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Formatter Specification

In order to keep the design of treefmt simple, we only supports formatters that adhere to a certain standard. This document outlines that standard.

If the formatter you would like to use doesn't comply with the rules, it's often possible to create a wrapper script that transforms the usage to match the specification.

In this design, we rely on treefmt to do the tree traversal, and only invoke the code formatter on the selected files.


In order for the formatter to comply to this spec, it MUST comply with the following:

1. Files passed as arguments

In order to be integrated with treefmt's workflow, the formatter's CLI must be of the form:

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


  • <command> is the name of the formatting tool.
  • [options] is any number of flags and options that the formatter accepts.
  • [...<files>] is one or more files given to the formatter for processing.


$ rustfmt --edition 2018 src/ src/


It SHOULD processes only the specified files. Files that are not passed SHOULD never be formatted.

2. Write to changed files

Whenever there is a change to the code formatting, the code formatter MUST write to the changes back to the original location.

If there is no changes to the original file, the formatter MUST NOT write to the original location.

3. Idempotent

The code formatter SHOULD be indempotent. Meaning that it produces stable outputs.

4. Reliable

We expect the formatter to be reliable and not break the semantics of the formatted files.

Released under the MIT License.