Add missing type annotations that do not break binary compatibility

Some missing type annotations can be added without breaking binary compatibility; they are added in this pull request, letting #655 concentrate on adding type annotations that _do_ break binary compatibility (for Scala 3).

Removed MiMa exclusions that are no longer needed.

Removed Gitter link to a non-existing room.

Added links to the compiler implementation of the `SymbolicXMLBuilder` for different versions of Scala.

Author: dubinsky

README.md

@@ -2,12 +2,17 @@ scala-xml
 [![latest release for 2.12](https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-xml_2.12.svg?label=scala+2.12)](http://mvnrepository.com/artifact/org.scala-lang.modules/scala-xml_2.12)
 [![latest release for 2.13](https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-xml_2.13.svg?label=scala+2.13)](http://mvnrepository.com/artifact/org.scala-lang.modules/scala-xml_2.13)
 [![latest release for 3.0](https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-xml_3.svg?label=scala+3)](http://mvnrepository.com/artifact/org.scala-lang.modules/scala-xml_3)
-[![Gitter](https://badges.gitter.im/Join+Chat.svg)](https://gitter.im/scala/scala-xml)
 =========
 
 The standard Scala XML library. Please file XML issues here, not at https://github.com/scala/bug/issues or http://github.com/lampepfl/dotty/issues.
 
-The decoupling of scala-xml from the Scala compiler and standard library is possible because the compiler desugars XML literals in Scala source code into a set of method calls. Alternative implementations of these calls are welcome! (This [implementation](https://github.com/scala/scala/blob/2.11.x/src/compiler/scala/tools/nsc/ast/parser/SymbolicXMLBuilder.scala) shows the calls needed.)
+The decoupling of scala-xml from the Scala compiler and standard library is possible because the compiler desugars XML literals in Scala source code into a set of method calls.
+Alternative implementations of these calls are welcome!
+Compiler code that shows the calls needed: 
+  [Scala 2.11](https://github.com/scala/scala/blob/2.11.x/src/compiler/scala/tools/nsc/ast/parser/SymbolicXMLBuilder.scala),
+  [Scala 2.12](https://github.com/scala/scala/blob/2.12.x/src/compiler/scala/tools/nsc/ast/parser/SymbolicXMLBuilder.scala),
+  [Scala 2.13](https://github.com/scala/scala/blob/2.13.x/src/compiler/scala/tools/nsc/ast/parser/SymbolicXMLBuilder.scala),
+  [Scala 3](https://github.com/lampepfl/dotty/blob/main/compiler/src/dotty/tools/dotc/parsing/xml/SymbolicXMLBuilder.scala).
 
 API documentation is available [here](https://javadoc.io/doc/org.scala-lang.modules/scala-xml_2.13/).
 

build.sbt

@@ -68,14 +68,6 @@ lazy val xml = crossProject(JSPlatform, JVMPlatform, NativePlatform)
       import com.typesafe.tools.mima.core._
       import com.typesafe.tools.mima.core.ProblemFilters._
       Seq(
-        // afaict this is just a JDK 8 vs 16 difference, producing a false positive when
-        // we compare classes built on JDK 16 (which we only do on CI, not at release time)
-        // to previous-version artifacts that were built on 8.  see scala/scala-xml#501
-        exclude[DirectMissingMethodProblem]("scala.xml.include.sax.XIncluder.declaration"),
-
-        // necessitated by the switch from DefaultHandler to DefaultHandler2 in FactoryAdapter:
-        exclude[MissingTypesProblem]("scala.xml.parsing.FactoryAdapter"),                         // see #549
-
         // necessitated by the introduction of new abstract methods in FactoryAdapter:
         exclude[ReversedMissingMethodProblem]("scala.xml.parsing.FactoryAdapter.createComment"),  // see #549
         exclude[ReversedMissingMethodProblem]("scala.xml.parsing.FactoryAdapter.createPCData")    // see #558

shared/src/main/scala-2.13-/scala/xml/ScalaVersionSpecific.scala

@@ -12,21 +12,21 @@
 
 package scala.xml
 
-import scala.collection.SeqLike
+import scala.collection.{SeqLike, mutable}
 import scala.collection.generic.CanBuildFrom
 
 private[xml] object ScalaVersionSpecific {
   import NodeSeq.Coll
   type CBF[-From, -A, +C] = CanBuildFrom[From, A, C]
   object NodeSeqCBF extends CanBuildFrom[Coll, Node, NodeSeq] {
-    override def apply(from: Coll) /* TODO type annotation */ = NodeSeq.newBuilder
-    override def apply() /* TODO type annotation */ = NodeSeq.newBuilder
+    override def apply(from: Coll): mutable.Builder[Node, NodeSeq] = NodeSeq.newBuilder
+    override def apply(): mutable.Builder[Node, NodeSeq] = NodeSeq.newBuilder
   }
 }
 
 private[xml] trait ScalaVersionSpecificNodeSeq extends SeqLike[Node, NodeSeq] { self: NodeSeq =>
   /** Creates a list buffer as builder for this class */
-  override protected[this] def newBuilder /* TODO type annotation */ = NodeSeq.newBuilder
+  override protected[this] def newBuilder: mutable.Builder[Node, NodeSeq] = NodeSeq.newBuilder
 }
 
 private[xml] trait ScalaVersionSpecificNodeBuffer { self: NodeBuffer =>

shared/src/main/scala/scala/xml/Null.scala

@@ -24,7 +24,7 @@ import scala.collection.Seq
  *  @author  Burak Emir
  */
 case object Null extends MetaData {
-  override def iterator /* TODO type annotation */ = Iterator.empty
+  override def iterator: Iterator[Nothing] = Iterator.empty
   override def size: Int = 0
   override def append(m: MetaData, scope: NamespaceBinding = TopScope): MetaData = m
   override def filter(f: MetaData => Boolean): MetaData = this