Separate compilation enabled in the test framework for compileDir

`compileDir` ignores the `A_1.scala` `B_2.scala` syntax intended
to specify the order of sources compilation when we want to compile them
in several separate runs. This commit leaves the default behavior unchanged
but provides for an option to explicitly specify the separate compilation
mode.

Author: anatoliykmetyuk

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -96,7 +96,8 @@ class CompilationTests extends ParallelTesting {
       "tests/neg-custom-args/fatal-warnings/xfatalWarnings.scala",
       defaultOptions.and("-nowarn", "-Xfatal-warnings")
     ) +
-    compileFile("tests/pos-special/typeclass-scaling.scala", defaultOptions.and("-Xmax-inlines", "40"))
+    compileFile("tests/pos-special/typeclass-scaling.scala", defaultOptions.and("-Xmax-inlines", "40")) +
+    compileDir("tests/pos-special/i6152", defaultOptions, separateCompilation = true)
   }.checkCompile()
 
   @Test def posTwice: Unit = {

compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -1257,30 +1257,34 @@ trait ParallelTesting extends RunnerOrchestration { self =>
    *  By default, files are compiled in alphabetical order. An optional seed
    *  can be used for randomization.
    */
-  def compileDir(f: String, flags: TestFlags, randomOrder: Option[Int] = None, recursive: Boolean = true)(implicit testGroup: TestGroup): CompilationTest = {
+  def compileDir(f: String, flags: TestFlags, randomOrder: Option[Int] = None, recursive: Boolean = true, separateCompilation: Boolean = false)(implicit testGroup: TestGroup): CompilationTest = {
     val outDir = defaultOutputDir + testGroup + JFile.separator
     val sourceDir = new JFile(f)
+    val name = s"compiling '$f' in test '$testGroup'"
     checkRequirements(f, sourceDir, outDir)
 
-    def flatten(f: JFile): Array[JFile] =
-      if (f.isDirectory) {
-        val files = f.listFiles
-        if (recursive) files.flatMap(flatten) else files
-      }
-      else Array(f)
-
-    // Sort files either alphabetically or randomly using the provided seed:
-    val sortedFiles = flatten(sourceDir).sorted
-    val randomized  = randomOrder match {
-      case None       => sortedFiles
-      case Some(seed) => new Random(seed).shuffle(sortedFiles.toList).toArray
-    }
-
     // Directories in which to compile all containing files with `flags`:
     val targetDir = new JFile(outDir + JFile.separator + sourceDir.getName + JFile.separator)
     targetDir.mkdirs()
 
-    val target = JointCompilationSource(s"compiling '$f' in test '$testGroup'", randomized, flags, targetDir)
+    val target =
+      if (separateCompilation) SeparateCompilationSource(name, sourceDir, flags, targetDir) else {
+        def flatten(f: JFile): Array[JFile] =
+          if (f.isDirectory) {
+            val files = f.listFiles
+            if (recursive) files.flatMap(flatten) else files
+          }
+          else Array(f)
+
+        // Sort files either alphabetically or randomly using the provided seed:
+        val sortedFiles = flatten(sourceDir).sorted
+        val randomized  = randomOrder match {
+          case None       => sortedFiles
+          case Some(seed) => new Random(seed).shuffle(sortedFiles.toList).toArray
+        }
+
+        JointCompilationSource(name, randomized, flags, targetDir)
+      }
     new CompilationTest(target)
   }