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Add compiler error when trying to use concat metavar expr in repetitions ## 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](https://github.com/rust-lang/rust/issues/124225) 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 rust-lang/rust#140479.