fixes mirrorThatLoaded in JavaMirrors

`mirrorThatLoaded(sym: Symbol): Mirror` in JavaMirrors used to iterate over
`lazy val mirrors = new WeakHashMap[ClassLoader, WeakReference[JavaMirror]]()`
to find out what mirror has loaded a certain symbol.

It worked okay until yesterday when I noticed failing tests, which crashed
when weak references in mirrors were dereferenced with get.get. Apparently
some mirrors were collected, and the logic in JavaMirror didn't account for
that possibility.

When fixing this bug, I noticed that mirrors can become unreachable even
if there are still reachable symbols created by those mirrors. That doesn't
make sense, therefore I fixed this bug as well by introducing a strong ref
from root symbols to the enclosing mirror. Therefore, any active symbol
will have a strong reference to the enclosing mirror by the virtue of the
owner chain.

1 files changed, 3 insertions, 10 deletions

diff --git a/src/reflect/scala/reflect/runtime/JavaMirrors.scala b/src/reflect/scala/reflect/runtime/JavaMirrors.scala
index 0d9e90d3a6..2f12ba59a2 100644
--- a/src/reflect/scala/reflect/runtime/JavaMirrors.scala
+++ b/src/reflect/scala/reflect/runtime/JavaMirrors.scala
@@ -1236,16 +1236,9 @@ trait JavaMirrors extends internal.SymbolTable with api.JavaUniverse { thisUnive
   override def scopeTransform(owner: Symbol)(op: => Scope): Scope =
     if (owner.isPackageClass) owner.info.decls else op
 
-  private lazy val rootToLoader = new WeakHashMap[Symbol, ClassLoader]
-
-  override def mirrorThatLoaded(sym: Symbol): Mirror = {
-    val root = sym.enclosingRootClass
-    def findLoader = {
-      val loaders = (mirrors collect { case (cl, ref) if ref.get.get.RootClass == root => cl })
-      assert(loaders.nonEmpty, sym)
-      loaders.head
-    }
-    mirrors(rootToLoader getOrElseUpdate(root, findLoader)).get.get
+  override def mirrorThatLoaded(sym: Symbol): Mirror = sym.enclosingRootClass match {
+    case root: RootSymbol => root.mirror
+    case _ => abort(s"${sym}.enclosingRootClass = ${sym.enclosingRootClass}, which is not a RootSymbol")
   }
 
   private lazy val syntheticCoreClasses: Map[(String, Name), Symbol] = {