Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::float_equality_without_abs: float equality check without `.abs()`

What it meansChecks for statements of the form (a - b) < f32::EPSILON or (a - b) < f64::EPSILON. Note the missing .abs(). It is in the suspicious group and warns by default.

What you see

warning: float equality check without `.abs()`
 --> main.rs:4:5
  |
4 |     (a - b) < f32::EPSILON
  |     -------^^^^^^^^^^^^^^^
  |     |
  |     help: add `.abs()`: `(a - b).abs()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#float_equality_without_abs
  = note: `#[warn(clippy::float_equality_without_abs)]` on by default

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 code without .abs() is more likely to have a bug.

Code that is most likely wrong or useless. Warns by default.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

pub fn is_roughly_equal(a: f32, b: f32) -> bool {
    (a - b) < f32::EPSILON
}

Write this instead

pub fn is_roughly_equal(a: f32, b: f32) -> bool {
    (a - b).abs() < f32::EPSILON
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
float_equality_without_abs = "allow"

One run

cargo clippy -- -A clippy::float-equality-without-abs

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.

Suspiciousgroup

warndefault level

nofixed by cargo clippy --fix

1.48.0added in Rust

Clippy's own documentation

What it does

Checks for statements of the form (a - b) < f32::EPSILON or (a - b) < f64::EPSILON. Note the missing .abs().

Why is this bad?

The code without .abs() is more likely to have a bug.

Known problems

If the user can ensure that b is larger than a, the .abs() is technically unnecessary. However, it will make the code more robust and doesn’t have any large performance implications. If the abs call was deliberately left out for performance reasons, it is probably better to state this explicitly in the code, which then can be done with an allow.

Example

pub fn is_roughly_equal(a: f32, b: f32) -> bool {
    (a - b) < f32::EPSILON
}

Use instead:

pub fn is_roughly_equal(a: f32, b: f32) -> bool {
    (a - b).abs() < f32::EPSILON
}

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

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