Support code exclusion from coverage instrumentation

[jozic]

Original scalac scoverage plugin supports excluding code from instrumentation and reports
https://github.com/scoverage/scalac-scoverage-plugin#excluding-code-from-coverage-stats
Scala 3 can support it as well, to make transition from Scala 2 easier.

Also, any hints for volunteers on where to start are highly appreciated :)
Thank you!

[TheElectronWill]

Okay, here's some hint :)
Coverage instrumentation is done by the phase instrumentCoverage in compiler/src/dotty/tools/dotc/transform/InstrumentCoverage.scala.

The most performant form of exclusion is to exclude files. The current "compilation unit" provides the source that is being compiled:

ctx.compilationUnit.source

This could be used to disable the instrumentation phase according to a collection of regex-like strings. We have -coverage-out to configure the output of scoverage files, we could have -coverage-exclude-files or something like that. Such options are defined in ScalaSettings, you can see how -coverage-out has been defined.

Excluding packages (or worse, classes) looks a bit more complicated.

[tOverney]

It would be great to have the coverage exclusion supported in Scala3;
We mainly rely on it to remove any instrumentation from generated classes in src_managed.
We do this with coverageExcludedPackages

[george-wilson-rea]

I would love to see coverage checking exclusion for Scala 3. This is the last thing that is preventing me from upgrading many projects to Scala 3.

[jtjeferreira]

I would like to add that any ignoring mechanism should also support // $COVERAGE-OFF$ comments...

[TonioGela]

@TheElectronWill do you know how also this feature can be supported by the compiler?

[TheElectronWill]

This one could be more difficult to implement because it would require to modify the Parser and to pass down the exclusion information to the InstrumentCoverage phase. IMO an annotation would be easier (because annotations are already taken care of by the Parser and won't require a special case for the comments).

[TonioGela]

I see, thanks for the pointer

[jozic]

Annotations can be easier to implement but harder to use. Comments allow to exclude anything, while annotations can be applied to elements only afaik. So comments would be a preferred solution for users (at least for me :)), especially given that it works like that for Scala 2 for years. Cross compiling also would be easier.

Having said that I do understand we need to find a balance.

[maslovalex]

Does anybody knows if #19727 planned to be back-ported to LTS branch ?