fix manual_ignore_case_cmp chars iterator cmp

[cyphercodes]

changelog: [manual_ignore_case_cmp]: detect case-folded .chars() iterators compared with Iterator::cmp

Fixes #16961

Summary

• Detect .chars().map(|c| c.to_ascii_lowercase()/to_ascii_uppercase()).cmp(...).is_eq() forms.
• Also handle the corresponding .is_ne() form by suggesting !eq_ignore_ascii_case.
• Add UI coverage for borrowed strings, owned strings, and the to_str().is_some_and(...) closure reproducer.

Tests

• cargo fmt --check -- clippy_lints/src/manual_ignore_case_cmp.rs tests/ui/manual_ignore_case_cmp.rs clippy_utils/src/sym.rs
• TESTNAME=manual_ignore_case_cmp cargo uibless
• TESTNAME=manual_ignore_case_cmp cargo uitest
• cargo test --test dogfood
• git diff --check

[rustbot]

r? @samueltardieu

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The reviewer was selected based on:

• Owners of files modified in this PR: 7 candidates
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• Random selection from Jarcho, dswij, llogiq, samueltardieu

[samueltardieu]

You should probably be stricter than that, and check if this is a (possibly multi-level) reference to str. Otherwise, you may end up generating fixes that are incorrect, such as in this (convoluted) example:

struct S;

impl S {
    fn eq_ignore_case_cmp(self) {}
}

impl std::ops::Deref for S {
    type Target = str;
    fn deref(&self) -> &Self::Target {
        ""
    }
}

fn bogus(s: S) -> bool {
    s.chars()
        .map(|c| c.to_ascii_lowercase())
        .cmp(s.chars().map(|c| c.to_ascii_lowercase()))
        .is_eq()
}

This will suggest s.eq_ignore_ascii_case(s) which won't compile.

View changes since the review

[rustbot]

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

[cyphercodes]

Addressed the review feedback by making the .chars().map(...).cmp(...) matcher stricter: it now only lints receivers whose unadjusted type is str/references to str or String, avoiding arbitrary Deref<Target = str> types that could produce invalid suggestions.

Added a regression case for a custom Deref<Target = str> type.

Verification:

• cargo fmt -- clippy_lints/src/manual_ignore_case_cmp.rs tests/ui/manual_ignore_case_cmp.rs
• TESTNAME=manual_ignore_case_cmp cargo uibless
• TESTNAME=manual_ignore_case_cmp cargo uitest
• cargo check -p clippy_lints
• git diff --check

[samueltardieu]

Verification:

* `cargo fmt -- clippy_lints/src/manual_ignore_case_cmp.rs tests/ui/manual_ignore_case_cmp.rs`

* `TESTNAME=manual_ignore_case_cmp cargo uibless`

* `TESTNAME=manual_ignore_case_cmp cargo uitest`

* `cargo check -p clippy_lints`

* `git diff --check`

Please avoid this LLM generated noise, especially when answering review comments.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[cyphercodes]

@rustbot ready