Fix scala#2809: Invalidate and unlink symbols for artifact classfiles

This is similar to scala/scala#5952, we do not
need to parse artifact classfiles (that is, classfiles produced by
scalac or dotty that do not contain pickling information), so we discard
them as soon as possible.
This fixes various IDE crashes when trying to complete packages such as
"scala."

Author: smarter

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -19,6 +19,10 @@ import scala.util.control.NonFatal
 object ClassfileParser {
   /** Marker trait for unpicklers that can be embedded in classfiles. */
   trait Embedded
+
+  /** Indicate that there is nothing to unpickle and the corresponding symbols can
+    * be invalidated. */
+  object NoEmbedded extends Embedded
 }
 
 class ClassfileParser(
@@ -147,6 +151,17 @@ class ClassfileParser(
 
       setClassInfo(classRoot, classInfo)
       setClassInfo(moduleRoot, staticInfo)
+    } else if (result == Some(NoEmbedded)) {
+      val scope = classRoot.owner.unforcedDecls.openForMutations
+
+      for (sym <- List(moduleRoot.sourceModule.symbol, moduleRoot.symbol, classRoot.symbol)) {
+        scope.unlink(sym)
+        if (classRoot.owner == defn.ScalaShadowingPackageClass) {
+          // Symbols in scalaShadowing are also added to scala
+          defn.ScalaPackageClass.unforcedDecls.openForMutations.unlink(sym)
+        }
+        sym.markAbsent()
+      }
     }
 
     // eager load java enum definitions for exhaustivity check of pattern match
@@ -692,6 +707,10 @@ class ClassfileParser(
     }
   }
 
+  // Nothing$ and Null$ were incorrectly emitted with a Scala attribute
+  // instead of ScalaSignature before 2.13.0-M2, see https://github.com/scala/scala/pull/5952
+  private[this] val scalaUnpickleWhitelist = List(tpnme.nothingClass, tpnme.nullClass)
+
   /** Parse inner classes. Expects `in.bp` to point to the superclass entry.
    *  Restores the old `bp`.
    *  @return true iff classfile is from Scala, so no Java info needs to be read.
@@ -752,6 +771,12 @@ class ClassfileParser(
         return unpickleTASTY(in.nextBytes(attrLen))
       }
 
+      if (scan(tpnme.ScalaATTR) && !scalaUnpickleWhitelist.contains(classRoot.name)) {
+        // If the Scala attribute is present but the TASTY attribute isn't, this
+        // classfile is a compilation artifact.
+        return Some(NoEmbedded)
+      }
+
       if (scan(tpnme.RuntimeAnnotationATTR)) {
         val attrLen = in.nextInt
         val nAnnots = in.nextChar