Get more info on Heisenbugs

1) Check that searched scope is consistent
2) Do a linear search for symbol with name, and report
   if something was found that way.

Author: odersky

src/dotty/tools/dotc/core/Scopes.scala

@@ -142,6 +142,8 @@ object Scopes {
     def openForMutations: MutableScope = unsupported("openForMutations")
 
     final def toText(printer: Printer): Text = printer.toText(this)
+
+    def checkConsistent()(implicit ctx: Context) = ()
   }
 
   /** A subclass of Scope that defines methods for entering and
@@ -240,6 +242,7 @@ object Scopes {
       else {
         hashTable = new Array[ScopeEntry](tableSize)
         enterAllInHash(lastEntry)
+        // checkConsistent() // DEBUG
       }
 
     private def enterAllInHash(e: ScopeEntry, n: Int = 0)(implicit ctx: Context): Unit = {
@@ -378,6 +381,17 @@ object Scopes {
     }
 
     override def openForMutations: MutableScope = this
+
+    /** Check that all symbols in this scope are in their correct hashtable buckets. */
+    override def checkConsistent()(implicit ctx: Context) = {
+      var e = lastEntry
+      while (e != null) {
+        var e1 = lookupEntry(e.name)
+        while (e1 != e && e1 != null) e1 = lookupNextEntry(e1)
+        assert(e1 == e, s"PANIC: Entry ${e.name} is badly linked")
+        e = e.prev
+      }
+    }
   }
 
   /** Create a new scope */

src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -383,6 +383,9 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
           if (sym.exists || owner.ne(defn.ObjectClass)) sym else declIn(defn.AnyClass)
       }
 
+      def slowSearch(name: Name): Symbol =
+        owner.info.decls.find(_.name == name).getOrElse(NoSymbol)
+
       def nestedObjectSymbol: Symbol = {
         // If the owner is overloaded (i.e. a method), it's not possible to select the
         // right member, so return NoSymbol. This can only happen when unpickling a tree.
@@ -414,13 +417,16 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
         fromName(name.toTermName.expandedName(owner)) orElse {
           // (3) Try as a nested object symbol.
           nestedObjectSymbol orElse {
-            //              // (4) Call the mirror's "missing" hook.
+            // (4) Call the mirror's "missing" hook.
             adjust(ctx.base.missingHook(owner, name)) orElse {
               // println(owner.info.decls.toList.map(_.debugString).mkString("\n  ")) // !!! DEBUG
               //              }
               // (5) Create a stub symbol to defer hard failure a little longer.
               System.err.println(i"***** missing reference, looking for $name in $owner")
               System.err.println(i"decls = ${owner.info.decls}")
+              owner.info.decls.checkConsistent()
+              if (slowSearch(name).exists)
+                System.err.println(i"**** slow search found: ${slowSearch(name)}")
               new Exception().printStackTrace()
               ctx.newStubSymbol(owner, name, source)
             }