Improve useless_conversion .into_iter() suggestion for nested references

[keirsalterego]

Description

I updated the useless_conversion lint to stop applying adjustment prefixes once it reaches the final target type.

Previously, the lint would continue applying adjustments even after the type requirements were met, which often resulted in over-borrowed suggestions like &**y. By breaking the loop early once the target type is reached, we now emit the minimal, idiomatic suggestion (e.g., *y).

fixes #14847

Test Plan

I added a targeted UI regression test: tests/ui/useless_conversion.rs.
This covers .into_iter() usage on nested slice references (&&[T]) and verifies that Clippy now suggests *items instead of the previous incorrect suggestion.

Checklist

• Added passing UI tests (including committed .stderr file)
• cargo test passes locally
• Run cargo dev fmt

---

changelog: [useless_conversion]: refine .into_iter() suggestions to stop at the final target type (fixing over-borrowed suggestions like &**y)

[rustbot]

r? @Jarcho

rustbot has assigned @Jarcho.
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

[Copilot]

Pull request overview

This PR refines the useless_conversion lint to fix over-borrowed suggestions when dealing with .into_iter() calls on nested references. The core change modifies the adjustments function to include early-break logic that stops processing adjustments once the target type is reached.

• Adds early break logic in the adjustments function to stop at the target type
• Introduces a regression test for .into_iter() on nested slice references (&&[T])

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
clippy_lints/src/useless_conversion.rs	Modified adjustments function to break early when reaching the target type, preventing over-application of adjustment prefixes
tests/ui/useless_conversion_into_iter_adjustments.rs	Added regression test for issue #14847 testing .into_iter() on &&[i32]
tests/ui/useless_conversion_into_iter_adjustments.stderr	Expected test output showing the corrected suggestion (*items instead of over-borrowed alternatives)

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

View changes since this review

[Jarcho]

The added tests should be added to test/ui/useless_conversion.rs, not in there own file. Can you please also squash this into a single commit.

View changes since this review

[rustbot]

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

[Jarcho]

Missed something on the last review.

View changes since this review