Make incremental compilation aware of synthesized mirrors

compiler/src/dotty/tools/dotc/CompilationUnit.scala

@@ -73,11 +73,20 @@ class CompilationUnit protected (val source: SourceFile) {
   /** List of all comments present in this compilation unit */
   var comments: List[Comment] = Nil
 
+  /** This is used to record dependencies to invalidate during incremental
+   *  compilation, but only if `ctx.runZincPhases` is true.
+   */
+  val depRecorder: sbt.DependencyRecorder = sbt.DependencyRecorder()
+
   /** Suspends the compilation unit by thowing a SuspendException
    *  and recording the suspended compilation unit
    */
   def suspend()(using Context): Nothing =
     assert(isSuspendable)
+    // Clear references to symbols that may become stale. No need to call
+    // `depRecorder.sendToZinc()` since all compilation phases will be rerun
+    // when this unit is unsuspended.
+    depRecorder.clear()
     if !suspended then
       if (ctx.settings.XprintSuspension.value)
         report.echo(i"suspended: $this")

compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala

@@ -676,11 +676,16 @@ private class ExtractAPICollector(using Context) extends ThunkHolder {
 
     // In the Scala2 ExtractAPI phase we only extract annotations that extend
     // StaticAnnotation, but in Dotty we currently pickle all annotations so we
-    // extract everything (except annotations missing from the classpath which
-    // we simply skip over, and inline body annotations which are handled above).
+    // extract everything, except:
+    // - annotations missing from the classpath which we simply skip over
+    // - inline body annotations which are handled above
+    // - the Child annotation since we already extract children via
+    //   `api.ClassLike#childrenOfSealedClass` and adding this annotation would
+    //   lead to overcompilation when using zinc's
+    //   `IncOptions#useOptimizedSealed`.
     s.annotations.foreach { annot =>
       val sym = annot.symbol
-      if sym.exists && sym != defn.BodyAnnot then
+      if sym.exists && sym != defn.BodyAnnot && sym != defn.ChildAnnot then
         annots += apiAnnotation(annot)
     }