error later.

[huonw]

I believe that's the plan; if we are deleting to retire, we theoretically achieve that by only adding incrementally to the end of the code declarations (I guess we'd have to be a bit careful if an entry last in the list is retired).

[ghost]

Yes, we should be removing unused codes. Sorry for not keeping up with these, busy times. :) Removing the code from diagnostics.rs is sufficient.

Error codes are definitely not supposed to be reused as Steve is saying. As long as we only keep adding codes to the end of the list, we're set, although some kind of an informatory comment in the file wouldn't hurt.