fastfmt

cargo fmt, then put the one-liners back.

Stable rustfmt expands constructs such as short function bodies, statement-position if/else, and short match or struct bodies. Some options for keeping them compact are nightly-only. Others have no rustfmt option. fastfmt runs rustfmt through stdin, then rejoins comment-free constructs within its size and width limits, giving fastai-style compact Rust from a stable toolchain.

Install and use

pip install fastfmt

The wheel contains one cargo-fastfmt binary and no Python code. With the binary on PATH, Cargo recognizes it as a subcommand:

cargo fastfmt            # format the current directory tree in place
cargo fastfmt --check    # exit 1 listing files that would change; write nothing
cargo fastfmt src lib.rs # format specific files or directories

rustfmt must also be on PATH. Install it with rustup component add rustfmt.

--width N sets the line cap for rustfmt and joined one-liners, which defaults to 160.

On a normal run, fastfmt creates or updates the target project's rustfmt.toml with disable_all_formatting = true. This prevents cargo fmt or editor rustfmt runs from undoing the compact formatting. fastfmt overrides the setting for its own pass. No pre-existing config file is needed.

--check never writes the config. It reports a missing or disabled guard as a required update.

Comment joining

Opt in to joining hard-wrapped comment paragraphs in Cargo.toml:

[package.metadata.fastfmt]
join_comments = true

Use [workspace.metadata.fastfmt] for workspace-wide defaults. Package settings override the workspace setting, including join_comments = false. Comment joining is off when neither sets it.

Consecutive standalone //, ///, or //! comments with matching indentation and markers are treated as Markdown. Soft paragraph breaks become spaces, including wrapped continuations within list items and block quotes. Paragraphs can exceed --width; fastfmt does not rewrap them.

Lists, code blocks, tables, headings, inline code, raw HTML, and explicit Markdown hard breaks (a trailing backslash or two spaces) retain their structure. Use lists for separate points and code fences for diagrams, disabled code, or other layout-sensitive text. Unmarked line breaks within prose are joined. Trailing comments and block comments are not joined.

Joining rules

A construct can join onto one line when it fits the width cap and contains no comments or multiline tokens. The supported constructs are:

• Ordinary blocks, including function, closure, if/else, and loop bodies, with at most one statement, optionally followed by a tail expression.
• match, struct, and enum bodies with up to three arms, fields, or variants.
• Single-item impl blocks.

Semicolons are preserved exactly. A statement-position else starts on a new line. An if/else used as a value stays on one line.

Joins run innermost-first and repeat until nothing more can join. This allows nested constructs to collapse fully. Blocks used as match-arm bodies stay expanded. Blocks containing comments or multiline strings retain rustfmt's output.