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)

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.

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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 😅

nia-e · 2026-08-04T18:11:06Z

(apologies for the squash-and-rebase, nothing meaningful to review should have changed between force-pushes)

clarfonthey · 2026-08-06T04:54:08Z

-        if self.len() > source.len() {
-            self.split_off(source.len());
-        }
-        for (elem, source_elem) in self.iter_mut().zip(&mut source_iter) {
+        for elem in self.iter_mut() {
+            let Some(source_elem) = source_iter.next() else {
+                break;
+            };
            elem.clone_from(source_elem);
        }
+        while self.len() > source.len() {
+            self.pop_back();
+        }


Unreasonably upset at this code just because of how limited the LinkedList API is. Should be doable, but isn't… :(

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samesies qwq

Darksonn · 2026-08-06T09:31:50Z

This PR seems to make multiple unrelated changes. Is it possible to get an overview of all the changes it's making? Or even better, perhaps we could split out this into multiple PRs so that we can merge the things we agree on now, without blocking them on the things still being discussed?

nia-e · 2026-08-06T09:52:56Z

I'm unsure how i'd go about splitting this up, since many of the miscellaneous library changes are necessary for the API to be sound. (ofc, if you have an idea that'd lighten the review burden, i'm happy to do it and i think @clarfonthey would be relieved too)

In short, the changes made are:

split out requirements from Allocator into the 2 new traits StaticAllocator and AllocatorClone, as not all allocators we want to support can obey those semantics
- StaticAllocator is required for pinning and acting as a global allocator, effectively being a promise that this allocator will never free memory without an explicit deallocate call (and not from e.g. dropping)
- AllocatorClone promises semantics about what cloning an allocator does, namely that it requires the clone not to unwind and that the new allocator is interchangeable with the old one. this is
update std APIs to use these new trait bounds where necessary for soundness
implement said traits for std pointer types where doing so is sound, to facilitate using methods bound by them even if the new traits take a longer while to be proposed for stabilisation than the main one (i guess this could be split out?)

i'm quite happy with the fact that (beyond adding the ban on unwinding out of an allocating method or drop) none of this tightens requirements on Allocator implementors at all.

maxdexh · 2026-08-06T12:48:17Z

Also, unlike the old allocator api, we think this is sound (modulo some details in the docs we haven't discussed fully) :D

yk what we can do btree later raaaaagh pain and suffering *unleaks your box* whats thiSegmentation fault (core dumped) straight up testing it. and by it, well, let's just say. "my allocator" linked and listed the unnecesary bits? gone. reduced to atoms

rustbot · 2026-08-09T15:38:43Z

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.

nia-e · 2026-08-09T15:38:48Z

(squashed & rebased since #56935 conflicted; only change was renaming PinCoerceUnsized to PinSafePointer per that PR)

rustbot assigned clarfonthey Jun 4, 2026

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

nia-e added the A-allocators Area: Custom and system allocators label Jun 4, 2026

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nia-e commented 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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maxdexh reviewed Jul 28, 2026

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RalfJung mentioned this pull request Aug 3, 2026

Writing an invalid value through a reference the caller can never use again rust-lang/unsafe-code-guidelines#618

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nia-e force-pushed the allocator-refactor branch from a41958f to acb148f Compare August 4, 2026 18:10

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nia-e mentioned this pull request Aug 8, 2026

Roadmap 2.0 rust-lang/wg-allocators#150

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nia-e force-pushed the allocator-refactor branch from 791e139 to f4d4ee7 Compare August 9, 2026 15:38

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here's your damn trait name

7cab1f0

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