Mixin: fix the initialization of traits

Before this commit, the following code:
trait Hello {
  println("Hello")
  val x: Int = 1
  println("World")
}

Became:
<trait> trait Hello extends Object {
  def <init>(): Hello = {
    {
      ()
    }
    this
  }
  <accessor> def x(): Int
  protected def initial$x(): Int = {
    println("Hello")
    1
  }
}

Notice that the initialization statements after the last getter were
missing, this is now fixed:
<trait> trait Hello extends Object {
  def <init>(): Hello = {
    {
      println("World")
      ()
    }
    this
  }
  <accessor> def x(): Int
  protected def initial$x(): Int = {
    println("Hello")
    1
  }
}

Author: smarter

src/dotty/tools/dotc/transform/Mixin.scala

@@ -96,7 +96,7 @@ class Mixin extends MiniPhaseTransform with SymTransformer { thisTransform =>
 
     def traitDefs(stats: List[Tree]): List[Tree] = {
       val initBuf = new mutable.ListBuffer[Tree]
-      stats flatMap {
+      stats.flatMap({
         case stat: DefDef if stat.symbol.isGetter && !stat.rhs.isEmpty && !stat.symbol.is(Flags.Lazy)  =>
           // make initializer that has all effects of previous getter,
           // replace getter rhs with empty tree.
@@ -114,7 +114,7 @@ class Mixin extends MiniPhaseTransform with SymTransformer { thisTransform =>
         case stat =>
           initBuf += stat
           Nil
-      }
+      }) ++ initBuf
     }
 
     def transformSuper(tree: Tree): Tree = {

tests/run/traitInit.check

@@ -0,0 +1,2 @@
+Hello
+World

tests/run/traitInit.scala

@@ -0,0 +1,13 @@
+trait Hello {
+  println("Hello")
+  val x: Int = 1
+  println("World")
+}
+
+class A extends Hello
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    new A
+  }
+}