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.