Avoid name table pollution with fresh existentials

During large compilations runs, the large numbers of globally unique
fresh names for existentials captured from prefixes of `asSeenFrom`.
is a) somewhat wasteful (all these names are interned in the name table)
, and, b) form a pathological case for the current implementation of
`Names#hashValue`, which leads to overfull hash-buckets in the name table.

`hashValue` should probably be improved, but my attempts to do so have
shown a small performance degradation in some benchmarks. So this commit
starts by being more frugal with these names, only uniquely naming
within an `asSeenFrom` operation.

References scala/scala-dev#246

1 files changed, 3 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/internal/tpe/TypeMaps.scala b/src/reflect/scala/reflect/internal/tpe/TypeMaps.scala
index ba4f2bec4b..08219c0634 100644
--- a/src/reflect/scala/reflect/internal/tpe/TypeMaps.scala
+++ b/src/reflect/scala/reflect/internal/tpe/TypeMaps.scala
@@ -512,6 +512,8 @@ private[internal] trait TypeMaps {
         && isBaseClassOfEnclosingClass(sym.owner)
       )
 
+    private var capturedThisIds= 0
+    private def nextCapturedThisId() = { capturedThisIds += 1; capturedThisIds }
     /** Creates an existential representing a type parameter which appears
       *  in the prefix of a ThisType.
       */
@@ -519,7 +521,7 @@ private[internal] trait TypeMaps {
       capturedParams find (_.owner == clazz) match {
         case Some(p) => p.tpe
         case _       =>
-          val qvar = clazz freshExistential nme.SINGLETON_SUFFIX setInfo singletonBounds(pre)
+          val qvar = clazz.freshExistential(nme.SINGLETON_SUFFIX, nextCapturedThisId()) setInfo singletonBounds(pre)
           _capturedParams ::= qvar
           debuglog(s"Captured This(${clazz.fullNameString}) seen from $seenFromPrefix: ${qvar.defString}")
           qvar.tpe