allocator: refactor for stabilisation by nia-e · Pull Request #157428 · rust-lang/rust

nia-e · 2026-06-04T11:47:58Z

Adds my current proposal per the doc in #156882 and follow-up Zulip conversations (notably for dyn-compat) unstably.

r? libs

nia-e · 2026-06-04T14:08:19Z

Note that the no-panic bounds introduced close #156490 and #155746. However, if we want to relax them in the future, we may need to adjust our collection types to be more resilient.

clarfonthey · 2026-06-04T19:19:49Z

@rustbot author

(mostly so you can more clearly signal when you think things are ready; I've commented here already so I'll see any additional changes for review as they're made)

nia-e · 2026-07-26T16:48:05Z

so, cc @maxdexh @theemathas - do you suspect there will be significantly more soundness bugs to iron out on the core traits? i'd lightly push towards landing this on nightly soonish and adjusting stuff in followup PRs.

in the same spirit, i ditched AllocatorEq since it's more in the realm of "potential future extensions" & isn't necessary right now.

maxdexh · 2026-07-26T16:53:40Z

I think the feature is in a great state now, thanks for the awesome work :D

I'll be giving the safety requirements another read and will try to break it again when I get time at the end of next week, but I'm pretty happy with how the API looks right now (and glad that it wasn't stabilized in the old state 😅)

Also if this hits nightly it'll be much easier to work with for me as well, so I'm strongly in favor of that.

maxdexh · 2026-07-26T16:59:37Z

Did you plan to add the A: Send + Sync bounds for Arc in this PR or separately?

theemathas · 2026-07-26T17:00:00Z

Other than the opsem stuff (which I haven't been following at all and therefore don't know the state of), and btree stuff (which is a later problem), I don't think there are other soundness issues. I'll probably have a few more looks at it before stabilization though.

nia-e · 2026-07-26T17:04:33Z

Did you plan to add the A: Send + Sync bounds for Arc in this PR or separately?

i'd leave that for the PR that actually moves the allocator out of the arc, which would be a blocker for stabilising anything like Arc::new_in

maxdexh · 2026-07-26T17:07:54Z

i'd leave that for the PR that actually moves the allocator out of the arc, which would be a blocker for stabilising anything like Arc::new_in

Such a PR will probably be blocked by all the other changes being made to Arc and Rc right now (#132553, which is itself blocked by #141348).

But it's fine if the allocator generic on Arc/Rc stays unstable (I don't like the idea of stabilizing it, but keeping new_in unstable, since people might already start generalizing their APIs)

nia-e · 2026-07-26T17:11:11Z

the plan is for everything allocator-y about Rc and Arc to stay unstable for now, no worries ^^

maxdexh · 2026-07-26T17:12:12Z

Institutional knowledge yay

nia-e · 2026-07-26T17:15:57Z

see the stabilisation doc on the main stabilisation PR (deets somewhat out of date now, but the plan for what to stabilise hasn't been changed) - i've been trying to avoid making too much of this institutional knowledge :D

maxdexh · 2026-07-26T19:26:04Z

For the record, LinkedList is also sensitive to alloc.clone() unwinding. Not filing an issue because I assume this is getting merged soon (unless you want me to file an issue 👀)

nia-e · 2026-07-26T19:27:29Z

please do file issues for everything distinct! we need to keep track of what's outstanding as we go and stabilise things

RalfJung · 2026-07-27T06:54:16Z

Other than the opsem stuff (which I haven't been following at all and therefore don't know the state of), and btree stuff (which is a later problem), I don't think there are other soundness issues. I'll probably have a few more looks at it before stabilization though.

Which opsem stuff? There should be none left since we delegated all opsem magic to a hypothetical future NativeAllocator wrapper.

And there's the unleaking problem but not much we can do about that, that just is fundamentally a tricky footgun.

clarfonthey · 2026-07-27T11:30:32Z

(Have been mostly just waiting for the discussion to settle before doing a proper review, and poking at random simple changes to the code where they don't require much thinking to verify they're correct.)

maxdexh · 2026-07-28T17:31:36Z

@@ -74,7 +74,8 @@ impl fmt::Display for AllocError {
 /// * Moving, subtyping, unsize-coercing, or trait-upcasting an allocator does not change
 ///   what the allocator is equivalent to.
 /// * Copying or cloning allocator results in an allocator that's


Do we require copies to be equivalent? If so, this relies on the orphan rule for Copy (which might be fine since Copy is special), but I'm confused because the AllocatorClone docs do not talk about requirements for copies.

View changes since the review

I thought there was wording to the effect of "if Copy could be implemented for an AllocatorClone type, that must also obey its semantics", but if not I will add it. ty!

What do you mean by "could"? Can we rely on Drop + Copy being disallowed for soundness? I think we didn't discuss this part enough 😅

rustbot assigned clarfonthey Jun 4, 2026

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theemathas mentioned this pull request Jun 5, 2026

Add more safety requirements for Allocator impls #156544

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trait soup reduces again

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straight up testing it. and by it,

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well, let's just say. "my allocator"

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linked and listed

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maxdexh mentioned this pull request Jul 26, 2026

LinkedList::split_off breaks length when allocator clone panics #159982

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the unnecesary bits? gone. reduced to atoms

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doc wording......

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