fixed VecDeque::splice() not filling the buffer correctly when resizing the buffer on start = end range by asder8215 · Pull Request #152258 · rust-lang/rust

asder8215 · 2026-02-06T22:22:04Z

This PR fixes #151758. The issue came from Splice::move_tail, which as joboet pointed out:

The issue is in move_tail, which resizes the buffer, but fails to account for the resulting hole.

The problem with reserving more space through VecDeque's buf.reserve() is that it doesn't update VecDeque's head, which means that this code in move_tail:

deque.wrap_copy(
    deque.to_physical_idx(tail_start),
    deque.to_physical_idx(new_tail_start),
    self.tail_len,
);

could move over the section of data that tail_start..tail_start + self.tail_len of the buffer is supposed to be held at to the incorrect destination since all .to_physical_idx() is doing is a wrapping add on the VecDeque's head with the passed in idx value.

To avoid this I decided to use VecDeque::reserve to both allocate more space into the VecDeque if necessary and update head appropriately. However, VecDeque::reserve internally relies on the VecDeque's len field. Earlier in VecDeque::splice, it modifies the VecDeque's len constructing the drain via Drain::new (as it does a mem::replace on deque.len with the start bound of the passed in range). The VecDeque's len can also be potentially modified in the earlier Splice::fill() call if it does any replacement towards elements within the passed in range value for VecDeque::splice(). I needed to temporarily restore the VecDeque's len to its actual len, so that VecDeque::reserve can work properly. Afterward, you can bring back the VecDeque's len to what its value was before and fill the gap appropriately with the rest of the replace_with content.

r? @joboet

rustbot · 2026-02-06T22:22:10Z

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joboet

I'm working on a patch that is more clever about copying the elements, but this is certainly a good fix for the meantime. There's an issue though...

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library/alloc/src/collections/vec_deque/splice.rs

rustbot · 2026-02-07T15:32:47Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

library/alloc/src/collections/vec_deque/splice.rs

asder8215 · 2026-02-07T15:54:08Z

@rustbot ready

library/alloc/src/collections/vec_deque/splice.rs

joboet · 2026-02-23T12:58:34Z

Looks great, thank you! Could you squash your commits, please? I'll approve after that.

asder8215 · 2026-02-23T16:09:52Z

@joboet put the change into one commit. Lmk if there's anything else I need to do!

library/alloc/src/collections/vec_deque/splice.rs

…ng the buffer on start = end range

asder8215 · 2026-03-01T03:30:30Z

Hope the new comment/rationale on the change is okay!

asder8215 · 2026-03-11T21:48:13Z

@joboet I apologize for the ping, was wondering if it would be possible for you to review this within a couple of days?

joboet · 2026-03-12T22:52:45Z

@bors r+

rust-bors · 2026-03-12T22:52:49Z

📌 Commit d0a33ab has been approved by joboet

It is now in the queue for this repository.

@joboet

…r=joboet fixed VecDeque::splice() not filling the buffer correctly when resizing the buffer on start = end range This PR fixes rust-lang#151758. The issue came from `Splice::move_tail`, which as joboet pointed out: > The issue is in move_tail, which resizes the buffer, but fails to account for the resulting hole. The problem with reserving more space through `VecDeque`'s `buf.reserve()` is that it doesn't update `VecDeque`'s `head`, which means that this code in `move_tail`: ```rust deque.wrap_copy( deque.to_physical_idx(tail_start), deque.to_physical_idx(new_tail_start), self.tail_len, ); ``` could move over the section of data that `tail_start..tail_start + self.tail_len` of the buffer is supposed to be held at to the incorrect destination since all `.to_physical_idx()` is doing is a wrapping add on the `VecDeque`'s head with the passed in `idx` value. To avoid this I decided to use `VecDeque::reserve` to both allocate more space into the `VecDeque` if necessary and update head appropriately. However, `VecDeque::reserve` internally relies on the `VecDeque`'s `len` field. Earlier in `VecDeque::splice`, it modifies the `VecDeque`'s `len` constructing the drain via `Drain::new` (as it does a `mem::replace` on `deque.len` with the start bound of the passed in `range`). The `VecDeque`'s `len` can also be potentially modified in the earlier `Splice::fill()` call if it does any replacement towards elements within the passed in `range` value for `VecDeque::splice()`. I needed to temporarily restore the `VecDeque`'s `len` to its actual len, so that `VecDeque::reserve` can work properly. Afterward, you can bring back the `VecDeque`'s `len` to what its value was before and fill the gap appropriately with the rest of the `replace_with` content. r? @joboet

Rollup of 5 pull requests Successful merges: - #152258 (fixed VecDeque::splice() not filling the buffer correctly when resizing the buffer on start = end range) - #153691 (Miscellaneous tweaks) - #153766 (`try_execute_query` tweaks) - #153188 (implementation of `-Z min-recursion-limit`) - #153428 (Avoid duplicated query modifier comments.)

Rollup of 5 pull requests Successful merges: - rust-lang/rust#152258 (fixed VecDeque::splice() not filling the buffer correctly when resizing the buffer on start = end range) - rust-lang/rust#153691 (Miscellaneous tweaks) - rust-lang/rust#153766 (`try_execute_query` tweaks) - rust-lang/rust#153188 (implementation of `-Z min-recursion-limit`) - rust-lang/rust#153428 (Avoid duplicated query modifier comments.)

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fixed VecDeque::splice() not filling the buffer correctly when resizi…

d0a33ab

…ng the buffer on start = end range

asder8215 force-pushed the vecdeque_splice_151758 branch from bb67ee2 to d0a33ab Compare March 1, 2026 03:29

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This was referenced Mar 13, 2026

Rollup of 5 pull requests #153802

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Rollup of 5 pull requests #153805

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rust-bors bot merged commit c5faf3a into rust-lang:main Mar 13, 2026
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rustbot added this to the 1.96.0 milestone Mar 13, 2026

joboet mentioned this pull request Mar 13, 2026

Use strongly typed wrapped indices in VecDeque #152112

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