Testing in Dotty

Running all tests in Dotty is as simple as:

$ sbt test

There are currently several forms of tests in Dotty. These can be split into two categories:

Unit tests

These tests can be found in <sub-project>/test and are used to check functionality of specific parts of the codebase in isolation e.g: parsing, scanning and message errors.

Running a single unit test class from sbt is as simple as:

> testOnly absolute.path.to.TestClass

You can further restrict the executed tests to a subset of TestClass methods as follows:

> testOnly absolute.path.to.TestClass -- *methodName

Integration tests

These tests are Scala source files expected to compile with Dotty (pos tests), along with their expected output (run tests) or errors (neg tests).

All of these tests are contained in the ./tests/* directories.


Historically these tests needed a structure which was generated by running the unit tests, and then that structure was in turn used by scala-partest to run compilation tests in parallel.

This test suite can still be used (and is currently a part of the CI to check that it has the same outcome as the new test suite). It is invoked from sbt by running one of the following commands:

> partest-only-no-bootstrap
> partest-only
> partest
  • partest-only-no-bootstrap will only run the integration tests
  • partest-only will bootstrap the compiler and run the integration tests
  • partest will bootstrap the compiler, run the unit tests and then the integration tests

dotty parallel test suite

The new test suite will soon become the standard integration test runner. It has several advantages over the old implementation:

  • integrates with JUnit, without the need for setup
  • reuses the same VM for compilation
  • allows filtering of tests
  • runs much faster (almost 2x)

Currently to run these tests you need to invoke from sbt:

> testOnly dotty.tools.dotc.CompilationTests

This might be aliased in the future. It is also possible to run tests filtered by using:

> vulpix i2147.scala

This will run both the test ./tests/pos/i2147.scala and ./tests/partest-test/i2147.scala since both of these match the given string. This also means that you could run vulpix with no arguments to run all integration tests.