Add documentation

Author: nicolasstucki

compiler/src/dotty/tools/dotc/typer/UnusedUtil.scala

@@ -12,15 +12,21 @@ import dotty.tools.dotc.core.Flags._
  */
 object UnusedUtil {
 
+  /** Transforms the tree into a its default tree.
+   *  Performed to shrink the tree that is known to be erased later.
+   */
   def normalizeUnusedExpr(tree: Tree, msg: String)(implicit ctx: Context): Tree = {
     if (!isPureExpr(tree))
       ctx.warning(msg + "This expression will not be evaluated.", tree.pos)
     defaultValue(tree.tpe)
   }
 
-  def normalizeUnusedRhs(tree: Tree, sym: Symbol)(implicit ctx: Context) = {
-    if (sym.is(Unused) && tree.tpe.exists) normalizeUnusedExpr(tree, "Expression is on the RHS of an `unused` " + sym.showKind + ". ")
-    else tree
+  /** Transforms the rhs tree into a its default tree if it is in an `unused` val/def.
+   *  Performed to shrink the tree that is known to be erased later.
+   */
+  def normalizeUnusedRhs(rhs: Tree, sym: Symbol)(implicit ctx: Context) = {
+    if (sym.is(Unused) && rhs.tpe.exists) normalizeUnusedExpr(rhs, "Expression is on the RHS of an `unused` " + sym.showKind + ". ")
+    else rhs
   }
 
 }