Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::useless_concat: unneeded use of `concat!` macro

What it meansChecks that the concat! macro has at least two arguments. It is in the complexity group and warns by default.

What you see

warning: unneeded use of `concat!` macro
 --> main.rs:3:9
  |
3 | let x = concat!("a");
  |         ^^^^^^^^^^^^ help: replace with: `"a"`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#useless_concat
  = note: `#[warn(clippy::useless_concat)]` 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?

If there are less than 2 arguments, then calling the macro is doing nothing.

Code that does something simple in a complicated way. 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

let x = concat!("a");

Write this instead

let x = "a";

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::useless_concat)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
useless_concat = "allow"

One run

cargo clippy -- -A clippy::useless-concat

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.

Complexitygroup

warndefault level

yesfixed by cargo clippy --fix

1.89.0added in Rust

Clippy's own documentation

What it does

Checks that the concat! macro has at least two arguments.

Why is this bad?

If there are less than 2 arguments, then calling the macro is doing nothing.

Example

let x = concat!("a");

Use instead:

let x = "a";

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

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