Clippy lint · Nursery · checked with Clippy 0.1.99

clippy::literal_string_with_formatting_args: this looks like a formatting argument but it is not part of a formatting macro

What it meansChecks if string literals have formatting arguments outside of macros using them (like format!). It is in the nursery group and off by default.

What you see

warning: this looks like a formatting argument but it is not part of a formatting macro
 --> main.rs:5:11
  |
5 | x.expect("{y:?}");
  |           ^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#literal_string_with_formatting_args
  = note: `-W clippy::literal-string-with-formatting-args` implied by `-W clippy::nursery`

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?

It will likely not generate the expected content.

New lints still being finished. Off by default and may have false positives.

How to fix it

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

Flagged

let x: Option<usize> = None;
let y = "hello";
x.expect("{y:?}");

Write this instead

let x: Option<usize> = None;
let y = "hello";
x.expect(&format!("{y:?}"));

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]
literal_string_with_formatting_args = "warn"

In code

#![warn(clippy::literal_string_with_formatting_args)]

One run

cargo clippy -- -W clippy::literal-string-with-formatting-args

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.

Nurserygroup

allowdefault level

nofixed by cargo clippy --fix

1.85.0added in Rust

Clippy's own documentation

What it does

Checks if string literals have formatting arguments outside of macros using them (like format!).

Why is this bad?

It will likely not generate the expected content.

Example

let x: Option<usize> = None;
let y = "hello";
x.expect("{y:?}");

Use instead:

let x: Option<usize> = None;
let y = "hello";
x.expect(&format!("{y:?}"));

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

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