Isolate scoverage modules from their parent modules (com-lihaoyi#3118)

## Context

I encountered an issue with the way scoverage plugin alters the dependency tree of the test module by changing its module dependency from the `outer` module to the `outer.scoverage` module.
This shadow modification of the dependency tree produces a side effect that impacts the IDEA configuration generation.
When the IDEA configuration generation resolves the dependencies instead of using the outer module as a dependency for the test project it depends on the `outer.scoverage` module which should not be generated as it does not exist factually but more virtually.

## Solution

Instead of modifying the dependency tree at compile-time I change it at runtime by removing the `outer.localRunClasspath()` from the `test.runClasspath()` and appending, instead, the `outer.scoverage.localRunClasspath()`

This resolves the issue of modifying the dependency tree but introduces one more compilation as when the `outer.localRunClasspath()` is resolved it will force the compilation of the outer module. In the previous version only the `outer.scoverage` was compiled and not both of them.

Pull request: com-lihaoyi#3118

contrib/scoverage/src/mill/contrib/scoverage/ScoverageModule.scala

@@ -3,8 +3,8 @@ package mill.contrib.scoverage
 import coursier.Repository
 import mill._
 import mill.api.{Loose, PathRef, Result}
-import mill.main.BuildInfo
 import mill.contrib.scoverage.api.ScoverageReportWorkerApi.ReportType
+import mill.main.BuildInfo
 import mill.scalalib.api.ZincWorkerUtil
 import mill.scalalib.{Dep, DepSyntax, JavaModule, ScalaModule}
 import mill.util.Util.millProjectModule
@@ -188,6 +188,7 @@ trait ScoverageModule extends ScalaModule { outer: ScalaModule =>
       PathRef(T.dest)
     }
 
+    override def compileResources: T[Seq[PathRef]] = outer.compileResources
     override def generatedSources: Target[Seq[PathRef]] = T { outer.generatedSources() }
     override def allSources: Target[Seq[PathRef]] = T { outer.allSources() }
     override def moduleDeps: Seq[JavaModule] = outer.moduleDeps
@@ -225,30 +226,25 @@ trait ScoverageModule extends ScalaModule { outer: ScalaModule =>
     def xmlCoberturaReport(): Command[Unit] = T.command { doReport(ReportType.XmlCobertura) }
     def consoleReport(): Command[Unit] = T.command { doReport(ReportType.Console) }
 
-    override def skipIdea = outer.skipIdea
+    override def skipIdea = true
   }
 
   trait ScoverageTests extends ScalaTests {
-    override def upstreamAssemblyClasspath = T {
-      super.upstreamAssemblyClasspath() ++
-        resolveDeps(T.task {
-          outer.scoverageRuntimeDeps().map(bindDependency())
-        })()
-    }
-    override def compileClasspath = T {
-      super.compileClasspath() ++
-        resolveDeps(T.task {
-          outer.scoverageRuntimeDeps().map(bindDependency())
-        })()
-    }
-    override def runClasspath = T {
-      super.runClasspath() ++
+
+    /**
+     * Alter classfiles and resources from upstream modules and dependencies
+     * by removing the ones from outer.localRunClasspath() and replacing them
+     * with outer.scoverage.localRunClasspath()
+     */
+    override def runClasspath: T[Seq[PathRef]] = T {
+      val outerLocalRunClasspath = outer.localRunClasspath().toSet
+      super.runClasspath().filterNot(
+        outerLocalRunClasspath
+      ) ++
+        outer.scoverage.localRunClasspath() ++
         resolveDeps(T.task {
           outer.scoverageRuntimeDeps().map(bindDependency())
         })()
     }
-
-    // Need the sources compiled with scoverage instrumentation to run.
-    override def moduleDeps: Seq[JavaModule] = Seq(outer.scoverage)
   }
 }

example/misc/7-contrib-scoverage/build.sc

@@ -0,0 +1,60 @@
+import mill._, scalalib._
+import $ivy.`com.lihaoyi::mill-contrib-scoverage:`
+
+import mill.contrib.scoverage._
+
+object foo extends RootModule with ScoverageModule {
+  def scoverageVersion = "2.1.0"
+  def scalaVersion = "2.13.11"
+  def ivyDeps = Agg(
+    ivy"com.lihaoyi::scalatags:0.12.0",
+    ivy"com.lihaoyi::mainargs:0.6.2"
+  )
+
+  object test extends ScoverageTests /*with TestModule.Utest */{
+    def ivyDeps = Agg(ivy"com.lihaoyi::utest:0.7.11")
+    def testFramework = "utest.runner.Framework"
+  }
+}
+
+// This is a basic Mill build for a single `ScalaModule`, enhanced with
+// Scoverage plugin. The root module extends the `ScoverageModule` and
+// specifies the version of scoverage version to use here: `2.1.0`. This
+// version can be changed if there is a newer one. Now you can call the
+// scoverage targets to produce coverage reports.
+// The sub test module extends `ScoverageTests` to transform the
+// execution of the various testXXX targets to use scoverage and produce
+// coverage data.
+// This lets us perform the coverage operations but before that you
+// must first run the test.
+// `./mill test` then `./mill scoverage.consoleReport` and get your
+// coverage into your console output.
+//
+// You can download this example project using the *download* link above
+// if you want to try out the commands below yourself. The only requirement is
+// that you have some version of the JVM installed; the `./mill` script takes
+// care of any further dependencies that need to be downloaded.
+
+/** Usage
+
+> ./mill test # Run the tests and produce the coverage data
+...
++ foo.FooTests.simple ...  <h1>hello</h1>
++ foo.FooTests.escaping ...  <h1>&lt;hello&gt;</h1>
+
+> ./mill resolve scoverage._ # List what tasks are available to run from scoverage
+...
+scoverage.consoleReport
+...
+scoverage.htmlReport
+...
+scoverage.xmlCoberturaReport
+...
+scoverage.xmlReport
+...
+
+> ./mill scoverage.consoleReport
+...
+Statement coverage.: 16.67%
+Branch coverage....: 100.00%
+*/

example/misc/7-contrib-scoverage/src/Foo.scala

@@ -0,0 +1,16 @@
+package foo
+import scalatags.Text.all._
+import mainargs.{main, ParserForMethods}
+
+object Foo {
+  def generateHtml(text: String) = {
+    h1(text).toString
+  }
+
+  @main
+  def main(text: String) = {
+    println(generateHtml(text))
+  }
+
+  def main(args: Array[String]): Unit = ParserForMethods(this).runOrExit(args)
+}

example/misc/7-contrib-scoverage/test/src/FooTests.scala

@@ -0,0 +1,16 @@
+package foo
+import utest._
+object FooTests extends TestSuite {
+  def tests = Tests {
+    test("simple") {
+      val result = Foo.generateHtml("hello")
+      assert(result == "<h1>hello</h1>")
+      result
+    }
+    test("escaping") {
+      val result = Foo.generateHtml("<hello>")
+      assert(result == "<h1>&lt;hello&gt;</h1>")
+      result
+    }
+  }
+}