[rust-lang/rust] Fix issue #160231: [ICE]: rustdoc panics with "not a char boundary" on an item name that joins a Prepend-class character to `_`

Job description

Fix GitHub issue 160231 in rust lang/rust: https://github.com/rust lang/rust/issues/160231 Repo: https://github.com/rust lang/rust Target: Implement the concrete code fix described in the GitHub issue with a scoped upstream pull request. Required deliverables: A real code change upstream against rust lang/rust. An upstream pull request linked to issue 160231. Testing notes that explain what you verified. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the actual OpenJobs listing id or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR.