Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::const_is_empty: this expression always evaluates to true

What it meansIt identifies calls to .is_empty() on constant values. It is in the suspicious group and warns by default.

What you see

warning: this expression always evaluates to true
 --> main.rs:4:4
  |
4 | if value.is_empty() {
  |    ^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#const_is_empty
  = note: `#[warn(clippy::const_is_empty)]` 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?

String literals and constant values are known at compile time. Checking if they are empty will always return the same value. This might not be the intention of the expression.

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

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

let value = "";
if value.is_empty() {
    println!("the string is empty");
}

Write this instead

println!("the string 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::const_is_empty)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
const_is_empty = "allow"

One run

cargo clippy -- -A clippy::const-is-empty

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

1.79.0added in Rust

Clippy's own documentation

What it does

It identifies calls to .is_empty() on constant values.

Why is this bad?

String literals and constant values are known at compile time. Checking if they are empty will always return the same value. This might not be the intention of the expression.

Example

let value = "";
if value.is_empty() {
    println!("the string is empty");
}

Use instead:

println!("the string is empty");

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

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