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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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xizheyin commented 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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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 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 17, 2025
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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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xizheyin commented Jul 20, 2025
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@rustbot ready

tests/ui/borrowck/suggest-use-as-mut-for-map.rs
Comment on lines +16 to +18
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 +1460
.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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rust suggests using IndexMut for maps, even though they don't implement the trait
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