Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::useless_format: useless use of `format!`

What it meansChecks for the use of format!("string literal with no argument") and format!("{}", foo) where foo is a string. It is in the complexity group and warns by default.

What you see

warning: useless use of `format!`
 --> main.rs:4:1
  |
4 | format!("{}", foo);
  | ^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `foo.to_string()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#useless_format
  = note: `#[warn(clippy::useless_format)]` 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?

There is no point of doing that. format!("foo") can be replaced by "foo".to_owned() if you really need a String. The even worse &format!("foo") is often encountered in the wild. format!("{}", foo) can be replaced by foo.clone() if foo: String or foo.to_owned() if foo: &str.

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 foo = "foo";
format!("{}", foo);

Write this instead

let foo = "foo";
foo.to_owned();

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_format)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
useless_format = "allow"

One run

cargo clippy -- -A clippy::useless-format

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

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for the use of format!("string literal with no argument") and format!("{}", foo) where foo is a string.

Why is this bad?

There is no point of doing that. format!("foo") can be replaced by "foo".to_owned() if you really need a String. The even worse &format!("foo") is often encountered in the wild. format!("{}", foo) can be replaced by foo.clone() if foo: String or foo.to_owned() if foo: &str.

Examples

let foo = "foo";
format!("{}", foo);

Use instead:

let foo = "foo";
foo.to_owned();

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

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