Suggest mutable method when iterating over binding - #160001

estebank · 2026-07-27T09:37:06Z

When encountering a for loop that requires a mutable iterator item, we previously suggested changing the method on the iterated expression, but ignored bindings. We now go to the binding's definition and suggest changing the method there too.

error[E0594]: cannot assign to `v.v`, which is behind a `&` reference
  --> $DIR/suggest-mut-method-for-loop-hashmap.rs:31:9
   |
LL |     for (_k, v) in x {
   |                    - this iterator yields `&` references
...
LL |         v.v += 1;
   |         ^^^^^^^^ `v` is a `&` reference, so it cannot be written to
   |
help: use mutable method
   |
LL |     let mut x = map.iter_mut();
   |                         ++++

Fix #49839.

When encountering a for loop that requires a mutable iterator item, we previously suggested changing the method on the iterated expression, but ignored bindings. We now go to the binding's definition and suggest changing the method there too. ``` error[E0594]: cannot assign to `v.v`, which is behind a `&` reference --> $DIR/suggest-mut-method-for-loop-hashmap.rs:31:9 | LL | for (_k, v) in x { | - this iterator yields `&` references ... LL | v.v += 1; | ^^^^^^^^ `v` is a `&` reference, so it cannot be written to | help: use mutable method | LL | let mut x = map.iter_mut(); | ++++ ```

rustbot · 2026-07-27T09:37:10Z

r? @mati865

rustbot has assigned @mati865.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: borrowck, compiler
borrowck, compiler expanded to 74 candidates
Random selection from 18 candidates

estebank · 2026-07-27T13:16:18Z

+                            assoc_items
+                                .in_definition_order()
+                                .map(|assoc_item_def| assoc_item_def.ident(tcx))
+                                .filter(|&ident| {
+                                    let original_method_ident = path_segment.ident;
+                                    original_method_ident != ident
+                                        && ident
+                                            .as_str()
+                                            .starts_with(&original_method_ident.name.to_string())
+                                })
+                                .map(|ident| format!("{ident}()"))
+                                .peekable()


I would want us to make this logic more principled and actually check that the associated function will give back an appropriate iterator of mutable items, even if the name is different.

View changes since the review

That would be better but more involved. Could be a nice followup.

mati865 · 2026-07-28T12:35:32Z

@rustbot reroll

adwinwhite · 2026-08-04T03:40:39Z

@bors r+ rollup

rust-bors · 2026-08-04T03:40:42Z

📌 Commit 9fb185d has been approved by adwinwhite

It is now in the queue for this repository.

Suggest mutable method when iterating over binding When encountering a for loop that requires a mutable iterator item, we previously suggested changing the method on the iterated expression, but ignored bindings. We now go to the binding's definition and suggest changing the method there too. ``` error[E0594]: cannot assign to `v.v`, which is behind a `&` reference --> $DIR/suggest-mut-method-for-loop-hashmap.rs:31:9 | LL | for (_k, v) in x { | - this iterator yields `&` references ... LL | v.v += 1; | ^^^^^^^^ `v` is a `&` reference, so it cannot be written to | help: use mutable method | LL | let mut x = map.iter_mut(); | ++++ ``` Fix rust-lang#49839.

…uwer Rollup of 22 pull requests Successful merges: - #160426 (`rust-analyzer` subtree update) - #160372 (Derive the allocator used by tools from rustc's allocator) - #146882 (fully deprecate the legacy integral modules) - #158727 (std: use `readdir` on nearly all UNIX platforms) - #159727 (Various steps in moving away from the big reflection enum to reflection functions) - #160443 (normalize in relations, not generalize, when relating infer with alias) - #160457 (implement -Zllvm-target-feature) - #160480 (Single-pass ASCII lower/upper case conversion) - #160502 (Reduce number of miri tests executed on PR CI) - #157430 (std::random: use little-endian for reproducibility) - #158110 (fix macro attribute feature-gate span) - #159975 (Use real ThinVec in StmtDebugInfos) - #160001 (Suggest mutable method when iterating over binding) - #160024 (Fix mono reachability with no-op landing pads) - #160154 (Add regression test for HRTB associated type projection closure) - #160176 (No more `tests/ui/issues`!) - #160326 (Remove hidden_glob_reexports) - #160407 (Add regression tests for a number of ICEs and diagnostics issues labelled `E-needs-test`) - #160430 (bootstrap: Don't produce mutated/filtered PathSets during command-line matching) - #160472 (Minor fixes to `core::io` & `alloc::io` Documentation) - #160486 (Remove unused `FreeRegionsVisitor`) - #160496 (clarify non-determinism docs for algebraic operations) Failed merges: - #160501 (Add bootstrap CLI snapshot test for testing miri)

Rollup merge of #160001 - estebank:issue-49839, r=adwinwhite Suggest mutable method when iterating over binding When encountering a for loop that requires a mutable iterator item, we previously suggested changing the method on the iterated expression, but ignored bindings. We now go to the binding's definition and suggest changing the method there too. ``` error[E0594]: cannot assign to `v.v`, which is behind a `&` reference --> $DIR/suggest-mut-method-for-loop-hashmap.rs:31:9 | LL | for (_k, v) in x { | - this iterator yields `&` references ... LL | v.v += 1; | ^^^^^^^^ `v` is a `&` reference, so it cannot be written to | help: use mutable method | LL | let mut x = map.iter_mut(); | ++++ ``` Fix #49839.

…uwer Rollup of 22 pull requests Successful merges: - rust-lang/rust#160426 (`rust-analyzer` subtree update) - rust-lang/rust#160372 (Derive the allocator used by tools from rustc's allocator) - rust-lang/rust#146882 (fully deprecate the legacy integral modules) - rust-lang/rust#158727 (std: use `readdir` on nearly all UNIX platforms) - rust-lang/rust#159727 (Various steps in moving away from the big reflection enum to reflection functions) - rust-lang/rust#160443 (normalize in relations, not generalize, when relating infer with alias) - rust-lang/rust#160457 (implement -Zllvm-target-feature) - rust-lang/rust#160480 (Single-pass ASCII lower/upper case conversion) - rust-lang/rust#160502 (Reduce number of miri tests executed on PR CI) - rust-lang/rust#157430 (std::random: use little-endian for reproducibility) - rust-lang/rust#158110 (fix macro attribute feature-gate span) - rust-lang/rust#159975 (Use real ThinVec in StmtDebugInfos) - rust-lang/rust#160001 (Suggest mutable method when iterating over binding) - rust-lang/rust#160024 (Fix mono reachability with no-op landing pads) - rust-lang/rust#160154 (Add regression test for HRTB associated type projection closure) - rust-lang/rust#160176 (No more `tests/ui/issues`!) - rust-lang/rust#160326 (Remove hidden_glob_reexports) - rust-lang/rust#160407 (Add regression tests for a number of ICEs and diagnostics issues labelled `E-needs-test`) - rust-lang/rust#160430 (bootstrap: Don't produce mutated/filtered PathSets during command-line matching) - rust-lang/rust#160472 (Minor fixes to `core::io` & `alloc::io` Documentation) - rust-lang/rust#160486 (Remove unused `FreeRegionsVisitor`) - rust-lang/rust#160496 (clarify non-determinism docs for algebraic operations) Failed merges: - rust-lang/rust#160501 (Add bootstrap CLI snapshot test for testing miri)

rustbot assigned mati865 Jul 27, 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. labels Jul 27, 2026

estebank commented Jul 27, 2026

View reviewed changes

rustbot assigned adwinwhite and unassigned mati865 Jul 28, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 4, 2026

This was referenced Aug 4, 2026

Rollup of 14 pull requests #160507

Closed

Rollup of 22 pull requests #160517

Merged

rust-bors Bot merged commit 02a459b into rust-lang:main Aug 5, 2026
13 checks passed

rustbot added this to the 1.99.0 milestone Aug 5, 2026

Navigation Menu

Uh oh!

Uh oh!

Suggest mutable method when iterating over binding - #160001