Experiment: Reborrow traits #151753

aapoalas · 2026-01-27T16:53:01Z

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

Signed-off-by: Xiangfei Ding <[email protected]>

…nmail.ch>

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

cc @fee1-dead

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

aapoalas · 2026-02-02T21:16:53Z

r? compiler

RalfJung · 2026-02-03T08:29:27Z

compiler/rustc_middle/src/mir/syntax.rs

    /// Wraps a value in an unsafe binder.
    WrapUnsafeBinder(Operand<'tcx>, Ty<'tcx>),
+
+    /// A reborrowable type being reborrowed


Please explain the runtime behavior of this operation.

The interpreter implementation seems to indicate that this is equivalent to a normal assignment of Copy($place). Why is it even a separate statement?

Right, my apologies, I definitely should've added more information around this: I added a link to the experiment tracking issue into the opening comment which contains some more information and links but basically: in the trivial case (#[repr(transparent)]-compatible memory layout of types and a single lifetime) Reborrow is a Copy plus extra borrow checking analysis to perform the same reborrowing operation as happens with &'a mut T, but now on some user-defined ADT struct Custom<'a>(..). CoerceShared is then the &'a mut T -> &'a T equivalent of this operation and thus also requires a user-defined target type.

This PR does not implement any non-trivial cases so in both cases the runtime effect is exactly equivalent to a Copy, but future expansions should make CoerceShared possible even if the layouts of the source and target types do not match, including the possibility for the target type to drop out some fields from the source type. (This has a requested use-case where a custom exclusive reference type has some write-time metadata which is then not necessary in a read-only reference.)

I added a link to the experiment tracking issue

That link is broken btw.

future expansions should make CoerceShared possible even if the layouts of the source and target types do not match, including the possibility for the target type to drop out some fields from the source type.

That sounds like an operation that is too complicated to be a single MIR statement, it should instead be encoded as a sequence of simpler operations. Otherwise every MIR consumer has to re-implement all that logic, partially defeating the point of having an IR like MIR in the first place.

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

dingxiangfei2009 · 2026-02-03T10:37:43Z

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

                block.and(PlaceBuilder::from(temp))
            }
+            ExprKind::Reborrow { source: _, mutability: _, ty: _ } => {
+                todo!();


We should generate a temporary. Therefore, we can merge this case one above, in other words build a temporary place, so that the reborrowing receives its own region "token."

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 :)

rustbot assigned tmandry Jan 27, 2026

aapoalas and others added 25 commits January 27, 2026 18:54

Add basic trait resolving

d6b0c00

Reborrow adjustment

8cb2709

further work: reborrow now in your local MIR

a6964fb

fmt

e11633b

fix: Implement proper Reborrow coercion

819dbb7

changes

a161c7b

fmt

a830c43

fix rebase mistake

3fa1f2f

fix

989b63c

try actually gather borrows for Rvalue::Reborrow

8116826

actually nearly kind of sort of working

06311ec

cleanup and fmt

a35386c

Teach borrowck that reborrow is borrowing

daa8aab

Signed-off-by: Xiangfei Ding <[email protected]>

Start CoerceShared impl

39621bd

temp

ebae325

coerce_unsized_info

66a54cf

[PATCH] use the right region by Xiangfei Ding <dingxiangfei2009@proto…

de7dee9

…nmail.ch>

Coherence for CoerceShared probably working, actual MIR not working

14c8827

not quite working anymore

d52465d

fix CoerceShared borrow_set

27dacbc

make CoerceShared work

f7c4d07

Make Reborrow work

c2411da

errors handled

77a6156

Fix lifetime of reborrow by Ding Xiang Fei

1f968e7

Start adding more comprehensive tests

3b1957a

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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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fix after rebase

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aapoalas added 2 commits January 29, 2026 21:00

fix &mut -> & CoerceShared

1e7b514

some tests

37bdf23

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aapoalas added 4 commits February 2, 2026 21:39

self-review fixes

2f3dde6

move back to marker

a6ea388

fix and add tests

92ff599

remove Reborrow derive for now

235d471

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

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 2, 2026

rustbot assigned jackh726 and unassigned tmandry Feb 2, 2026

RalfJung reviewed Feb 3, 2026

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dingxiangfei2009 reviewed Feb 3, 2026

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rustbot assigned BoxyUwU and unassigned jackh726 Feb 3, 2026

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