Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::mismatched_bit_width_type: possible buggy implementation of `bit_width`

What it meansChecks for usage of T::BITS - x.leading_zeros() where T and x are of different types. It is in the suspicious group and warns by default.

What you see

warning: possible buggy implementation of `bit_width`
 --> main.rs:4:17
  |
4 | let bit_width = u32::BITS - x.leading_zeros();
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: in order to calculate the bit width, `T::BITS` should match the type of the value calling `.leading_zeros()`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#mismatched_bit_width_type
  = note: `#[warn(clippy::mismatched_bit_width_type)]` on by default
help: if you meant to use `u64::BITS`, use
  |
4 - let bit_width = u32::BITS - x.leading_zeros();
4 + let bit_width = x.bit_width();
  |

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?

Substracting leading_zeros from the number of bits of another type might be a buggy implementation of the bit_width method.

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

let x: u64 = 5;
let bit_width = u32::BITS - x.leading_zeros();

Write this instead

let x: u64 = 5;
let bit_width = x.bit_width();

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
mismatched_bit_width_type = "allow"

One run

cargo clippy -- -A clippy::mismatched-bit-width-type

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

1.98.0added in Rust

Clippy's own documentation

What it does

Checks for usage of T::BITS - x.leading_zeros() where T and x are of different types.

Why is this bad?

Substracting leading_zeros from the number of bits of another type might be a buggy implementation of the bit_width method.

Example

let x: u64 = 5;
let bit_width = u32::BITS - x.leading_zeros();

Use instead:

let x: u64 = 5;
let bit_width = x.bit_width();

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

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