11182: fix: don't panic on seeing an unexpected offset r=Veykril a=dimbleby
Intended as a fix, or at least a sticking plaster, for #11081.
I have arranged that [offset()](1ba9a924d7/crates/ide_db/src/line_index.rs (L105-L107)) returns `Option<TextSize>` instead of going out of bounds; other changes are the result of following the compiler after doing this.
Perhaps there's still an issue here - I suppose the server and client have gotten out of sync and that probably shouldn't happen in the first place? I see that https://github.com/rust-analyzer/rust-analyzer/issues/10138#issuecomment-913727554 suggests what sounds like a more substantial fix which I think might be aimed in this direction. So perhaps that one should be left open to cover such things?
Meanwhile, I hope that not-crashing is a good improvement: and I can confirm that it works out just fine in the repro I have at #11081.
Co-authored-by: David Hotham <david.hotham@metaswitch.com>
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