fix: Overlapping spans in delimited meta-vars

[EliseZeroTwo]

Closes #118786

Delimited meta-vars inside of MBE's spans were set to have the same opening and closing position resulting in an ICE when debug assertions were enabled and an error was present in the templated code.

This ensures that the spans do not overlap, whilst still having the spans point at the usage of the meta-var inside the macro definition.

It includes a regression test.

🖤

[rustbot]

r? @cjgillot

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[petrochenkov]

I have a PR in progress that removes this pile of hacks (if tts.len() == rhs.tts.len() { ... }) entirely.
It should be fine to land this for the test though.

[cjgillot]

I'd have asked for a comment explaining the logic, but let's avoid the bother if this code is going away soon.
@bors r+ rollup

[bors]

📌 Commit 770013d has been approved by cjgillot

It is now in the queue for this repository.

[EliseZeroTwo]

I'd have asked for a comment explaining the logic, but let's avoid the bother if this code is going away soon. @bors r+ rollup

The branch taken in the match statement without this change set both the open and close of the dspan to the entire span of the metavar which resulted in an overlapping span. With this change it sets open to the entire span of the metavar, and the close to the metavar span shrunk to just hi, which no longer resulted in an overlapping span.

Sorry for not providing more of an explanation of the changes in my pull request, I will try to make more of an explanation of the changes in future PRs.

[petrochenkov]

I have a PR in progress that removes this pile of hacks (if tts.len() == rhs.tts.len() { ... }) entirely.

Submitted in #119204.