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

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Some changes occurred to constck

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Some changes occurred to the CTFE / Miri interpreter

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Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

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aapoalas · 2026-02-02T21:16:53Z

r? compiler

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

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

wesleywiser · 2026-04-16T14:12:02Z

Hi @oli-obk, it looks like this is ready for another round of review when you get a chance. Thanks! 🙂

oli-obk · 2026-04-21T10:34:07Z

                let right_ty = rvalue.ty(&self.body.local_decls, self.tcx);

+                if matches!(rvalue, Rvalue::Reborrow(..)) {
+                    // Trust me bro: Reborrow/CoerceShared is only inserted if types match.


wait, but this is validation :D

double-checking seems good, especially to catch optimizations from messing up

View changes since the review

Yeah; the thinking here is that because we only produce an Rvalue::Reborrow from an adjustment, and we only produce such an adjustment if we're looking at an assignment where we're going either from T -> T where T: Reborrow, or we're going from T -> U where T: CoerceShared<U>, then at this point we know we're necessarily doing a T -> T or T -> U and types match.

But double-checking is fine by me, and it does also offer up a future improvement of the implementation where we drop the adjustment and instead perform Reborrow somewhere at a deeper level on every use/access of a Reborrow or CoerceShared type.

Okay; I added checking that the Rvalue::Reborrow(target, ..) type matches left_ty; an early return is still needed to skip calling super_assignment as something there (which I don't quite remember right now) doesn't work with Rvalue::Reborrow.

oli-obk · 2026-04-21T10:39:53Z

+    if !errors.is_empty() { Err(errors) } else { Ok(()) }
+}
+
+pub(crate) fn coerce_shared_info<'tcx>(


how much is the logic of this copied from similar logic on the other coercion traits? Can we share some of the algorithm or sth else here?

View changes since the review

The basic structure and error handling is copied from CoerceUnsized. I'll take a look at possible deduplications.

I deduplicated some code between CoerceShared and Reborrow handling, but didn't find anything significant to dedupe between other coercion traits. Most of the work is in the nitty-gritty field type checking, which is generally pretty unique for each trait.

aapoalas · 2026-04-30T05:32:21Z

@rustbot ready

…idate

rustbot · 2026-05-01T17:48:05Z

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.

rust-bors · 2026-05-02T21:11:29Z

☔ The latest upstream changes (presumably #154341) made this pull request unmergeable. Please resolve the merge conflicts.

oli-obk · 2026-05-05T09:44:43Z

+                    // CoerceShared changes the type from right_ty to target_ty.
+                    *target_ty


that's what rvalue.ty would return, so you can just use the normal code path

View changes since the review

oli-obk · 2026-05-05T09:48:33Z

                }
+
+                if matches!(rvalue, Rvalue::Reborrow(..)) {
+                    // Reborrow/CoerceShared must not call into super_statement.


why? it looks like super_statement just calls super_assign, which calls visit_rvalue, which you already defined as a noop for Rvalue::Reborrow. so the only thing I can think of is the dest being problematic somehow via visit_place?

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rustbot assigned tmandry Jan 27, 2026

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aapoalas force-pushed the aapoalas/reborrow-and-coerce-shared-first-version branch from e4c08f7 to 3b1957a Compare January 27, 2026 17:03

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

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aapoalas force-pushed the aapoalas/reborrow-and-coerce-shared-first-version branch from 7cdb66f to 339fc61 Compare April 16, 2026 17:16

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aapoalas added 8 commits May 1, 2026 20:37

Reborrow traits

91588fe

minor fixme churn

83194b8

fix rebase error

d708f82

review

4f29005

Remove copied-over comments

bb072f5

Add target_ty to left_ty type checking for Reborrow statements in val…

7cf4f19

…idate

Remove some duplication

64c9a03

fix all but one Unnormalized related error

b26bf4c

aapoalas force-pushed the aapoalas/reborrow-and-coerce-shared-first-version branch from aa831d0 to b26bf4c Compare May 1, 2026 17:48

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Remove structurally_normalize_ty in coerce_shared_info

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oli-obk reviewed May 5, 2026

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oli-obk added I-types-nominated Nominated for discussion during a types team meeting. T-types Relevant to the types team, which will review and decide on the PR/issue. and removed T-clippy Relevant to the Clippy team. labels May 5, 2026

		// CoerceShared changes the type from right_ty to target_ty.
		*target_ty

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