Clippy lint · Style · checked with Clippy 0.1.99

clippy::manual_is_ascii_check: manual check for common ascii range

What it meansSuggests to use dedicated built-in methods, is_ascii_(lowercase|uppercase|digit|hexdigit) for checking on corresponding ascii range It is in the style group and warns by default.

What you see

warning: manual check for common ascii range
 --> main.rs:2:13
  |
2 |     assert!(matches!('x', 'a'..='z'));
  |             ^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#manual_is_ascii_check
  = note: `#[warn(clippy::manual_is_ascii_check)]` on by default
help: try
  |
2 -     assert!(matches!('x', 'a'..='z'));
2 +     assert!('x'.is_ascii_lowercase());
  |

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?

Using the built-in functions is more readable and makes it clear that it’s not a specific subset of characters, but all ASCII (lowercase|uppercase|digit|hexdigit) characters.

Code that works but is not idiomatic Rust. 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 main() {
    assert!(matches!('x', 'a'..='z'));
    assert!(matches!(b'X', b'A'..=b'Z'));
    assert!(matches!('2', '0'..='9'));
    assert!(matches!('x', 'A'..='Z' | 'a'..='z'));
    assert!(matches!('C', '0'..='9' | 'a'..='f' | 'A'..='F'));

    ('0'..='9').contains(&'0');
    ('a'..='z').contains(&'a');
    ('A'..='Z').contains(&'A');
}

Write this instead

fn main() {
    assert!('x'.is_ascii_lowercase());
    assert!(b'X'.is_ascii_uppercase());
    assert!('2'.is_ascii_digit());
    assert!('x'.is_ascii_alphabetic());
    assert!('C'.is_ascii_hexdigit());

    '0'.is_ascii_digit();
    'a'.is_ascii_lowercase();
    'A'.is_ascii_uppercase();
}

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_is_ascii_check)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
manual_is_ascii_check = "allow"

One run

cargo clippy -- -A clippy::manual-is-ascii-check

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.

Settings in clippy.toml

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Stylegroup

warndefault level

yesfixed by cargo clippy --fix

1.67.0added in Rust

Clippy's own documentation

What it does

Suggests to use dedicated built-in methods, is_ascii_(lowercase|uppercase|digit|hexdigit) for checking on corresponding ascii range

Why is this bad?

Using the built-in functions is more readable and makes it clear that it’s not a specific subset of characters, but all ASCII (lowercase|uppercase|digit|hexdigit) characters.

Example

fn main() {
    assert!(matches!('x', 'a'..='z'));
    assert!(matches!(b'X', b'A'..=b'Z'));
    assert!(matches!('2', '0'..='9'));
    assert!(matches!('x', 'A'..='Z' | 'a'..='z'));
    assert!(matches!('C', '0'..='9' | 'a'..='f' | 'A'..='F'));

    ('0'..='9').contains(&'0');
    ('a'..='z').contains(&'a');
    ('A'..='Z').contains(&'A');
}

Use instead:

fn main() {
    assert!('x'.is_ascii_lowercase());
    assert!(b'X'.is_ascii_uppercase());
    assert!('2'.is_ascii_digit());
    assert!('x'.is_ascii_alphabetic());
    assert!('C'.is_ascii_hexdigit());

    '0'.is_ascii_digit();
    'a'.is_ascii_lowercase();
    'A'.is_ascii_uppercase();
}

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

(default: current version)

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

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