rustfmt

A tool for formatting Rust code according to style guidelines.

If you'd like to help out (and you should, it's a fun project!), see Contributing.md and our Code of Conduct.

We are changing the default style used by rustfmt. There is an ongoing RFC process. The last version using the old style was 0.8.6. From 0.9 onwards, the RFC style is the default. If you want the old style back, you can use legacy-rustfmt.toml as your rustfmt.toml.

The current master branch uses libsyntax (part of the compiler). It is published as rustfmt-nightly. The syntex branch uses Syntex instead of libsyntax, it is published (for now) as rustfmt. Most development happens on the master branch, however, this only supports nightly toolchains. If you use stable or beta Rust toolchains, you must use the Syntex version (which is likely to be a bit out of date). Version 0.1 of rustfmt-nightly is forked from version 0.9 of the syntex branch.

You can use rustfmt in Travis CI builds. We provide a minimal Travis CI configuration (see here) and verify its status using another repository. The status of that repository's build is reported by the "travis example" badge above.

Quick start

You can use rustfmt on Rust 1.24 and above.

To install:

rustup component add rustfmt-preview

to run on a cargo project in the current working directory:

cargo fmt

For the latest and greatest rustfmt (nightly required):

rustup component add rustfmt-preview --toolchain nightly

To run:

cargo +nightly fmt

Limitations

Rustfmt tries to work on as much Rust code as possible, sometimes, the code doesn't even need to compile! As we approach a 1.0 release we are also looking to limit areas of instability; in particular, post-1.0, the formatting of most code should not change as Rustfmt improves. However, there are some things that Rustfmt can't do or can't do well (and thus where formatting might change significantly, even post-1.0). We would like to reduce the list of limitations over time.

The following list enumerates areas where Rustfmt does not work or where the stability guarantees do not apply (we don't make a distinction between the two because in the future Rustfmt might work on code where it currently does not):

• a program where any part of the program does not parse (parsing is an early stage of compilation and in Rust includes macro expansion).
• Macro declarations and uses (current status: some macro declarations and uses are formatted).
• Comments, including any AST node with a comment 'inside' (Rustfmt does not currently attempt to format comments, it does format code with comments inside, but that formatting may change in the future).
• Rust code in code blocks in comments.
• Any fragment of a program (i.e., stability guarantees only apply to whole programs, even where fragments of a program can be formatted today).
• Code containing non-ascii unicode characters (we believe Rustfmt mostly works here, but do not have the test coverage or experience to be 100% sure).
• Bugs in Rustfmt (like any software, Rustfmt has bugs, we do not consider bug fixes to break our stability guarantees).

Installation

rustup component add rustfmt-preview

Installing from source

To install from source (nightly required), first checkout to the tag or branch you want to install, then issue

cargo install --path .

This will install rustfmt in your ~/.cargo/bin. Make sure to add ~/.cargo/bin directory to your PATH variable.

Running

You can run Rustfmt by just typing rustfmt filename if you used cargo install. This runs rustfmt on the given file, if the file includes out of line modules, then we reformat those too. So to run on a whole module or crate, you just need to run on the root file (usually mod.rs or lib.rs). Rustfmt can also read data from stdin. Alternatively, you can use cargo fmt to format all binary and library targets of your crate.

You'll probably want to specify the write mode. Currently, there are modes for diff, replace, overwrite, display, coverage, checkstyle, and plain.

• overwrite Is the default and overwrites the original files without creating backups.
• replace Overwrites the original files after creating backups of the files.
• display Will print the formatted files to stdout.
• plain Also writes to stdout, but with no metadata.
• diff Will print a diff between the original files and formatted files to stdout. Will also exit with an error code if there are any differences.
• checkstyle Will output the lines that need to be corrected as a checkstyle XML file, that can be used by tools like Jenkins.

The write mode can be set by passing the --write-mode flag on the command line. For example rustfmt --write-mode=display src/filename.rs

cargo fmt uses --write-mode=overwrite by default.

If you want to restrict reformatting to specific sets of lines, you can use the --file-lines option. Its argument is a JSON array of objects with file and range properties, where file is a file name, and range is an array representing a range of lines like [7,13]. Ranges are 1-based and inclusive of both end points. Specifying an empty array will result in no files being formatted. For example,

rustfmt --file-lines '[
    {"file":"src/lib.rs","range":[7,13]},
    {"file":"src/lib.rs","range":[21,29]},
    {"file":"src/foo.rs","range":[10,11]},
    {"file":"src/foo.rs","range":[15,15]}]'

would format lines 7-13 and 21-29 of src/lib.rs, and lines 10-11, and 15 of src/foo.rs. No other files would be formatted, even if they are included as out of line modules from src/lib.rs.

If rustfmt successfully reformatted the code it will exit with 0 exit status. Exit status 1 signals some unexpected error, like an unknown option or a failure to read a file. Exit status 2 is returned if there are syntax errors in the input files. rustfmt can't format syntactically invalid code. Finally, exit status 3 is returned if there are some issues which can't be resolved automatically. For example, if you have a very long comment line rustfmt doesn't split it. Instead it prints a warning and exits with 3.

You can run rustfmt --help for more information.

Running Rustfmt from your editor

• Vim
• Emacs
• Sublime Text 3
• Atom
• Visual Studio Code using vscode-rust, vsc-rustfmt or rls_vscode through RLS.

Checking style on a CI server

To keep your code base consistently formatted, it can be helpful to fail the CI build when a pull request contains unformatted code. Using --write-mode=diff instructs rustfmt to exit with an error code if the input is not formatted correctly. It will also print any found differences.

A minimal Travis setup could look like this (requires Rust 1.24.0 or greater):

language: rust
before_script:
- rustup component add rustfmt-preview
script:
- cargo fmt --all -- --write-mode=diff
- cargo build
- cargo test

How to build and test

cargo build to build.

cargo test to run all tests.

To run rustfmt after this, use cargo run --bin rustfmt -- filename. See the notes above on running rustfmt.

Configuring Rustfmt

Rustfmt is designed to be very configurable. You can create a TOML file called rustfmt.toml or .rustfmt.toml, place it in the project or any other parent directory and it will apply the options in that file. See rustfmt --config-help for the options which are available, or if you prefer to see visual style previews, Configurations.md.

By default, Rustfmt uses a style which conforms to the Rust style guide that has been formalized through the style RFC process.

Configuration options are either stable or unstable. Stable options can always be used, while unstable ones are only available on a nightly toolchain, and opt-in. See Configurations.md for details.

Tips

• For things you do not want rustfmt to mangle, use one of

  #[rustfmt_skip]  // requires nightly and #![feature(custom_attribute)] in crate root
  #[cfg_attr(rustfmt, rustfmt_skip)]  // works in stable
  

• When you run rustfmt, place a file named rustfmt.toml or .rustfmt.toml in target file directory or its parents to override the default settings of rustfmt. You can generate a file containing the default configuration with rustfmt --dump-default-config rustfmt.toml and customize as needed.

• After successful compilation, a rustfmt executable can be found in the target directory.

• If you're having issues compiling Rustfmt (or compile errors when trying to install), make sure you have the most recent version of Rust installed.

• If you get an error like error while loading shared libraries while starting up rustfmt you should try the following:

  On Linux:

  export LD_LIBRARY_PATH=$(rustc --print sysroot)/lib:$LD_LIBRARY_PATH
  

  On MacOS:

  export DYLD_LIBRARY_PATH=$(rustc --print sysroot)/lib:$DYLD_LIBRARY_PATH
  

  On Windows (Git Bash/Mingw):

  export PATH=$(rustc --print sysroot)/lib/rustlib/x86_64-pc-windows-gnu/lib/:$PATH
  

  (Substitute x86_64 by i686 and gnu by msvc depending on which version of rustc was used to install rustfmt).

License

Rustfmt is distributed under the terms of both the MIT license and the Apache License (Version 2.0).

See LICENSE-APACHE and LICENSE-MIT for details.