SI-6546 InnerClasses attribute refers to absent class

At issue is that the optimizer would eliminate closure classes
completely, then neglect to eliminate those classes from the
container's InnerClasses attribute. This breaks tooling which
expects those entries to correspond to real classes.

The code change is essentially mgarcia's - I minimized it and
put the caches in perRunCaches, and added the test case which
verifies that after being compiled under -optimise, there are
no inner classes. Before/after:

  7,8d6
  <   InnerClasses:
  <        public final #22; //class A_1$$anonfun$f$1
  37,45c35,40
  <   #21 = Utf8               A_1$$anonfun$f$1
  <   #22 = Class              #21            //  A_1$$anonfun$f$1
  <   #23 = Utf8               Code
  ---
  >   #21 = Utf8               Code

2 files changed, 10 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala b/src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala
index 7c46d648fe..97140aca2e 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala
@@ -86,6 +86,7 @@ abstract class GenASM extends SubComponent with BytecodeWriters with GenJVMASM {
       if (settings.Xdce.value)
         for ((sym, cls) <- icodes.classes if inliner.isClosureClass(sym) && !deadCode.liveClosures(sym)) {
           log(s"Optimizer eliminated ${sym.fullNameString}")
+          deadCode.elidedClosures += sym
           icodes.classes -= sym
         }
 
@@ -624,7 +625,8 @@ abstract class GenASM extends SubComponent with BytecodeWriters with GenJVMASM {
           innerClassBuffer += m
       }
 
-      val allInners: List[Symbol] = innerClassBuffer.toList
+      val allInners: List[Symbol] = innerClassBuffer.toList filterNot deadCode.elidedClosures
+
       if (allInners.nonEmpty) {
         debuglog(csym.fullName('.') + " contains " + allInners.size + " inner classes.")
 
diff --git a/src/compiler/scala/tools/nsc/backend/opt/DeadCodeElimination.scala b/src/compiler/scala/tools/nsc/backend/opt/DeadCodeElimination.scala
index 1beed3f420..db56f61f16 100644
--- a/src/compiler/scala/tools/nsc/backend/opt/DeadCodeElimination.scala
+++ b/src/compiler/scala/tools/nsc/backend/opt/DeadCodeElimination.scala
@@ -40,7 +40,13 @@ abstract class DeadCodeElimination extends SubComponent {
   }
 
   /** closures that are instantiated at least once, after dead code elimination */
-  val liveClosures: mutable.Set[Symbol] = new mutable.HashSet()
+  val liveClosures = perRunCaches.newSet[Symbol]()
+
+  /** closures that are eliminated, populated by GenASM.AsmPhase.run()
+   *  these class symbols won't have a .class physical file, thus shouldn't be included in InnerClasses JVM attribute,
+   *  otherwise some tools get confused or slow (SI-6546)
+   * */
+  val elidedClosures = perRunCaches.newSet[Symbol]()
 
   /** Remove dead code.
    */