Add migration warning for XML literals in language future (#19101)

First step towards #19100

Author: nicolasstucki

compiler/src/dotty/tools/dotc/config/MigrationVersion.scala

@@ -38,5 +38,6 @@ object MigrationVersion:
   val ImportWildcard = MigrationVersion(future, future)
   val ImportRename = MigrationVersion(future, future)
   val ParameterEnclosedByParenthesis = MigrationVersion(future, future)
+  val XmlLiteral = MigrationVersion(future, future)
 
 end MigrationVersion

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -571,8 +571,16 @@ object Parsers {
 
     object symbXMLBuilder extends xml.SymbolicXMLBuilder(this, true) // DEBUG choices
 
-    def xmlLiteral() : Tree = xmlp.xLiteral
-    def xmlLiteralPattern() : Tree = xmlp.xLiteralPattern
+    def xmlLiteral() : Tree = xmlDeprecationWarning(xmlp.xLiteral)
+    def xmlLiteralPattern() : Tree = xmlDeprecationWarning(xmlp.xLiteralPattern)
+
+    private def xmlDeprecationWarning(tree: Tree): Tree =
+      report.errorOrMigrationWarning(
+        em"""XML literals are no longer supported.
+             |See https://docs.scala-lang.org/scala3/reference/dropped-features/xml.html""",
+        tree.srcPos,
+        MigrationVersion.XmlLiteral)
+      tree
 
 /* -------- COMBINATORS -------------------------------------------------------- */
 

compiler/src/dotty/tools/dotc/parsing/xml/MarkupParsers.scala

@@ -277,7 +277,7 @@ object MarkupParsers {
      *                | xmlTag1 '/' '>'
      */
     def element: Tree = {
-      val start = curOffset
+      val start = curOffset // FIXME should be `curOffset - 1` (scalatest and tests/neg/i19100.scala must be updated)
       val (qname, attrMap) = xTag(())
       if (ch == '/') { // empty element
         xToken("/>")
@@ -435,7 +435,7 @@ object MarkupParsers {
      *                  | Name [S] '/' '>'
      */
     def xPattern: Tree = {
-      var start = curOffset
+      var start = curOffset // FIXME should be `curOffset - 1` (scalatest and tests/neg/i19100.scala must be updated)
       val qname = xName
       debugLastStartElement = (start, qname) :: debugLastStartElement
       xSpaceOpt()

compiler/src/dotty/tools/dotc/typer/TyperPhase.scala

@@ -12,6 +12,8 @@ import parsing.{Parser => ParserPhase}
 import config.Printers.typr
 import inlines.PrepareInlineable
 import util.Stats.*
+import dotty.tools.dotc.config.Feature
+import dotty.tools.dotc.config.SourceVersion
 
 /**
  *
@@ -83,7 +85,7 @@ class TyperPhase(addRootImports: Boolean = true) extends Phase {
 
     ctx.base.parserPhase match {
       case p: ParserPhase =>
-        if p.firstXmlPos.exists && !defn.ScalaXmlPackageClass.exists then
+        if p.firstXmlPos.exists && !defn.ScalaXmlPackageClass.exists && Feature.sourceVersion == SourceVersion.future then
           report.error(
             """To support XML literals, your project must depend on scala-xml.
               |See https://github.com/scala/scala-xml for more information.""".stripMargin,

tests/neg/i19100.check

@@ -0,0 +1,15 @@
+-- Error: tests/neg/i19100.scala:4:3 -----------------------------------------------------------------------------------
+4 |  <foo/> match // error
+  |   ^^^^^
+  |   XML literals are no longer supported.
+  |   See https://docs.scala-lang.org/scala3/reference/dropped-features/xml.html
+-- Error: tests/neg/i19100.scala:5:10 ----------------------------------------------------------------------------------
+5 |    case <foo/> => 1 // error
+  |          ^^^^^
+  |          XML literals are no longer supported.
+  |          See https://docs.scala-lang.org/scala3/reference/dropped-features/xml.html
+-- Error: tests/neg/i19100.scala:6:3 -----------------------------------------------------------------------------------
+6 |  <bar></bar> // error
+  |   ^^^^^^^^^^
+  |   XML literals are no longer supported.
+  |   See https://docs.scala-lang.org/scala3/reference/dropped-features/xml.html

tests/neg/i19100.scala

@@ -0,0 +1,6 @@
+import scala.language.future
+
+def test =
+  <foo/> match // error
+    case <foo/> => 1 // error
+  <bar></bar> // error