Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::unused_trait_names: importing trait that is only used anonymously

What it meansChecks for use Trait where the Trait is only used for its methods and not referenced by a path directly. It is in the restriction group and off by default.

What you see

warning: importing trait that is only used anonymously
 --> main.rs:1:15
  |
1 | use std::fmt::Write;
  |               ^^^^^ help: use: `Write as _`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unused_trait_names
  = note: requested on the command line with `-W clippy::unused-trait-names`

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?

Traits imported that aren’t used directly can be imported anonymously with use Trait as _. It is more explicit, avoids polluting the current scope with unused names and can be useful to show which imports are required for traits.

Lints that forbid language features for specific codebases (no unwrap, no indexing...). Off by default; pick them one by one, never the whole group.

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

use std::fmt::Write;

fn main() {
    let mut s = String::new();
    let _ = write!(s, "hello, world!");
    println!("{s}");
}

Write this instead

use std::fmt::Write as _;

fn main() {
    let mut s = String::new();
    let _ = write!(s, "hello, world!");
    println!("{s}");
}

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]
unused_trait_names = "warn"

In code

#![warn(clippy::unused_trait_names)]

One run

cargo clippy -- -W clippy::unused-trait-names

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.

Settings in clippy.toml

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Restrictiongroup

allowdefault level

yesfixed by cargo clippy --fix

1.83.0added in Rust

Clippy's own documentation

What it does

Checks for use Trait where the Trait is only used for its methods and not referenced by a path directly.

Why is this bad?

Traits imported that aren’t used directly can be imported anonymously with use Trait as _. It is more explicit, avoids polluting the current scope with unused names and can be useful to show which imports are required for traits.

Example

use std::fmt::Write;

fn main() {
    let mut s = String::new();
    let _ = write!(s, "hello, world!");
    println!("{s}");
}

Use instead:

use std::fmt::Write as _;

fn main() {
    let mut s = String::new();
    let _ = write!(s, "hello, world!");
    println!("{s}");
}

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.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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