Eliminate rustc_attrs::builtin::handle_errors in favor of emitting errors directly

[dtolnay]

Suggested in #116773 (review).

This handle_errors function is originally from #34531, in which it was useful because it allowed error messages and error codes (E0542) for multiple occurrences of the same error to be centralized in one place. For example rather than repeating this diagnostic in 2 places:

span_err!(diagnostic, attr.span, E0542, "missing 'since'");

one could repeat this instead:

handle_errors(diagnostic, attr.span, AttrError::MissingSince);

ensuring that all "missing 'since'" errors always remained consistent in message and error code.

Over time as error messages and error codes got factored to fluent diagnostics (#100836), this rationale no longer applies. The new code has the same benefit while being less verbose (+73, -128).

sess.emit_err(session_diagnostics::MissingSince { span: attr.span });

r? @cjgillot

[cjgillot]

@bors r+ rollup

[bors]

📌 Commit 2878529 has been approved by cjgillot

It is now in the queue for this repository.

[cjgillot]

@bors r+

[bors]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: Avoid a track_errors by bubbling up most errors from check_well_formed #116849

[bors]

📌 Commit 2878529 has been approved by cjgillot

It is now in the queue for this repository.