Add compiler error when trying to use concat metavar expr in repetitions

[jullanggit]

Disclaimer

This is my first PR to rust, so if I missed/could improve something about this PR, please excuse and tell me!

The improvement

The metavar_expr_concat feature currently does not seem to support nested repetitions, and throws an ICE without much explanation if the relevant code path is hit.
This PR adds a draft compiler error that attempts to explain the issue. I am not 100% sure what all the ways of triggering this error are, so the message is currently pretty generic, please do correct me if there's something wrong with it or it could be improved.

Thank you for you time!

Fixes #140479.

[rustbot]

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[jullanggit]

@rustbot label F-macro_metavar_expr_concat

[fmease]

Please add a regression test, squash your commits and make the error message start with a lower case letter (https://rustc-dev-guide.rust-lang.org/diagnostics.html?highlight=diagnostic#diagnostic-output-style-guide)

[jullanggit]

@fmease like this?

[jullanggit]

I seem to have fixed the crash from #140479, and already covered the issue in my regression test. Should I just remove the crash test?

[rustbot]

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

[jullanggit]

Is everything alright now?

[fmease]

Thanks, we're good!

@bors r+ rollup

[bors]

📌 Commit f6e7c81 has been approved by fmease

It is now in the queue for this repository.