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Rust format strings - syntax, width, precision, and traits

A Rust format string mixes literal text with replacement fields. The smallest field is {}: it selects the next argument and formats it with Display. Add syntax after : when you need width, alignment, precision, flags, or a different formatting trait.

EXAMPLE: CI-CHECKEDTEST ASSERTED · examples/format-output/src/reference_examples.rs#format-basic
assert_eq!(format!("Hello, {}!", "Rust"), "Hello, Rust!");

Rust format string syntax

Section titled “Rust format string syntax”

The official grammar defines a field as an optional argument followed by an optional format specification:

Format specification grammar
{ [argument] [ : [[fill]align][sign][#][0][width][.precision][type] ] }

The colon only introduces formatting options. With no explicit argument, Rust consumes the next argument. An integer selects a positional argument; an identifier selects a named argument or captures a variable from scope.

EXAMPLE: CI-CHECKEDTEST ASSERTED · examples/format-output/src/reference_examples.rs#format-arguments
let language = "Rust";
assert_eq!(format!("{language} {0}", "prints"), "Rust prints");

Width

Section titled “Width”

Width is a minimum, not a forced size. Short values are padded; longer values remain intact.

EXAMPLE: CI-CHECKEDTEST ASSERTED · examples/format-output/src/reference_examples.rs#format-width
assert_eq!(format!("|{:>8}|", "rust"), "|    rust|");
assert_eq!(format!("|{:*^8}|", "rust"), "|**rust**|");

Width can come from another usize argument by adding $:

EXAMPLE: CI-CHECKEDTEST ASSERTED · examples/format-output/src/reference_examples.rs#format-dynamic-width
assert_eq!(format!("|{:width$}|", "rust", width = 8), "|rust    |");

Precision

Section titled “Precision”

Precision means decimal places for floating-point values and maximum output width for strings. Integral formatters ignore it.

EXAMPLE: CI-CHECKEDTEST ASSERTED · examples/format-output/src/reference_examples.rs#format-precision
assert_eq!(format!("{:.2}", 3.14159), "3.14");
assert_eq!(format!("{:.4}", "rustacean"), "rust");

Escaping braces

Section titled “Escaping braces”

Double a brace to emit it literally: {{ becomes { and }} becomes }.

EXAMPLE: CI-CHECKEDTEST ASSERTED · examples/format-output/src/reference_examples.rs#format-escaping
assert_eq!(format!("{{value}}"), "{value}");

Fix invalid format string errors

Section titled “Fix invalid format string errors”

Rust parses the format string at compile time. Unmatched braces or malformed replacement fields therefore stop compilation instead of becoming a runtime formatting failure.

EXAMPLE: CI-CHECKED FAILURECOMPILE-FAIL ASSERTED · examples/compile-fail/invalid-format-string.rs#invalid-format-string
fn main() {
    println!("{");
}

The failure above is compiled intentionally in CI and must continue to produce an invalid format string diagnostic. When you need literal braces in the output, escape them as {{ and }} rather than leaving a single brace in the format text.

Go deeper

Section titled “Go deeper”

Use the Rust Format Explorer to split a field into its grammar tokens.