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Guide to Scala 3 Compiler Contribution

Testing Your Changes


It is important to add tests before a pull request, to verify that everything is working as expected, and act as proof of what is valid/invalid Scala code (in case it is broken in the future). In this section you will see the testing procedures in Scala 3.

Running all Tests

To run all tests of Scala 3, including for compiler, REPL, libraries and more, run the following in sbt:

$ sbt
sbt:scala3> scala3-bootstrapped/test

Often however it is not necessary to test everything if your changes are localised to one area, you will see in the following sections the different kinds of tests, and how to run individual tests.

Compilation Tests

Compilation tests run the compiler over input files, using various settings. Input files are found within the tests/ directory at the root of the compiler repo.

Test input files are categorised further by placing them in the subdirectories of the tests/ directory. A small selection of test categories include:

  • tests/pos – tests that should compile: pass if compiles successfully.
  • tests/neg – should not compile: pass if fails compilation. Useful, e.g., to test an expected compiler error.
  • tests/run – these tests not only compile but are also run.

Naming and Running a Test Case

Tests are, by convention, named after the number of the issue they are fixing. e.g. if you are fixing issue 101, then the test should be named i101.scala, for a single-file test, or be within a directory called i101/ for a multi-file test.

To run the test, invoke the sbt command testCompilation i101 (this will match all tests with "i101" in the name, so it is useful to use a unique name)

The test groups – pos, neg, etc. – are defined in CompilationTests. If you want to run a group of tests, e.g. pos, you can do so via testOnly *CompilationTests -- *pos command.

Testing a Single Input File

If your issue is reproducible by only one file, put that file under an appropriate category. For example, if your issue is about getting rid of a spurious compiler error (that is a code that doesn’t compile should, in fact, compile), you can create a file tests/pos/i101.scala.

Testing Multiple Input Files

If you need more than one file to reproduce an issue, create a directory instead of a file e.g. tests/pos/i101/, and put all the Scala files that are needed to reproduce the issue there. There are two ways to organise the input files within:

1: Requiring classpath dependency: Sometimes issues require one file to be compiled after the other, (e.g. if the issue only happens with a library dependency, like with Java interop). In this case, the outputs of the first file compiled will be available to the next file compiled, available via the classpath. This is called separate compilation.

To achieve this, within tests/pos/i101/, add a suffix _n to each file name, where n is an integer defining the order in which the file will compile. E.g. if you have two files, Lib.scala and Main.scala, and you need them compiled separately – Lib first, Main second, then name them Lib_1.scala and Main_2.scala.

2: Without classpath dependency: If your issue does not require a classpath dependency, your files can be compiled in a single run, this is called joint compilation. In this case use file names without the _n suffix.

Checking Program Output

tests/run tests verify the run-time behaviour of a test case. The output is checked by invoking a main method on a class Test, this can be done with either

@main def Test: Unit = assert(1 > 0)


object Test extends scala.App:
  assert(1 > 0)

If your program also prints output, this can be compared against *.check files. These contain the expected output of a program. Checkfiles are named after the issue they are checking, e.g. tests/run/i101.check will check either tests/run/i101.scala or tests/run/i101/.

Checking Compilation Errors

tests/neg tests verify that a file does not compile, and user-facing errors are produced. There are other neg categories such as neg-custom-args, i.e. with neg prefixing the directory name. Test files in the neg* categories require annotations for the lines where errors are expected. To do this add one // error token to the end of a line for each expected error. For example, if there are three expected errors, the end of the line should contain // error // error // error.

You can verify the content of the error messages with a *.check file. These contain the expected output of the compiler. Checkfiles are named after the issue they are checking, e.g. i101.check will check either tests/neg/i101.scala or tests/neg/i101/. Note: checkfiles are not required for the test to pass, however they do add stronger constraints that the errors are as expected.

If Checkfiles do not Match Output

If the actual output mismatches the expected output, the test framework will dump the actual output in the file *.check.out and fail the test suite. It will also output the instructions to quickly replace the expected output with the actual output, in the following format:

Test output dumped in: tests/neg/Sample.check.out
  See diff of the checkfile
    > diff tests/neg/Sample.check tests/neg/Sample.check.out
  Replace checkfile with current output
    > mv tests/neg/Sample.check.out tests/neg/Sample.check

Tips for creating Checkfiles

To create a checkfile for a test, you can do one of the following:

  1. Create an empty checkfile
    • then add arbitrary content
    • run the test
    • when it fails, use the mv command reported by the test to replace the initial checkfile with the actual output.
  2. Manually compile the file you are testing with scala3/scalac
    • copy-paste whatever console output the compiler produces to the checkfile.

Automatically Updating Checkfiles

When complex or many checkfiles must be updated, testCompilation can run in a mode where it overrides the checkfiles with the test outputs.

$ sbt
> testCompilation --update-checkfiles

Use --help to see all the options

$ sbt
> testCompilation --help

Bootstrapped-only tests

To run testCompilation on a bootstrapped Dotty compiler, use scala3-compiler-bootstrapped/testCompilation (with the same syntax as above). Some tests can only be run in bootstrapped compilers; that includes all tests with with-compiler in their name.

From TASTy tests

testCompilation has an additional mode to run tests that compile code from a .tasty file. Modify the lists in compiler/test/dotc to enable or disable tests from .tasty files.

$ sbt
> testCompilation --from-tasty

Unit Tests

Unit tests cover the other areas of the compiler, such as interactions with the REPL, scripting tools and more. They are defined in compiler/test, so if your use case isn’t covered by this guide, you may need to consult the codebase. Some common areas are highlighted below:

SemanticDB tests

To test the SemanticDB output from the extractSemanticDB phase (enabled with the -Xsemanticdb flag), run the following sbt command:

$ sbt
sbt:scala3> scala3-compiler-bootstrapped/testOnly

SemanticdbTests uses source files in tests/semanticdb/expect to generate “expect files”: these verify both

  • SemanticDB symbol occurrences inline in sourcecode (*.expect.scala)
  • complete output of all SemanticDB information (metac.expect).

Expect files are used as regression tests to detect changes in the compiler. Their correctness is determined by human inspection.

If expect files change then SemanticdbTests will fail, and generate new expect files, providing instructions for comparing the differences and replacing the outdated expect files.

If you are planning to update the SemanticDB output, you can do it in bulk by running the command

$ sbt
sbt:scala3> scala3-compiler/Test/runMain

then compare the changes via version control.

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