Backport "Warn when @volatile is used on vals" to LTS (#21051)

Backports #19462 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -204,6 +204,9 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case VarArgsParamCannotBeGivenID // errorNumber: 188
   case ExtractorNotFoundID // errorNumber: 189
   case PureUnitExpressionID // errorNumber: 190
+  case MatchTypeLegacyPatternID // errorNumber: 191 - unused in LTS
+  case UnstableInlineAccessorID // errorNumber: 192 - unused in LTS
+  case VolatileOnValID // errorNumber: 193
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3068,3 +3068,8 @@ class MatchTypeScrutineeCannotBeHigherKinded(tp: Type)(using Context)
   extends TypeMsg(MatchTypeScrutineeCannotBeHigherKindedID) :
     def msg(using Context) = i"the scrutinee of a match type cannot be higher-kinded"
     def explain(using Context) = ""
+
+class VolatileOnVal()(using Context)
+extends SyntaxMsg(VolatileOnValID):
+  protected def msg(using Context): String = "values cannot be volatile"
+  protected def explain(using Context): String = ""

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

@@ -966,6 +966,10 @@ object RefChecks {
           report.error(em"private $sym cannot override ${other.showLocated}", sym.srcPos)
   end checkNoPrivateOverrides
 
+  def checkVolatile(sym: Symbol)(using Context): Unit =
+    if sym.isVolatile && !sym.is(Mutable) then 
+      report.warning(VolatileOnVal(), sym.srcPos)
+  
   /** Check that unary method definition do not receive parameters.
    *  They can only receive inferred parameters such as type parameters and implicit parameters.
    */
@@ -1148,6 +1152,7 @@ class RefChecks extends MiniPhase { thisPhase =>
     if tree.symbol.exists then
       checkNoPrivateOverrides(tree)
       val sym = tree.symbol
+      checkVolatile(sym)
       if (sym.exists && sym.owner.isTerm) {
         tree.rhs match {
           case Ident(nme.WILDCARD) => report.error(UnboundPlaceholderParameter(), sym.srcPos)

compiler/src/dotty/tools/io/ZipArchive.scala

@@ -159,7 +159,7 @@ final class FileZipArchive(jpath: JPath, release: Option[String]) extends ZipArc
     override def sizeOption: Option[Int] = Some(zipEntry.getSize.toInt)
   }
 
-  @volatile lazy val (root, allDirs): (DirEntry, collection.Map[String, DirEntry]) = {
+  lazy val (root, allDirs): (DirEntry, collection.Map[String, DirEntry]) = {
     val root = new DirEntry("/", null)
     val dirs = mutable.HashMap[String, DirEntry]("/" -> root)
     val zipFile = openZipFile()

sbt-test/sbt-dotty/scaladoc/src/main/scala/AutoParamTupling.scala

@@ -11,7 +11,7 @@ object AutoParamTupling {
       * In order to get thread safety, you need to put @volatile before lazy vals.
       * https://dotty.epfl.ch/docs/reference/changed-features/lazy-vals.html
       */
-    @volatile lazy val xs: List[String] = List("d", "o", "t", "t", "y")
+    lazy val xs: List[String] = List("d", "o", "t", "t", "y")
 
     /**
       * Current behaviour in Scala 2.12.2 :

tests/warn/i19416.check

@@ -0,0 +1,4 @@
+-- [E193] Syntax Warning: tests/warn/i19416.scala:2:18 -----------------------------------------------------------------
+2 |    @volatile val x: Int = ??? // warn
+  |                  ^
+  |                  values cannot be volatile

tests/warn/i19416.scala

@@ -0,0 +1,2 @@
+class Foo:
+    @volatile val x: Int = ??? // warn