Add manual_ignore_cast_cmp lint

[nyurik]

// bad
fn compare(a: &str, b: &str) -> bool {
    a.to_ascii_lowercase() == b.to_ascii_lowercase()
    || a.to_ascii_lowercase() == "abc"
}

// good
fn compare(a: &str, b: &str) -> bool {
   a.eq_ignore_ascii_case(b)
   || a.eq_ignore_ascii_case("abc")
}

• Followed [lint naming conventions][lint_naming]
• Added passing UI tests (including committed .stderr file)
• cargo test passes locally
• Executed cargo dev update_lints
• Added lint documentation
• Run cargo dev fmt

changelog: [`manual_ignore_case_cmp`]: replace `to_ascii_lowercase`-style comparisons with `eq_ignore_ascii_case`

Closes #13204

[rustbot]

r? @xFrednet

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[bors]

☔ The latest upstream changes (presumably #13247) made this pull request unmergeable. Please resolve the merge conflicts.

[xFrednet]

Solid start, let me know if anything is unclear :D

[xFrednet]

Can you explain these lines?

They feel a bit weird. The peel_refs removes all potential references from the type, which means that some ascii types will fail the needs_ref_to_cmp function even if ty_raw would pass it.

[nyurik]

@xFrednet what's the better way to do this comparison?

[nyurik]

i was thinking some more on this, and tried to come up with a unit test that would fail, but was not able to do that... are you certain this is an issue?

[xFrednet]

Not really worth supporting. I would expect them to still trigger the lint though 🤔

[nyurik]

oki, will keep them as part of tests to track progress if they ever change, but won't fix them in the lint

[bors]

☔ The latest upstream changes (presumably #12987) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #13322) made this pull request unmergeable. Please resolve the merge conflicts.