Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::comparison_to_empty: Checks for comparing to an empty slice such as "" or [], and suggests using .is_empty() where applicable
What it meansChecks for comparing to an empty slice such as "" or [], 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 checking for equality. 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 s == "" {
..
}
if arr == [] {
..
}
Write this instead
if s.is_empty() {
..
}
if arr.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::comparison_to_empty)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
comparison_to_empty = "allow"One run
cargo clippy -- -A clippy::comparison-to-emptyThe [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 comparing to an empty slice such as "" or [],
and suggests using .is_empty() where applicable.
Why is this bad?
Some structures can answer .is_empty() much faster
than checking for equality. 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 s == "" {
..
}
if arr == [] {
..
}
Use instead:
if s.is_empty() {
..
}
if arr.is_empty() {
..
}
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
Related lints
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- new_without_defaultChecks for public types with a pub fn new() -> Self method and no implementation of Default
- len_zeroChecks for getting the length of something via .len() just to compare to zero, and suggests using .is_empty() where applicable
- cmp_nullThis lint checks for equality comparisons with ptr::null or ptr::null_mut
- collapsible_ifChecks for nested if statements which can be collapsed by &&-combining their conditions
- collapsible_matchthis `match` can be collapsed into the outer `match`
- default_instead_of_iter_empty`std::iter::empty()` is the more idiomatic way
- disallowed_fieldsDenies the configured fields in clippy.toml Note: Even though this lint is warn-by-default, it will only trigger if fields are defined in the clippy.toml file
- disallowed_macrosDenies the configured macros in clippy.toml Note: Even though this lint is warn-by-default, it will only trigger if macros are defined in the clippy.toml file
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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