Use strongly typed wrapped indices in `VecDeque` by Kobzol · Pull Request #152112 · rust-lang/rust

Kobzol · 2026-02-04T15:35:06Z

This is far from perfect, but it would have prevented #151769.

joboet

I love the general approach, though I have a couple of nits. And personally I'd chose BufferIndex over WrappedIndex, but I'm fine with whichever you choose.

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joboet · 2026-02-07T15:02:16Z

+    pub(super) struct WrappedIndex(usize);
+
+    impl WrappedIndex {
+        /// Safety invariant: the newly constructed index must be in-bounds for the VecDeque


I don't think this safety invariant is particularly useful, as all functions that act on the buffer still need to require that the WrappedIndex is specific to that particular VecDeque with a given capacity.

Yeah, it's a relatively weak invariant. That being said, I still thought it would be a good idea to provide a "SLOW DOWN" sign when someone wants to conjure the index out of the blue, which is why I marked the function unsafe.

I'm just not a big fan of redundant unsafe (this can neither cause UB itself nor be used in safety proofs). Isn't the WrappedIndex::new call enough of a stop-sign?

Okay, fair enough. Maybe we could just name it something like from_arbitrary_number? 😆 Should I also remove unsafe from add and sub?

Yeah, that sounds good. add and sub can stay unsafe, that would allow changing the adds and subs to unchanged in the future (if that turns out to be profitable).

joboet · 2026-02-07T15:03:24Z

+        }
+
+        #[inline(always)]
+        pub(super) fn abs_diff(self, other: Self) -> Self {


The return type should be usize, right? It isn't a buffer index but rather a length.

Right, the type represents something like "value in-bounds". My thought process was that if we have two in-bounds values, and we do an absolute diff of them, the result also has to be in-bounds (w.r.t. the same VecDeque). But we do treat it as usize everywhere the function is currently used, so.. I'll change it to usize.

Kobzol · 2026-02-07T20:12:07Z

I did name if BufferIndex index originally 😆 I guess both works.

Kobzol · 2026-02-07T20:21:19Z

I do slightly prefer the Wrapped name, I think that the buffer part is implied by the word "index". The main idea is to express that the index was wrapped to not go over bounds, which is how it is created through most means.

@rustbot ready

joboet · 2026-03-13T16:14:14Z

This probably needs a rebase atop #152258. Sorry for taking so long, staring at so many lines makes me dizzy, I've not been in the right mind to do it...

Kobzol · 2026-03-17T10:10:29Z

Huh, the rebase went cleanly without any conflict, surprising.. 😆

No problem, I know from experience how draining it can be to review large diffs. Thank you very much for taking a look!

joboet · 2026-05-07T10:49:38Z

-        let tail_len = tail;
+        let head_len = cap - head.as_index();
+
+        // Safety: tail <= head_len <= len <= capacity


tail <= head_len isn't correct (tail <= head is, though).

View changes since the review

Good catch. And also the <= len comparison doesn't hold either. Changed it to tail <= head < capacity.

joboet · 2026-05-07T10:53:11Z

+    pub(super) struct WrappedIndex(usize);
+
+    impl WrappedIndex {
+        /// Safety invariant: the newly constructed index must be in-bounds for the VecDeque


Yeah, that sounds good. add and sub can stay unsafe, that would allow changing the adds and subs to unchanged in the future (if that turns out to be profitable).

rustbot · 2026-05-09T10:55:26Z

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.

Kobzol · 2026-05-09T10:57:23Z

@rustbot ready

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rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Feb 4, 2026

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Kobzol force-pushed the vec-deque-strongly-typed branch from 38ead54 to 5aed3d9 Compare February 6, 2026 19:35

joboet requested changes Feb 7, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 7, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 7, 2026

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Kobzol force-pushed the vec-deque-strongly-typed branch from 72a5f8f to 841763e Compare February 7, 2026 20:40

Kobzol force-pushed the vec-deque-strongly-typed branch from 841763e to 9c52ec0 Compare March 17, 2026 10:10

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joboet requested changes May 7, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 7, 2026

Kobzol added 3 commits May 9, 2026 11:55

Use strongly typed wrapped indices in VecDeque

e36f74d

Update debugger visualizers

a8e2a74

Change abs_diff return type to usize

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Kobzol force-pushed the vec-deque-strongly-typed branch from 9c52ec0 to 31abab1 Compare May 9, 2026 10:55

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 9, 2026

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Kobzol force-pushed the vec-deque-strongly-typed branch from 31abab1 to 8044982 Compare May 9, 2026 11:26

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Kobzol force-pushed the vec-deque-strongly-typed branch from 8044982 to 5ee2f79 Compare May 10, 2026 07:20

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Rename WrappedIndex::new to from_arbitrary_number and resolve rev…

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Kobzol force-pushed the vec-deque-strongly-typed branch from 5ee2f79 to aa9f27e Compare May 10, 2026 08:11

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