Clippy lint · Performance · checked with Clippy 0.1.99

clippy::manual_ignore_case_cmp: manual case-insensitive ASCII comparison

What it meansChecks for manual case-insensitive ASCII comparison. It is in the performance group and warns by default.

What you see

warning: manual case-insensitive ASCII comparison
 --> main.rs:4:5
  |
4 |     a.to_ascii_lowercase() == b.to_ascii_lowercase() || a.to_ascii_lowercase() == "abc"
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#manual_ignore_case_cmp
  = note: `#[warn(clippy::manual_ignore_case_cmp)]` on by default
help: consider using `.eq_ignore_ascii_case()` instead
  |
4 -     a.to_ascii_lowercase() == b.to_ascii_lowercase() || a.to_ascii_lowercase() == "abc"
4 +     a.eq_ignore_ascii_case(b) || a.to_ascii_lowercase() == "abc"
  |

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

The eq_ignore_ascii_case method is faster because it does not allocate memory for the new strings, and it is more readable.

Code that could be written to run faster. Warns by default.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

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

Write this instead

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

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::manual_ignore_case_cmp)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
manual_ignore_case_cmp = "allow"

One run

cargo clippy -- -A clippy::manual-ignore-case-cmp

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Performancegroup

warndefault level

yesfixed by cargo clippy --fix

1.84.0added in Rust

Clippy's own documentation

What it does

Checks for manual case-insensitive ASCII comparison.

Why is this bad?

The eq_ignore_ascii_case method is faster because it does not allocate memory for the new strings, and it is more readable.

Example

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

Use instead:

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

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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