fix(reborrow): recursive implementation by aapoalas · Pull Request #159103 · rust-lang/rust

aapoalas · 2026-07-10T18:29:04Z

If Reborrow finds '&'a mut T' fields where 'a is the Reborrowed type's first lifetime parameter (currently the only lifetime that is allowed to reborrow) then it inserts a Deref and borrow of the T, and likewise if it finds a 'T: Reborrow' field then the field type is recursed into.

This makes Reborrow always produce the correct borrow checking logic at the cost of most probably being inconsiderately expensive. The thinking is that performance will be a followup consideration.

r? @oli-obk

rustbot · 2026-07-10T18:29:08Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes MIR

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

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to constck

cc @fee1-dead

This PR changes rustc_public

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

rustbot · 2026-07-10T19:21:26Z

rustc_codegen_cranelift is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_cranelift instead.

cc @bjorn3

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

oli-obk

Started a review a few days ago, but other things came in between, so nothing architectural yet, but docs are always useful

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rustbot · 2026-07-23T19:38:13Z

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

If Reborrow finds '&'a mut T' fields then it inserts a Deref and borrow of the T, and likewise if it finds a 'T: Reborrow' field then the field type is recursed into. This makes Reborrow always produce the correct borrow checking logic at the cost of most probably being inconsiderately expensive. The thinking is that performance will be a followup consideration.

Co-authored-by: Oli Scherer <github35764891676564198441@oli-obk.de>

…make sense

rustbot · 2026-08-13T16:00:24Z

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.

oli-obk

I don't fully grok all this yet, but I have enough of an understanding that I'm confident with landing this. It does seem like a coherent thing to me and it will fully "click" in my mind at some point

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oli-obk · 2026-08-18T13:31:16Z

+    /// producing a borrow on an indirect place, producing a value that can be returned from the
+    /// function since it does not capture any local place. If no such field exists, then
+    /// reborrowing the type must dereference the type itself to find an indirect place, but
+    /// generally such types will not implements `Deref`. Therefore, in borrow checking we instead


Suggested change

/// generally such types will not implements `Deref`. Therefore, in borrow checking we instead

/// generally such types will not implement `Deref`. Therefore, in borrow checking we instead

oli-obk · 2026-08-18T13:34:08Z

@@ -0,0 +1,17 @@
+//@ check-fail


this is the default setting for ui tests and redundant (also more of those below)

oli-obk · 2026-08-18T13:40:11Z

+            }
+        }
+        if !did_reborrow {
+            // If source contained no reference, perform a phantom dereference.


Ah this is the key point here: we need the rest of borrowck to act as if there is a deref here 🤔

oli-obk · 2026-08-18T13:52:07Z

+            // The reason why we do this obligation here (again: it's already done in THIR) is
+            // because we'd want to catch impls like `CoerceShared<Target<'static>> for Source<'a>`
+            // and, in those cases, correctly generate a `'a: 'static` bound.
+            //
+            // I'm not sure what would be the right way to resolve this conundrum.


this is exactly what you need to do in borrowck's "typeck". You redo the ops, this time with lifetimes, and ICE on any type conflicts.

I think what you should be doing (instead of manually running an ocx), is to use the usual fully_perform actions, but otherwise this is exactly The Right Thing ™️

oli-obk · 2026-08-18T13:54:21Z

@bors delegate+

r=me with remaining things resolved

rust-bors · 2026-08-18T13:54:25Z

✌️ @aapoalas, you can now approve this pull request!

If @oli-obk told you to "r=me" after making some further change, then please make that change and post @bors r=oli-obk.

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aapoalas requested review from dingxiangfei2009 and oli-obk July 10, 2026 18:29

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. labels Jul 10, 2026

rustbot assigned oli-obk Jul 10, 2026