clippy::uninlined_format_args: Detect when a variable is not inlined in a format string, and suggests to inline it

format!("{}", var);
format!("{:?}", var);
format!("{v:?}", v = var);
format!("{0} {0}", var);
format!("{0:1$}", var, width);
format!("{:.*}", prec, var);

format!("{var}");
format!("{var:?}");
format!("{var:?}");
format!("{var} {var}");
format!("{var:width$}");
format!("{var:.prec$}");

[lints.clippy]
uninlined_format_args = "warn"

#![warn(clippy::uninlined_format_args)]

cargo clippy -- -W clippy::uninlined-format-args

Clippy's own documentation

What it does

Detect when a variable is not inlined in a format string, and suggests to inline it.

Why is this bad?

Non-inlined code is slightly more difficult to read and understand, as it requires arguments to be matched against the format string. The inlined syntax, where allowed, is simpler.

Example

format!("{}", var);
format!("{:?}", var);
format!("{v:?}", v = var);
format!("{0} {0}", var);
format!("{0:1$}", var, width);
format!("{:.*}", prec, var);

Use instead:

format!("{var}");
format!("{var:?}");
format!("{var:?}");
format!("{var} {var}");
format!("{var:width$}");
format!("{var:.prec$}");

If allow-mixed-uninlined-format-args is set to false in clippy.toml, the following code will also trigger the lint:

format!("{} {}", var, 1+2);

Use instead:

format!("{var} {}", 1+2);

Known Problems

If a format string contains a numbered argument that cannot be inlined nothing will be suggested, e.g. println!("{0}={1}", var, 1+2).

Configuration

• allow-mixed-uninlined-format-args: Whether to allow mixed uninlined format args, e.g. format!("{} {}", a, foo.bar)

  (default: true)

• 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.

Related lints

• too_many_linesChecks for functions with a large amount of lines
• missing_errors_docChecks the doc comments of publicly visible functions that return a Result type and warns if there is no # Errors section
• must_use_candidateChecks for public functions that have no #[must_use] attribute, but return something not already marked must-use, have no mutable arg and mutate no statics
• trivially_copy_pass_by_refthis argument (4 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
• unchecked_time_subtractionLints subtraction between an Instant and a Duration, or between two Duration values
• unicode_not_nfcChecks for string literals that contain Unicode in a form that is not equal to its NFC-recomposition
• unnecessary_box_returnsboxed return of the sized type `std::string::String`
• unnecessary_debug_formattingChecks for Debug formatting ({:?}) applied to an OsStr or Path. This includes: Format specifiers on format_args!() (width, precision have no effect) Format width too small for the format trait (e.g
• unnecessary_joincalled `.collect::<Vec<String>>().join("")` on an iterator

All 847 Clippy lints · Rust compiler errors · Cargo.toml checker