Experiment: Reborrow traits by aapoalas · Pull Request #151753 · rust-lang/rust

aapoalas · 2026-01-27T16:53:01Z

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With this PR we now have basic functional Reborrow and CoerceShared traits. The current limitations are:

Reborrowable types can only have one lifetime parameter, so as to avoid having to figure out and store in rmeta the information of which lifetimes weaken during reborrowing.
Reborrowing of &mut wrappers is working (though I've not tested generic wrappers like Option<&mut T> yet), but CoerceShared of &mut wrappers currently causes an ICE.

The remaining tasks to complete before I'd consider this PR mergeable are:

Fix ICE on CoerceShared. Unfortunately this might require dipping into rmeta.
Expand the tests to give a more complete view of the current state of the experiment.

Reborrow traits experiment: #145612

Co-authored by @dingxiangfei2009

rustbot · 2026-02-02T21:16:50Z

Some changes occurred in rustc_ty_utils::consts.rs

cc @BoxyUwU

Some changes occurred in match checking

cc @Nadrieril

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to constck

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Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred in compiler/rustc_codegen_cranelift

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Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred in src/tools/clippy

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aapoalas · 2026-02-02T21:16:53Z

r? compiler

compiler/rustc_middle/src/mir/syntax.rs

dingxiangfei2009 · 2026-02-03T10:32:08Z

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

                }
+
+                adjustment::Adjust::GenericReborrow(_reborrow) => {
+                    // Do the magic!


Oh we should call delegates. See the ReborrowPin arm and that is basically what we want to do.

And eventually we should be able to subsume ReborrowPin case hopefully.

dingxiangfei2009 · 2026-02-03T10:34:21Z

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

                    // `&mut T: DerefMut` tho, so it's kinda moot.
                }
+                Adjust::GenericReborrow(_) => {
+                    // No effects to enforce here.


We should do something here. The type would change after a generic reborrow, ie the CoerceShared case.

BoxyUwU · 2026-02-03T10:37:43Z

Where can I read context on what CoerceShared is/why it exists, and what the design for reborrow traits is

compiler/rustc_mir_build/src/builder/expr/as_place.rs

aapoalas · 2026-02-03T11:06:27Z

Where can I read context on what CoerceShared is/why it exists, and what the design for reborrow traits is

I added a link to the experiment tracking issue but put it very shortly: Reborrow is to struct X<'a>(..) what &mut *x is for &mut T, and CoerceShared is the equivalent operation as &*x. CoerceShared takes a type argument instead of having an associated type in expectation of it being possible to reborrow types like struct Double(&mut A, &mut B) which would then have three different "shared reference" variants: struct DR1(&A, &mut B), struct DR2(&mut A, &B), and struct DR3(&A, &B), though I am currently doubting the wisdom of this choice and it may return to using an associated type.

BoxyUwU · 2026-02-03T11:55:47Z

I think I'll just
r? me
on this since I know jack is quite busy with coercions work right now and this should probably have a types reviewer 🤔

aapoalas · 2026-02-03T12:05:25Z

I think I'll just r? me on this since I know jack is quite busy with coercions work right now and this should probably have a types reviewer 🤔

Thank you <3 my top secret plan (designed together with tmandry) was to get a compiler review and then reroll into types to that view as well so that works well :)

jackh726 · 2026-02-03T17:26:57Z

I think I'll just r? me on this since I know jack is quite busy with coercions work right now and this should probably have a types reviewer 🤔

Appreciated.

oli-obk

Not sure this would be better, but if there are also plans to generalize field projections later, it would probably be good to try to keep references and custom ref types in sync

View changes since this review

compiler/rustc_borrowck/src/type_check/mod.rs

rustbot · 2026-02-24T08:54:37Z

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

aapoalas · 2026-02-26T17:53:21Z

Stuck somewhere in the middle of a review, but submitting at this point as I gotta run

I have come back to "this should just be a BorrowKind::Custom", which I don't think will affect perf or complexity (and def not more than this PR does), but I can finish reviewing if you want before doing such a major refactor of the PR

View changes since this review

I would very much appreciate that: I'm very afraid of doing such a major rewrite of the entire of the feature. This isn't exactly a great reason, but from a personal point of view it is quite discouraging to have something working after months of work and then hear that nah, this'd actually better be done an entire different way :D in that sense I would personally prefer landing this and then starting on the rewrite while concurrently gathering feedback from users.

aapoalas · 2026-02-26T18:36:55Z

I have come back to "this should just be a BorrowKind::Custom", which I don't think will affect perf or complexity (and def not more than this PR does), but I can finish reviewing if you want before doing such a major refactor of the PR

We discussed this a little: mir/syntax.rs BorrowKind::Custom kind of seems make sense, but it does leak into a lot of places (like BorrowData in borrow_set.rs) and it might not make sense in all of these places... maybe?

The main question we had was: what was your thinking about usage of the BorrowKind? We'd presumably put use it in ExprKind::Borrow to make generic reborrow part of that family of expressions, but do we then take the logical route and use Rvalue::Ref as well? Therein things really start to make ... maybe less sense? The first payload of Rvalue::Ref is the lifetime '_ in &'_ T, but for Rvalue::Ref(_, BorrowKind::Custom, _) we would never use that lifetime because the correct lifetime to use is coming in the type.

EDIT:

We currently pass the T through ExprKind::Reborrow – if using ExprKind::Borrow we'd presumably have to recover the T through either the source ExprId or through the assignment. We'll try out removing the T type pass-through from ExprKind::Reborrow though, and see if it turns out to be possible to recover it through other means or if it really is necessary.

Turns out we weren't even using the T that we passed through, so that part was definitely unnecessary worries. The lifetime/region in Rvalue::Ref being unused is a thing we would still have.

aapoalas · 2026-02-26T21:57:10Z

@rustbot ready

rustbot · 2026-03-19T19:26:19Z

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.

aapoalas · 2026-03-19T19:29:33Z

@rustbot ready

rustbot assigned tmandry Jan 27, 2026

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aapoalas marked this pull request as ready for review February 2, 2026 21:16

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aapoalas mentioned this pull request Feb 28, 2026

Reborrow traits rust-lang/rust-project-goals#399

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Reborrow traits

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aapoalas requested review from RalfJung, dingxiangfei2009 and oli-obk March 19, 2026 19:32

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