Omit suggestions when spans overlap

[jdonszelmann]

Proposal

Currently, there are two cases in which suggestions in diagnostics are invalid: their spans span more than one file, or their span overlaps. In the former case, we skip the suggestion silently, in the latter we ICE.

This is the code that skips suggestions when they span multiple files: https://github.com/rust-lang/rust/blob/c8a31b780d5415358566a20b94912620a3f27067/compiler/rustc_errors/src/lib.rs#L371

This usually is good! It often means a broken diagnostic. However, this code shows that because users can change the spans on tokens, they can cause spans to overlap, and diagnostics to ICE:

rust-lang/rust@436c2eb

Currently, we ICE here because when running rustfix, it crashes when suggestions overlap.

My proposal is to, instead of ICEing, omit suggestions with invalid spans, but not silently.
This means we also don't emit these suggestions in JSON output; that way rustfix doesn't stumble over them.

This pull request shows what that would look like: rust-lang/rust#147849, and specifically here:
https://github.com/jdonszelmann/rust/blob/b876dd753e0dcc6be5c0b6c1ed6844b67266dc25/tests/ui/parser/attribute/invalid-delimeter-ice-146808.stderr#L8
is an example of what "non silent omission" could look like.

I think this is better than either crashing or silently omitting suggestions.

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