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Suggest use .get_mut instead of &mut when overloaded index type not impl IndexMut #144018

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Suggest use .get_mut instead of &mut when overloaded index type not impl IndexMut #144018

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@xizheyin xizheyin commented Jul 16, 2025 •
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Fixes #143732

r? compiler

@rustbot 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 16, 2025
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You need to compare the defid with the defid of the HashMap/BTreeMap diagnostic item and hard code the special case

@xizheyin xizheyin changed the title Suggest use .get_mut instead of &mut when occur overloaded index expression Suggest use .get_mut instead of &mut when occur HashMap and BtreeMap Jul 16, 2025
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The second commit shows the testcase difference.

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xizheyin commented Jul 16, 2025
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@Kobzol Kobzol mentioned this pull request Jul 16, 2025
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fee1-dead commented Jul 17, 2025 •
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In your first version you checked if the assignment on the type came from an overloaded index. You should check whether the overloaded index type implements IndexMut and return instead of passing the bool into the suggest_ampmut function if they don't implement IndexMut, then before returning you can suggest get_mut

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rustbot commented Jul 17, 2025

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

@xizheyin xizheyin force-pushed the 143732 branch 2 times, most recently from 8d3b265 to c43a91f Compare July 20, 2025 10:07
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@rustbot ready

tests/ui/borrowck/suggest-use-as-mut-for-map.rs Outdated
Comment on lines 16 to 38
let mut vec = vec![String::new(), String::new()];
let string = &vec[0];
string.push_str("test"); //~ ERROR cannot borrow `*string` as mutable, as it is behind a `&` reference [E0596]
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I add a case that type impls IndexMut, i.e. Vec.

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
Comment on lines +1449 to +1455
.type_implements_trait(
index_mut_trait,
[ty, index_ty],
self.infcx.param_env,
)
.must_apply_modulo_regions()
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I use type_implements_trait to judge if the type impl IndexMut.

@rustbot 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 Jul 20, 2025
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@xizheyin xizheyin changed the title Suggest use .get_mut instead of &mut when occur HashMap and BtreeMap Suggest use .get_mut instead of &mut when overloaded index type not impl IndexMut Jul 20, 2025
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#144018 (comment)

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@rustbot 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 Jul 21, 2025
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xizheyin commented Jul 21, 2025 •
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#144018 (comment)

This comment asks me to check the type which use overloaded index but not implement IndexMut, HashMap and BTreeMap are in that category, my understanding is that this is a enhancement over hardcoded. Why we should still hard-coding HashMap and BTreeMap? 😵‍💫

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because there are other types that aren't IndexMut and that aren't HashMap or BTreeMap?

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#143732 (comment)

@Kobzol suggest to consider all the types in this category? What do you think?

If we only consider a special case, we don't have to check whether it implements IndexMut. Because these two types didnt implement IndexMut, it seems to be enough to judge directly by using defid.

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Kobzol commented Jul 21, 2025

My suggestion was:

  • If T doesn't implement IndexMut, don't show the hint at all
  • If T is a Hashmap/HashSet, suggest get_mut

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This is clear. I forgot that some types do not have get_mut.

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@rustbot ready

This time, I considered different scenarios:

  1. suggest to use &mut
  2. suggest to use .get_mut
  3. dont suggest

I also added a new testcase.

@rustbot 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 Jul 21, 2025
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estebank reviewed Jul 23, 2025
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I spilt the testcase into two. One is marked run-rustfix, the other not.

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fee1-dead requested changes Jul 25, 2025
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compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
Comment on lines +1440 to +1444
if let Some(mir::Statement {
source_info: _,
kind: mir::StatementKind::Assign(box (_, mir::Rvalue::Ref(_, _, place))),
..
}) = self.body[location.block].statements.get(location.statement_index)
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this part is duplicated from the let opt_assignment_rhs_span part above - you should find a way to deduplicate

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
&self,
local: Local,
opt_assignment_rhs_span: Option<Span>,
) -> (bool, Option<AmpMutSugg>) {
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I don't like this too much, try ControlFlow<(), Option<AmpMutSugg>>

@rustbot 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 Jul 25, 2025
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rust suggests using IndexMut for maps, even though they don't implement the trait
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