Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::len_zero: Checks for getting the length of something via .len() just to compare to zero, and suggests using .is_empty() where applicable
What it meansChecks for getting the length of something via .len() just to compare to zero, and suggests using .is_empty() where applicable. It is in the style group and warns by default.
Does it matter?
Some structures can answer .is_empty() much faster than calculating their length. So it is good to get into the habit of using .is_empty(), and having it is cheap. Besides, it makes the intent clearer than a manual comparison in some contexts.
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
if x.len() == 0 {
..
}
if y.len() != 0 {
..
}
Write this instead
if x.is_empty() {
..
}
if !y.is_empty() {
..
}
Turn it off, or on
Allow it where it does not fit your code, as narrowly as you can:
On one item
#[allow(clippy::len_zero)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
len_zero = "allow"One run
cargo clippy -- -A clippy::len-zeroThe [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
Clippy's own documentation
What it does
Checks for getting the length of something via .len()
just to compare to zero, and suggests using .is_empty() where applicable.
Why is this bad?
Some structures can answer .is_empty() much faster
than calculating their length. So it is good to get into the habit of using
.is_empty(), and having it is cheap.
Besides, it makes the intent clearer than a manual comparison in some contexts.
Example
if x.len() == 0 {
..
}
if y.len() != 0 {
..
}
instead use
if x.is_empty() {
..
}
if !y.is_empty() {
..
}
Configuration
-
msrv: The minimum rust version that the project supports. Defaults to therust-versionfield inCargo.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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- legacy_numeric_constantsusage of a legacy numeric constant
- len_without_is_emptyChecks for items that implement .len() but not .is_empty()
- let_and_returnreturning the result of a `let` binding from a block
- let_unit_valuethis let-binding has unit value
- main_recursionrecursing into entrypoint `main`
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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