Formatter spec

In order to keep the design of treefmt simple, we want all of the formatters to adhere to a unified specification. This document outlines that spec.

Command-line interface

A formatter MUST adhere to the following interface:

<command> [options] [...<files>]


  • <command> is the name of the formatter.
  • [options] is any number of flags and options that the formatter wants to provide.
  • [...<files>] is one or more files that the formatter should process.

Whenever the program is invoked with the list of files, it MUST only process all the files that are passed and format them. Files that are not passed should never be formatted.

If, and only if, a file format has changed, the formatter MUST write the new content in place of the original file.


$ rustfmt --edition 2018 src/ src/

Formatting details

A formatter MUST preserve validity. This is a strong contract; the syntax and semantics must never be broken by the formatter.

A formatter SHOULD be idempotent. Meaning that if the formatter it run twice on a file, the file should not change on the second invocation.

The formatter MUST NOT do file traversal when a list of files is passed to it. This is the responsibility of treefmt.

Design notes

We assume that the code is stored in source control, which is why it's fine to write the new content in place.