Optimization to string concatenation.

with a primitive CONCAT and several opcodes, I'm assuming nobody will
oppose this on principle. Given an expression of the exact form "" + x
(that is, the NPE-immune and much less ugly way to call .toString) we
used to get 4x the bytecode and create an unneeded StringBuilder. Now
this:

0:	aload_1
1:	invokestatic	#20; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
4:	areturn

Closes #4160, review by dragos.

1 files changed, 15 insertions, 0 deletions

diff --git a/test/files/run/concat-two-strings.scala b/test/files/run/concat-two-strings.scala
new file mode 100644
index 0000000000..c8881aa146
--- /dev/null
+++ b/test/files/run/concat-two-strings.scala
@@ -0,0 +1,15 @@
+/** This doesn't test that the optimization is working, only that
+ *  nothing is exploding.
+ */
+object Test {
+  def f1(x: AnyRef)      = "" + x
+  def f2(x: Int)         = "" + x
+  def f3(x: Array[Char]) = "" + x
+  def f4(x: List[Int])   = "" + x
+  def f5(x: Any)         = "" + x
+  def f6(x: AnyVal)      = "" + x
+
+  def main(args: Array[String]): Unit = {
+    List(f1("a"), f2(5), f3(null), f3(Array('a')), f4(List(1)), f5(null), f6(55d)) mkString ""
+  }
+}