Make `BorrowedBuf` and `BorrowedCursor` generic over the data by joshtriplett · Pull Request #149749 · rust-lang/rust

joshtriplett · 2025-12-07T23:52:40Z

Replace uses of u8 with a generic type. This will allow reusing
BorrowedBuf elsewhere in the standard library, for purposes other than
byte buffers.

This currently requires T: Copy, to prevent T: Drop, which would require additional careful handling. We can consider relaxing that restriction in the future if we make T: Drop work (without pessimizing Copy types like u8).

ensure_init currently continues to require u8. We could potentially generalize it to T: Default, but we'd need to ensure it produced the same or better assembly for u8.

As discussed in libs-api; cc @Amanieu.

rustbot · 2025-12-07T23:52:45Z

r? @scottmcm

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Amanieu

r=me for the rest

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bors · 2025-12-17T23:10:15Z

☔ The latest upstream changes (presumably #150106) made this pull request unmergeable. Please resolve the merge conflicts.

scottmcm

I think it'd be fine to land this PR for generic-izing the type, then we can land another PR right afterwards to remove the default and update the uses.

Code looks good to me, and I tried to scan for anything making unsaid assumptions about T (being Copy, Freeze, etc) and didn't find any that weren't already updated.

So r=me with conflicts fixed.

Aside: excited to try this out with generic types. Might be a great way to solve some of the Iterator::next_chunk or Vec::push_within_existing_capacity scenarios.

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joshtriplett · 2026-03-09T23:11:55Z

I'm going to wait to update this until after #150129 is merged.

Replace uses of `u8` with a generic type. This will allow reusing `BorrowedBuf` elsewhere in the standard library, for purposes other than byte buffers. This currently requires `T: Copy`, to prevent `T: Drop`, which would require additional careful handling. We can consider relaxing that restriction in the future if we make `T: Drop` work (without pessimizing `Copy` types like `u8`). `ensure_init` currently continues to require `u8`. We could potentially generalize it to `T: Default`, but we'd need to ensure it produced the same or better assembly for `u8`.

rustbot · 2026-05-25T12:19:27Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

…tes"

This default simplified the commit making these types generic, but we don't actually want the default. Rewrite every user to specify `u8` explicitly.

joshtriplett · 2026-05-30T16:19:02Z

@rustbot ready

This is ready for review.

I would suggest looking at it commit by commit, as the change to remove the default T = u8 is very large and noisy, but largely mechanical.

joshtriplett · 2026-05-30T16:19:22Z

r? libs

Now that the `impl` block has a bound, the one on `append` became redundant.

joshtriplett · 2026-06-02T04:02:31Z

Pushing one additional commit with a minor fix (no semantic effect).

Caught by an experiment trying PR review with an AI; used for review only, no code. It noted seven issues, of which 1 was real (this one), 1 was obviously wrong (suggesting a test that would have called set_init with the buffer still uninitialized), 1 was subtly wrong (it would have provided a safe conjure_zst), and 4 were quibbles about code refactoring in diff context that this PR doesn't change.

joboet · 2026-06-11T18:11:01Z

Looks fine!
@bors r+ rollup

rust-bors · 2026-06-11T18:11:04Z

📌 Commit 5948b47 has been approved by joboet

It is now in the queue for this repository.

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Amanieu reviewed Dec 9, 2025

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Comment thread library/core/src/io/borrowed_buf.rs Outdated

Comment thread library/core/src/io/borrowed_buf.rs Outdated

Amanieu mentioned this pull request Dec 12, 2025

Implement Random for array #136732

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scottmcm mentioned this pull request Dec 21, 2025

Tracking Issue for core_io_borrowed_buf #117693

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scottmcm approved these changes Dec 21, 2025

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Comment thread library/core/src/io/borrowed_buf.rs

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scottmcm removed their assignment May 18, 2026

joshtriplett force-pushed the borrowed-buf-generic branch from c150931 to f7b1581 Compare May 25, 2026 12:19

briansmith reviewed May 26, 2026

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joshtriplett added 3 commits May 29, 2026 15:52

Avoid requiring T: Copy for Debug impls

542fcb1

BorrowedBuf: Rewrite docs to say "elements" rather than "data" or "by…

2c60b26

…tes"

Remove default T = u8 from BorrowedBuf and BorrowedCursor

a62ea44

This default simplified the commit making these types generic, but we don't actually want the default. Rewrite every user to specify `u8` explicitly.

rustbot added O-SGX Target: SGX O-unix Operating system: Unix-like O-wasi Operating system: Wasi, Webassembly System Interface O-wasm Target: WASM (WebAssembly), http://webassembly.org/ labels May 30, 2026

rustbot added the O-windows Operating system: Windows label May 30, 2026

joshtriplett added 2 commits May 30, 2026 07:36

BorrowedBuf: Add tests for non-u8 buffers/cursors

a870808

BorrowedBuf: Add tests for ZST buffers/cursors

b8fe7a8

joshtriplett removed O-windows Operating system: Windows O-wasm Target: WASM (WebAssembly), http://webassembly.org/ O-SGX Target: SGX O-unix Operating system: Unix-like O-wasi Operating system: Wasi, Webassembly System Interface labels May 30, 2026

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rustbot added the T-libs Relevant to the library team, which will review and decide on the PR/issue. label May 30, 2026

rustbot assigned joboet May 30, 2026

Remove redundant T: Copy bound in BorrowedCursor::append

5948b47

Now that the `impl` block has a bound, the one on `append` became redundant.

rustbot added O-SGX Target: SGX O-unix Operating system: Unix-like O-wasi Operating system: Wasi, Webassembly System Interface O-wasm Target: WASM (WebAssembly), http://webassembly.org/ O-windows Operating system: Windows labels Jun 2, 2026

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