fastfmt

cargo fmt, then put the one-liners back.

Stable rustfmt always breaks small constructs onto multiple lines: a single-expression fn body, a statement-position if/else, a short match or struct body. The options that would prevent this are nightly-only, and some shapes have no option at all. fastfmt runs rustfmt (via stdin, so no config file is needed), then re-joins those constructs when they are comment-free, small, and within a width cap, giving fastai-style compact Rust from a stable toolchain.

Install

pip install fastfmt

The wheel ships a single cargo-fastfmt binary and no Python code. With it on PATH, cargo picks it up 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

--width N sets the rustfmt line cap (default 130). Joined one-liners use tighter caps: 105 columns, or 80 for a two-statement block. rustfmt must be on PATH (rustup component add rustfmt).

On each normal run, fastfmt creates or updates the target project's rustfmt.toml with disable_all_formatting = true. This makes accidental cargo fmt and editor rustfmt runs harmless; fastfmt overrides the setting for its own rustfmt pass. In --check mode the config is never written, and a missing or disabled guard is reported as a required update.

What gets joined

A block joins onto one line only when nothing in it is a comment or spans multiple lines itself, and the result fits the cap: fn, if/else, and loop bodies with one statement or expression, match/struct/enum bodies with up to three arms, variants, or fields, and 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 to a fixpoint, so nested one-liners collapse fully. Match arms stay expanded, and any block containing a comment or a multi-line string is left exactly as rustfmt wrote it.