Avoid capturing context in lazy trees

Lazy trees can live longer than runs, so it is important
that they capture as little as possible. In particular they
should not capture contexts.

This change led with a ripple through effect to many changes
where operations now have to parameterzied with contexts, in
particular in what concerns tree folding.

The changes in turn uncovered some areas where dotc was
incompatible with scalac, and flagged correct things as errors.
These will be fixed in the next commits.

Another small twist: EmptyTrees will not be read in delayed mode,
so that one can check for lacking definitions without deserializing
the rhs.

1 files changed, 9 insertions, 7 deletions

diff --git a/src/dotty/tools/dotc/core/pickling/TreeUnpickler.scala b/src/dotty/tools/dotc/core/pickling/TreeUnpickler.scala
index c6c099f2e..72020012d 100644
--- a/src/dotty/tools/dotc/core/pickling/TreeUnpickler.scala
+++ b/src/dotty/tools/dotc/core/pickling/TreeUnpickler.scala
@@ -365,7 +365,7 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table, readPositio
             readByte()
             val end = readEnd()
             val sym = readType().typeSymbol
-            val lazyAnnotTree = readLater(end, _.readTerm())
+            val lazyAnnotTree = readLater(end, rdr => ctx => rdr.readTerm()(ctx))
             annots += Annotation.deferred(sym, _ => lazyAnnotTree.complete)
           case _ =>
             assert(false, s"illegal modifier tag at $currentAddr")
@@ -424,7 +424,9 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table, readPositio
         }
       }
       
-      def readRhs(implicit ctx: Context) = readLater(end, _.readTerm())
+      def readRhs(implicit ctx: Context) = 
+        if (nextByte == EMPTYTREE) EmptyTree
+        else readLater(end, rdr => ctx => rdr.readTerm()(ctx))
 
       def localCtx = ctx.fresh.setOwner(sym)
       
@@ -499,7 +501,7 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table, readPositio
         else EmptyValDef
       fork.indexStats(end)
       val constr = readIndexedDef().asInstanceOf[DefDef]
-      val lazyStats = readLater(end, _.readIndexedStats(localDummy, end))
+      val lazyStats = readLater(end, rdr => ctx => rdr.readIndexedStats(localDummy, end)(ctx))
       addAddr(start,
         untpd.Template(constr, parents, self, lazyStats)
           .withType(localDummy.nonMemberTermRef))
@@ -659,7 +661,7 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table, readPositio
       }
     }
 
-    def readLater[T](end: Addr, op: TreeReader => T)(implicit ctx: Context): Trees.Lazy[T] = {
+    def readLater[T <: AnyRef](end: Addr, op: TreeReader => Context => T): Trees.Lazy[T] = {
       val localReader = fork
       skipTo(end)
       new LazyReader(localReader, op)
@@ -704,9 +706,9 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table, readPositio
       }
   }
 
-  class LazyReader[T](reader: TreeReader, op: TreeReader => T)(implicit ctx: Context) extends Trees.Lazy[T] with DeferredPosition {
-    def complete: T = {
-      val res = op(reader)
+  class LazyReader[T <: AnyRef](reader: TreeReader, op: TreeReader => Context => T) extends Trees.Lazy[T] with DeferredPosition {
+    def complete(implicit ctx: Context): T = {
+      val res = op(reader)(ctx)
       normalizePos(res, parentPos)
       res
     }