Make sure symbols in annotation trees are fresh before pickling (#22002)

In a nutshell: when mapping annotated types, we can currently end up
with the same symbol being declared in distinct trees, which crashes the
pickler as it expects each symbol to be declared in a single place. See
#19957 (comment) and
#19957 (comment) for
more context.

This PR ensures that all symbols in annotation trees are different by
creating fresh symbols for all symbols in annotation tree during
`PostTyper`.

In my [previous
attempt](ab70f18)
which was discussed on #19957, I did it in `Annotations.mapWith`. Here,
it's only done once in `PostTyper`, so this is more lightweight.

Fixes #17939, fixes #19846 and fixes (partially?) #20272.

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -2,7 +2,7 @@ package dotty.tools
 package dotc
 package transform
 
-import dotty.tools.dotc.ast.{Trees, tpd, untpd, desugar}
+import dotty.tools.dotc.ast.{Trees, tpd, untpd, desugar, TreeTypeMap}
 import scala.collection.mutable
 import core.*
 import dotty.tools.dotc.typer.Checking
@@ -16,7 +16,7 @@ import Symbols.*, NameOps.*
 import ContextFunctionResults.annotateContextResults
 import config.Printers.typr
 import config.Feature
-import util.SrcPos
+import util.{SrcPos, Stats}
 import reporting.*
 import NameKinds.WildcardParamName
 import cc.*
@@ -154,7 +154,21 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
           case _ =>
       case _ =>
 
-    private def transformAnnot(annot: Tree)(using Context): Tree = {
+    /** Returns a copy of the given tree with all symbols fresh.
+     *
+     *  Used to guarantee that no symbols are shared between trees in different
+     *  annotations.
+     */
+    private def copySymbols(tree: Tree)(using Context) =
+      Stats.trackTime("Annotations copySymbols"):
+        val ttm =
+          new TreeTypeMap:
+            override def withMappedSyms(syms: List[Symbol]) =
+              withMappedSyms(syms, mapSymbols(syms, this, true))
+        ttm(tree)
+
+    /** Transforms the given annotation tree. */
+    private def transformAnnotTree(annot: Tree)(using Context): Tree = {
       val saved = inJavaAnnot
       inJavaAnnot = annot.symbol.is(JavaDefined)
       if (inJavaAnnot) checkValidJavaAnnotation(annot)
@@ -163,7 +177,19 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
     }
 
     private def transformAnnot(annot: Annotation)(using Context): Annotation =
-      annot.derivedAnnotation(transformAnnot(annot.tree))
+      val tree1 =
+        annot match
+          case _: BodyAnnotation => annot.tree
+          case _ => copySymbols(annot.tree)
+      annot.derivedAnnotation(transformAnnotTree(tree1))
+
+    /** Transforms all annotations in the given type. */
+    private def transformAnnotsIn(using Context) =
+      new TypeMap:
+        def apply(tp: Type) = tp match
+          case tp @ AnnotatedType(parent, annot) =>
+            tp.derivedAnnotatedType(mapOver(parent), transformAnnot(annot))
+          case _ => mapOver(tp)
 
     private def processMemberDef(tree: Tree)(using Context): tree.type = {
       val sym = tree.symbol
@@ -501,7 +527,7 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
           Checking.checkRealizable(tree.tpt.tpe, tree.srcPos, "SAM type")
           super.transform(tree)
         case tree @ Annotated(annotated, annot) =>
-          cpy.Annotated(tree)(transform(annotated), transformAnnot(annot))
+          cpy.Annotated(tree)(transform(annotated), transformAnnotTree(annot))
         case tree: AppliedTypeTree =>
           if (tree.tpt.symbol == defn.andType)
             Checking.checkNonCyclicInherited(tree.tpe, tree.args.tpes, EmptyScope, tree.srcPos)
@@ -524,11 +550,7 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
           super.transform(tree)
         case tree: TypeTree =>
           val tpe = if tree.isInferred then CleanupRetains()(tree.tpe) else tree.tpe
-          tree.withType:
-            tpe match
-              case AnnotatedType(parent, annot) =>
-                AnnotatedType(parent, transformAnnot(annot)) // TODO: Also map annotations embedded in type?
-              case _ => tpe
+          tree.withType(transformAnnotsIn(tpe))
         case Typed(Ident(nme.WILDCARD), _) =>
           withMode(Mode.Pattern)(super.transform(tree))
             // The added mode signals that bounds in a pattern need not

tests/pos/annot-17939.scala

@@ -0,0 +1,8 @@
+import scala.annotation.Annotation
+class myRefined[T](f: T => Boolean) extends Annotation
+
+class Box[T](val x: T)
+class Box2(val x: Int)
+
+class A(a: String @myRefined((x: Int) => Box(3).x == 3)) // crash
+class A2(a2: String @myRefined((x: Int) => Box2(3).x == 3)) // works

tests/pos/annot-19846.scala

@@ -0,0 +1,9 @@
+package dependentAnnotation
+
+class lambdaAnnot(g: () => Int) extends annotation.StaticAnnotation
+
+def f(x: Int): Int @lambdaAnnot(() => x + 1) = x
+
+@main def main =
+  val y: Int = 5
+  val z = f(y)

tests/pos/annot-19846b.scala

@@ -0,0 +1,8 @@
+class qualified[T](predicate: T => Boolean) extends annotation.StaticAnnotation
+
+class EqualPair(val x: Int, val y: Int @qualified[Int](it => it == x))
+
+@main def main =
+  val p = EqualPair(42, 42)
+  val y = p.y
+  println(42)

tests/pos/annot-body.scala

@@ -0,0 +1,15 @@
+// This test checks that symbols in `BodyAnnotation` are not copied in
+// `transformAnnot` during `PostTyper`.
+
+package json
+
+trait Reads[A] {
+  def reads(a: Any): A
+}
+
+object JsMacroImpl {
+  inline def reads[A]: Reads[A] =
+    new Reads[A] { self =>
+      def reads(a: Any) = ???
+    }
+}

tests/pos/annot-i20272a.scala

@@ -0,0 +1,20 @@
+import language.experimental.captureChecking
+
+ trait Iterable[T] { self: Iterable[T]^ =>
+   def map[U](f: T => U): Iterable[U]^{this, f}
+ }
+
+ object Test {
+   def assertEquals[A, B](a: A, b: B): Boolean = ???
+
+   def foo[T](level: Int, lines: Iterable[T]) =
+     lines.map(x => x)
+
+   def bar(messages: Iterable[String]) =
+     foo(1, messages)
+
+   val it: Iterable[String] = ???
+   val msgs = bar(it)
+
+   assertEquals(msgs, msgs)
+ }