Deterministically randomises test compilation order

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -788,7 +788,7 @@ object Trees {
     def complete(implicit ctx: Context): T
   }
 
-  // ----- Generic Tree Instances, inherited from  `tpt` and `untpd`.
+  // ----- Generic Tree Instances, inherited from `tpt` and `untpd`.
 
   abstract class Instance[T >: Untyped <: Type] extends DotClass { inst =>
 

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1793,7 +1793,7 @@ object Types {
     override def newLikeThis(prefix: Type)(implicit ctx: Context): TermRef =
       fixDenot(TermRef.withSig(prefix, name, sig), prefix)
 
-    override def shadowed(implicit ctx: Context): NamedType = 
+    override def shadowed(implicit ctx: Context): NamedType =
       fixDenot(TermRef.withSig(prefix, name.shadowedName, sig), prefix)
 
     override def equals(that: Any) = that match {

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

@@ -15,6 +15,7 @@ import scala.util.control.NonFatal
 import scala.util.Try
 import scala.collection.mutable
 import scala.util.matching.Regex
+import scala.util.Random
 
 import core.Contexts._
 import reporting.{ Reporter, TestReporter }
@@ -1010,8 +1011,11 @@ trait ParallelTesting { self =>
   /** Compiles a directory `f` using the supplied `flags`. This method does
    *  deep compilation, that is - it compiles all files and subdirectories
    *  contained within the directory `f`.
+   *
+   *  By default, files are compiled in alphabetical order. An optional seed
+   *  can be used for randomization.
    */
-  def compileDir(f: String, flags: Array[String])(implicit outDirectory: String): CompilationTest = {
+  def compileDir(f: String, flags: Array[String], randomOrder: Option[Int] = None)(implicit outDirectory: String): CompilationTest = {
     val callingMethod = getCallingMethod
     val outDir = outDirectory + callingMethod + "/"
     val sourceDir = new JFile(f)
@@ -1021,11 +1025,18 @@ trait ParallelTesting { self =>
       if (f.isDirectory) f.listFiles.flatMap(flatten)
       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 + "/" + sourceDir.getName + "/")
     targetDir.mkdirs()
 
-    val target = JointCompilationSource(callingMethod, flatten(sourceDir), flags, targetDir)
+    val target = JointCompilationSource(callingMethod, randomized, flags, targetDir)
     new CompilationTest(target)
   }