Clippy lints / Pedantic
Clippy lint · Pedantic · checked with Clippy 0.1.99
clippy::manual_ilog2: manually reimplementing `ilog2`
What it meansChecks for expressions like N - x.leading_zeros() (where N is one less than bit width of x) or x.ilog(2), which are manual reimplementations of x.ilog2() It is in the pedantic group and off by default.
What you see
warning: manually reimplementing `ilog2` --> main.rs:4:11 | 4 | let log = 31 - x.leading_zeros(); | ^^^^^^^^^^^^^^^^^^^^^^ help: try: `x.ilog2()` | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#manual_ilog2 = note: `-W clippy::manual-ilog2` implied by `-W clippy::pedantic`
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?
Manual reimplementations of ilog2 increase code complexity for little benefit.
Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.
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
let x: u32 = 5;
let log = 31 - x.leading_zeros();
let log = x.ilog(2);
Write this instead
let x: u32 = 5;
let log = x.ilog2();
let log = x.ilog2();
Turn it off, or on
It is off by default. To use it, set it to warn or deny:
Whole crate (Cargo.toml)
[lints.clippy]
manual_ilog2 = "warn"In code
#![warn(clippy::manual_ilog2)]One run
cargo clippy -- -W clippy::manual-ilog2The [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.
Clippy's own documentation
What it does
Checks for expressions like N - x.leading_zeros() (where N is one less than bit width
of x) or x.ilog(2), which are manual reimplementations of x.ilog2()
Why is this bad?
Manual reimplementations of ilog2 increase code complexity for little benefit.
Example
let x: u32 = 5;
let log = 31 - x.leading_zeros();
let log = x.ilog(2);
Use instead:
let x: u32 = 5;
let log = x.ilog2();
let log = x.ilog2();
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
Related lints
- too_many_linesChecks for functions with a large amount of lines
- missing_errors_docChecks the doc comments of publicly visible functions that return a Result type and warns if there is no # Errors section
- must_use_candidateChecks for public functions that have no #[must_use] attribute, but return something not already marked must-use, have no mutable arg and mutate no statics
- manual_assertonly a `panic!` in `if`-then statement
- manual_assert_eqused `assert!` with an equality comparison
- manual_bit_widthmanual implementation of `bit_width`
- manual_instant_elapsedmanual implementation of `Instant::elapsed`
- manual_is_power_of_twomanually reimplementing `is_power_of_two`
- manual_is_variant_andChecks for usage of option.map(f).unwrap_or_default() and result.map(f).unwrap_or_default() where f is a function or closure that returns the bool type. Also checks for equality comparisons like option.map(f) == Some(true) and result.map(f) == Ok(true), as well as result.ok().is_some_and(f)
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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