Format Rust code following rustfmt conventions with configurable edition and style options.
rustfmt is the official Rust formatter, installed with the Rust toolchain. It is a command-line program that runs locally: it parses a .rs file on your machine and re-emits it with the spacing, indentation, and line breaks defined by the Rust Style Guide - the code is never uploaded anywhere. Run rustfmt file.rs for one file, cargo fmt for a whole crate. The surprise most users hit: it refuses code it cannot parse, so a syntax error or an edition mismatch means the file is left untouched, and it only reflows layout - it never removes unused imports, renames anything, or rewrites comments.
A concrete input and expected output from the current implementation.
Input
fn main(){let x:Vec<u32>=vec![1,2,3];println!("{:?}",x);} Expected output
fn main() {
let x: Vec<u32> = vec![1, 2, 3];
println!("{:?}", x);
} rustfmt put a space before the opening brace, moved the body to its own line at four-space indentation, and normalized spacing: around the =, after the type colon, and after commas inside the vec! and println! arguments. Running the output through rustfmt again returns the same text unchanged.
rustfmt parses the source into a syntax tree and rebuilds each item by applying the layout rules from the Rust Style Guide: token spacing, indentation, where lines break, and whether a call fits on one line within the default 100-column max width. Code it cannot parse is rejected with an error instead of reformatted, so the output is always valid and behavior-neutral.
A rustfmt.toml or .rustfmt.toml in the crate root tunes decisions: max_width, tab_spaces, use_small_heuristics, and others. --edition selects the language edition (cargo fmt takes it from Cargo.toml automatically), and --check only reports whether the file is already formatted, which is how cargo fmt --check works as a CI gate. Stable rustfmt promises stable output across stable releases for a fixed configuration.
Run directly, rustfmt defaults to edition 2015, so newer syntax (let-else, for example) fails to parse until you pass --edition 2021 or newer. cargo fmt reads the edition from Cargo.toml automatically, which is why it usually works where plain rustfmt on a standalone file does not.
No. It changes only whitespace and line breaks, never semantics, and it will not remove unused imports, rename identifiers, or silence clippy lints. It guarantees consistent layout, not idiomaticity - lint suggestions and compiler-applicable fixes are the job of cargo clippy and cargo fix, and rustfmt does not attempt them.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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