Fix 'assign to data in an index of' collection suggestions by GTimothy · Pull Request #152216 · rust-lang/rust

GTimothy · 2026-02-06T12:15:41Z

fixes #150001
fixes rust-lang/rust-analyzer#16076
fixes #134917

The issues are threefold and linked:

Assigning data to a ~~collection~~BTreeMap/HashMap suggests 3 solutions all marked as MachineApplicable
The suggestions are slightly wrong with regards to their borrowing needs.
The suggestions are not guaranteed to produce code that is valid, and suggestion number two is equivalent to an update, not an insertion.

This PR:

splits the large triple suggestion into three
sets them to MaybeIncorrect
automatically determines the required borrowing to use.

I think this solution may not be very elegant, expecially the key typechecking / borrowing part, but it works. I am however very open to any improvement/change :)

edit: edited to replace 'collection' with BTreeMap/HashMap'

rustbot · 2026-02-06T12:15:47Z

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GTimothy · 2026-02-06T12:21:23Z

oops, meant to title "Fix 'assign to data in an index of' map suggestions"
Should probably rename the commit too.

edit for clarity: for now, these suggestions only apply to BTreeMap and HashMap.

rust/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

Lines 657 to 661 in fef627b

    
               fn suggest_map_index_mut_alternatives(&self, ty: Ty<'tcx>, err: &mut Diag<'infcx>, span: Span) { 
        
                   let Some(adt) = ty.ty_adt_def() else { return }; 
        
                   let did = adt.did(); 
        
                   if self.infcx.tcx.is_diagnostic_item(sym::HashMap, did) 
        
                       || self.infcx.tcx.is_diagnostic_item(sym::BTreeMap, did)

GTimothy · 2026-02-16T19:09:05Z

@JonathanBrouwer, I think this is ready for review. Do I need to change anything? Thanks in advance.

JonathanBrouwer · 2026-02-16T19:14:42Z

I'll take a look sometime in the next few days :)

JonathanBrouwer

Thanks for your first contribution to rustc!
The idea looks good, just would like the code to be slightly nicer :)

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rustbot · 2026-02-20T16:11:10Z

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

GTimothy · 2026-02-23T22:15:46Z

thank you @JonathanBrouwer for your remarks. I had made a completely wrong assumptions about the types in questions.
I have completely reworked the PR in a much more elegant and correct way i think.

GTimothy · 2026-02-24T19:04:12Z

@rustbot ready

splits the large triple suggestion into three sets them to MaybeIncorrect automatically determines the required/nicest borrowing to use.

…7 instead of E0594 a set of slightly different tests showing that E0594 can be reached in some cases via autodereferencing, but also that slight variations produce E0277 instead.

GTimothy · 2026-04-30T16:10:47Z

@rustbot ready
I have finally got the time to finish this.
The suggestions now use the minimal mount of & possible and the exact amount of * necessary.
I have added a test (tests/ui/borrowck/index-mut-help2.rs) describing peculiar auto-dereferencing behaviours encountered with slightly different cases.

JonathanBrouwer

Some more nits, but I think we're almost there :)

View changes since this review

added a valid example that violated the assumption in the proposed change: it is possible for a borrowee type to be a ref of it's borrowed type. removed the assumption, treated all cases appropriately.

GTimothy · 2026-05-02T12:44:58Z

@rustbot ready

JonathanBrouwer · 2026-05-03T13:39:58Z

@bors r+ rollup
Thanks for your hard work, this is an awesome first contribution <3

rust-bors · 2026-05-03T13:40:01Z

📌 Commit 5f1e962 has been approved by JonathanBrouwer

It is now in the queue for this repository.

…uwer Rollup of 4 pull requests Successful merges: - #156073 ([rustdoc] Fix `doc_cfg` feature on reexports) - #152216 (Fix 'assign to data in an index of' collection suggestions) - #155433 (Rip out rustc_layout_scalar_valid_range_* attribute support) - #156098 (Hand-implement `impl Debug for NumBuffer` to avoid constraining `T` or printing MaybeUninit)

Rollup merge of #152216 - GTimothy:map-diagnostics-fix, r=JonathanBrouwer Fix 'assign to data in an index of' collection suggestions fixes #150001 fixes rust-lang/rust-analyzer#16076 fixes #134917 The issues are threefold and linked: 1. Assigning data to a ~~collection~~BTreeMap/HashMap suggests 3 solutions all marked as `MachineApplicable` 2. The suggestions are slightly wrong with regards to their borrowing needs. 3. The suggestions are not guaranteed to produce code that is valid, and suggestion number two is equivalent to an update, not an insertion. This PR: - splits the large triple suggestion into three - sets them to `MaybeIncorrect` - automatically determines the required borrowing to use. I think this solution may not be very elegant, expecially the key typechecking / borrowing part, but it works. I am however very open to any improvement/change :) edit: edited to replace 'collection' with BTreeMap/HashMap'

rustbot assigned JonathanBrouwer Feb 6, 2026

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GTimothy force-pushed the map-diagnostics-fix branch from 8034656 to 2371906 Compare February 6, 2026 12:18

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GTimothy marked this pull request as draft February 6, 2026 13:03

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 6, 2026

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GTimothy force-pushed the map-diagnostics-fix branch from 0fedd40 to 77c2a6e Compare February 14, 2026 15:36

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GTimothy force-pushed the map-diagnostics-fix branch from 77c2a6e to 5d997ef Compare February 16, 2026 09:48

GTimothy marked this pull request as ready for review February 16, 2026 19:03

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GTimothy added 3 commits April 30, 2026 16:38

Fix 'assign to data in an index of' collection suggestions

cb37077

splits the large triple suggestion into three sets them to MaybeIncorrect automatically determines the required/nicest borrowing to use.

rebless the ui tests

f2bf6a8

add index-mut test case where mut index access to a map produces E027…

9de80d4

…7 instead of E0594 a set of slightly different tests showing that E0594 can be reached in some cases via autodereferencing, but also that slight variations produce E0277 instead.

GTimothy force-pushed the map-diagnostics-fix branch from b799514 to 9de80d4 Compare April 30, 2026 14:38

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 30, 2026

JonathanBrouwer reviewed May 1, 2026

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Comment thread compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs Outdated

JonathanBrouwer reviewed May 1, 2026

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Comment thread compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs Outdated

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 1, 2026

fix: wrong assumption could cause panic

5f1e962

added a valid example that violated the assumption in the proposed change: it is possible for a borrowee type to be a ref of it's borrowed type. removed the assumption, treated all cases appropriately.

GTimothy force-pushed the map-diagnostics-fix branch from 3c912e3 to 5f1e962 Compare May 2, 2026 11:38

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 May 2, 2026

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JonathanBrouwer mentioned this pull request May 3, 2026

Rollup of 4 pull requests #156111

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rust-bors Bot merged commit 11610db into rust-lang:main May 3, 2026
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rustbot added this to the 1.97.0 milestone May 3, 2026

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