Fix: false negative in useless_conversion on non-const items

[Shivangisharma4]

Fixes #16794

Background

In #14800, a fix was applied to suppress .into_iter() linting when the following chained method takes &mut self (like .any()). This successfully stopped a false positive when .into_iter() was called on const items (like a const R: Range = x..y), because removing the iteration call there triggers a const_item_mutation compiler warning (Issue #14656).

However, the guard added was too broad. It accidentally caused standard closures/expressions (like s.chars().into_iter().any(...)) to be entirely skipped by the useless_conversion lint.

What this PR does

• Adds a is_const_item HIR helper to verify if the expression corresponds to Res::Def(DefKind::Const | DefKind::AssocConst).
• Narrows the suppression guard by adding && is_const_item(cx, into_iter_recv) so that the strict checking only jumps in when protecting an actual const item.
• All other standard variables properly trigger the lint when .into_iter() can be safely stripped.
• Added regression tests in tests/ui/useless_conversion.rs to guard against this happening again.

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changelog: [useless_conversion]: fix false negative on non-const items when chaining &mut self methods

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[Copilot]

Pull request overview

Fixes a regression in Clippy’s useless_conversion lint where .into_iter() calls were incorrectly skipped for non-const receivers when followed by &mut self iterator methods (e.g. .any()), while preserving the intended suppression for const items to avoid const_item_mutation warnings.

Changes:

• Narrow useless_conversion’s suppression logic by introducing an is_const_item HIR helper and applying it to the &mut self + FnMut parent-method guard.
• Add a regression test ensuring .chars().into_iter().any(...) correctly triggers useless_conversion.
• Update UI test expectations (.stderr / .fixed) accordingly.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
clippy_lints/src/useless_conversion.rs	Narrows the .into_iter() suppression to only apply when the receiver is a const/AssocConst item.
tests/ui/useless_conversion.rs	Adds regression coverage for the non-const .into_iter().any(...) false negative while keeping const-item behavior covered.
tests/ui/useless_conversion.stderr	Updates expected UI output to include the newly restored useless_conversion diagnostic.
tests/ui/useless_conversion.fixed	Updates the rustfix expectation for the new regression case.
clippy_lints/src/eta_reduction.rs	Adds suppression for redundant_closure_for_method_calls when the method originates from an ambiguous external crate name (diamond dependency / multiple versions).

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[samueltardieu]

Over review capacity RN
r? clippy

[ada4a]

I'm sorry, was this PR description and the comments in the code created by an LLM?