Use toVector for XML literal sequences

[lrytz]

Forward-port of scala/scala#11065

In <a><b/><c/><a>, a NodeBuffer (which extends ArrayBuffer) is used to accumulate the children. The buffer is passed to new Elem($buf: _*), which only works thanks to the implicit collection.Seq[Node] => NodeSeq declared in scala-xml.

With -Vprint:typer:

scala> <a><b/></a>
[[syntax trees at end of                     typer]] // rs$line$1

    val res0: scala.xml.Elem =
      {
        new _root_.scala.xml.Elem(null, "a", _root_.scala.xml.Null,
          scala.xml.TopScope, false,
          {
            val $buf: scala.xml.NodeBuffer = new _root_.scala.xml.NodeBuffer()
            $buf.&+(
              {
                {
                  new _root_.scala.xml.Elem(null, "b", _root_.scala.xml.Null,
                    scala.xml.TopScope, true, [ : scala.xml.Node]*)
                }
              }
            )
            scala.xml.NodeSeq.seqToNodeSeq($buf)
          }*
        )
      }

The implicit was not inserted in 2.12 because the varargs parameter of Elem accepted a collection.Seq.

[lrytz]

I have a related PR to scala-xml which changes the API to return immutable.Seq instead of collection.Seq scala/scala-xml#760.

The current scala.xml.NodeSeq is only pretending to be immutable :)

$ scala repl -S 3 --dep org.scala-lang.modules::scala-xml:2.3.0

scala> import scala.xml._

scala> val x: collection.immutable.Seq[Node] = <a><b/></a>
val x: Seq[scala.xml.Node] = <a><b/></a>

scala> x.asInstanceOf[scala.xml.Node].child.asInstanceOf[scala.xml.NodeSeq].theSeq.asInstanceOf[NodeBuffer] += <c/>

scala> x
val res1: Seq[scala.xml.Node] = <a><b/><c/></a>

[lrytz]

There is code that relies on XML sequences <a/><b/> having type NodeBuffer

https://github.com/scalatest/scalatest/blob/release-3.2.19/jvm/core/src/main/scala/org/scalatest/tools/HtmlReporter.scala#L828

<table>...</table><table>...</table> &+ getHTMLForCause(cause)

Method &+ is defined in NodeBuffer.

I narrowed the change so that plain XML sequences keep type NodeBuffer. Other XML sequences (children passed to Elem, children of <xml:group> passed to Group, attribute values with entity references passed to UnprefixedAttribute / PrefixedAttribute ) are converted toVector.

[som-snytt]

I know you have your reasons. But the dust you raise makes me sneeze. Oh it almost rhymes.