Do not force symbols of annotations when unpickling

This lead to stale symbol errors in `tasty_tools` because the symbol
forcing was bringing forward symbols from the previous run before the
corresponding symbols for the current run were created.

We fix this by adding Annotations#deferredSymAndTree which behaves
similarly to Annotations#deferred but take a by-name symbol.

We also remove TreeUnpickler#LazyAnnotationReader which was apparently
never used.

Author: smarter

compiler/src/dotty/tools/dotc/core/Annotations.scala

@@ -34,14 +34,15 @@ object Annotations {
     def tree(implicit ctx: Context): Tree = t
   }
 
-  abstract case class LazyAnnotation(sym: Symbol) extends Annotation {
+  abstract class LazyAnnotation extends Annotation {
+    override def symbol(implicit ctx: Context): Symbol
+    def complete(implicit ctx: Context): Tree
+
     private var myTree: Tree = null
     def tree(implicit ctx: Context) = {
       if (myTree == null) myTree = complete(ctx)
       myTree
     }
-    def complete(implicit ctx: Context): Tree
-    override def symbol(implicit ctx: Context): Symbol = sym
   }
 
   /** An annotation indicating the body of a right-hand side,
@@ -108,8 +109,19 @@ object Annotations {
       apply(resolveConstructor(atp, args))
     }
 
+    /** Create an annotation where the tree is computed lazily. */
     def deferred(sym: Symbol, treeFn: Context => Tree)(implicit ctx: Context): Annotation =
-      new LazyAnnotation(sym) {
+      new LazyAnnotation {
+        override def symbol(implicit ctx: Context): Symbol = sym
+        def complete(implicit ctx: Context) = treeFn(ctx)
+      }
+
+    /** Create an annotation where the symbol and the tree are computed lazily. */
+    def deferredSymAndTree(sym: => Symbol, treeFn: Context => Tree)(implicit ctx: Context): Annotation =
+      new LazyAnnotation {
+        lazy val symf = sym
+
+        override def symbol(implicit ctx: Context): Symbol = symf
         def complete(implicit ctx: Context) = treeFn(ctx)
       }
 

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -541,9 +541,9 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table, posUnpickle
           case ANNOTATION =>
             readByte()
             val end = readEnd()
-            val sym = readType().typeSymbol
+            val tp = readType()
             val lazyAnnotTree = readLater(end, rdr => ctx => rdr.readTerm()(ctx))
-            annots += Annotation.deferred(sym, _ => lazyAnnotTree.complete)
+            annots += Annotation.deferredSymAndTree(tp.typeSymbol, _ => lazyAnnotTree.complete)
           case _ =>
             assert(false, s"illegal modifier tag at $currentAddr")
         }
@@ -1043,12 +1043,6 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table, posUnpickle
     }
   }
 
-  class LazyAnnotationReader(sym: Symbol, reader: TreeReader) extends LazyAnnotation(sym) {
-    def complete(implicit ctx: Context) = {
-      reader.readTerm()(ctx.withPhaseNoLater(ctx.picklerPhase))
-    }
-  }
-
   /** A lazy datastructure that records how definitions are nested in TASTY data.
    *  The structure is lazy because it needs to be computed only for forward references
    *  to symbols that happen before the referenced symbol is created (see `symbolAt`).