Add a per-run cache for member symbols

We are all used to calls to `definitions.PredefModule`, or
`defintions.Predef_???` to grab the symbol of some well known
entity. But you'll notice that some of these are lazy vals,
and others are defs.

Why is this so? In the presentation compiler, a member like
`Predef.???` will be assigned a new symbol after the user
browses into `Predef.scala`.

Mistakenly using vals in definitions leads to subtle
IDE bugs like SI-7678. We are able to trigger these
situations in tests, as noted in the comments of that issue.

Changing the vals to defs, on the other hand, has a performance
penalty. Some schemes to workaround this have shown up:
cache them per-implicit search, or compare method names and owners
rather than symbols on hot paths in the type checker.

This commit introduces a facility to cache these sort of symbols
per-run, and uses it to check for `Predef.conforms` and
and for the class/type tag materializers.

A followup pull request (WIP: https://github.com/retronym/scala/compare/ticket/7678-2)
will expand the use of to address the widespread and unsafe caching
of member symbols that I found while investigating SI-7678.

1 files changed, 4 insertions, 10 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
index 8a3bba7d0f..dc60631421 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
@@ -337,6 +337,9 @@ trait Implicits {
     val undetParams = if (isView) Nil else context.outer.undetparams
     val wildPt = approximate(pt)
 
+    private val runDefintions = currentRun.runDefinitions
+    import runDefintions.{ TagMaterializers, TagSymbols, Predef_conforms, PartialManifestClass, ManifestSymbols }
+
     def undet_s = if (undetParams.isEmpty) "" else undetParams.mkString(" inferring ", ", ", "")
     def tree_s = typeDebug ptTree tree
     def ctx_s = fullSiteString(context)
@@ -806,7 +809,7 @@ trait Implicits {
 
       private def isIneligible(info: ImplicitInfo) = (
            info.isCyclicOrErroneous
-        || isView && isPredefMemberNamed(info.sym, nme.conforms)
+        || isView && (info.sym eq Predef_conforms)
         || shadower.isShadowed(info.name)
         || (!context.macrosEnabled && info.sym.isTermMacro)
       )
@@ -1105,13 +1108,6 @@ trait Implicits {
       }
     }
 
-    private def TagSymbols =  TagMaterializers.keySet
-    private val TagMaterializers = Map[Symbol, Symbol](
-      ClassTagClass    -> materializeClassTag,
-      WeakTypeTagClass -> materializeWeakTypeTag,
-      TypeTagClass     -> materializeTypeTag
-    )
-
     /** Creates a tree will produce a tag of the requested flavor.
       * An EmptyTree is returned if materialization fails.
       */
@@ -1173,8 +1169,6 @@ trait Implicits {
       else SearchFailure
     }
 
-    private val ManifestSymbols = Set[Symbol](PartialManifestClass, FullManifestClass, OptManifestClass)
-
     /** Creates a tree that calls the relevant factory method in object
       * scala.reflect.Manifest for type 'tp'. An EmptyTree is returned if
       * no manifest is found. todo: make this instantiate take type params as well?