`BufWriter`: Make the soundness of `BorrowedBuf` usage clearer.#155321

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briansmith wants to merge 2 commits intorust-lang:mainfrom

briansmith:b/bufwriter-conservative

Commits on May 2, 2026

`BufWriter`: Make the soundness of `BorrowedBuf` usage clearer.
briansmith
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`BorrowedBuf` docs: Update docs for `is_init` with new semantics.
briansmith
committed