To develop custom scalafix rules, you will first need to create an sbt project.


Run the following commands to generate a skeleton project:

cd reponame # The project you want to implement rules for.

# --rule= should ideally match the GitHub repo name, to make
# it possible for users to run `scalafix "github:org/reponame/v1.0"`
sbt new scalacenter/scalafix.g8 --rule="reponame" --version="v1.0"
cd scalafix
sbt tests/test

Note that the scalafix directory is a self-contained sbt build and can be put into the root directory of your repo. The tests are written using scalafix-testkit.


scalafix-testkit is a module to help you write/run/debug/test scalafix rules.

The scalacenter/scalafix.g8 template boilerplate already setups a project which uses scalafix-testkit.

The anatomy of a scalafix-testkit project is like this

├── rules    // rule implementations
├── input    // code that runs through rule
├── output   // expected output from running input on rules
└── tests    // tiny project where unit tests run

scalafix-testkit features include:

  • Input code for rules and expected output is written in plain .scala files, with full IDE support.

  • Input and expected output files are kept in two separate sbt projects. Each project can have its own set of dependencies.

  • Each individual *.scala file in the input project can have a custom .scalafix.conf configuration provided in a comment at the top of the file.

  • Test failures are reported as unified diffs from the obtained output of the rule and the expected output in the output project.

  • Assert that a LintMessage is expected at a particular line by suffixing the line with the comment // assert: <LintCategory>. For an example, see the NoInfer test suite:

val x = List(1, "")// assert: NoInfer.any

It’s also possible to assert the offset position and the message contents with a multiline comment:

Option(1).get /* assert: Disable.get
Option.get is the root of all evils

If you must Option.get, wrap the code block with
// scalafix:off Option.get
// scalafix:on Option.get
  • The test can fail if:
    • An assert was added but it was not reported by the linter (unreported)
    • A lint message was reported but it was not asserted (unexpected)
    • An assert was added and a message was reported on that line but it does not match the message exactly. For example the offset can be wrong by a few column or you can have a typo in your assert message.