Support Scala 3 vararg splice syntax under -Xsource:3

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -922,6 +922,16 @@ self =>
         mkApply(Ident(op.encode), stripParens(left) :: arguments)
     }
 
+    /** Is current ident a `*`, and is it followed by a `)` or `, )`? */
+    def followingIsScala3Vararg(): Boolean =
+      currentRun.isScala3 && isRawStar && lookingAhead {
+        in.token == RPAREN ||
+        in.token == COMMA && {
+          in.nextToken()
+          in.token == RPAREN
+        }
+      }
+
     /* --------- OPERAND/OPERATOR STACK --------------------------------------- */
 
     /** Modes for infix types. */
@@ -1716,7 +1726,7 @@ self =>
       val base  = opstack
 
       @tailrec
-      def loop(top: Tree): Tree = if (!isIdent) top else {
+      def loop(top: Tree): Tree = if (!isIdent || followingIsScala3Vararg()) top else {
         pushOpInfo(reduceExprStack(base, top))
         newLineOptWhenFollowing(isExprIntroToken)
         if (isExprIntro)
@@ -1727,7 +1737,12 @@ self =>
         else finishPostfixOp(start, base, popOpInfo())
       }
 
-      reduceExprStack(base, loop(prefixExpr()))
+      val expr = reduceExprStack(base, loop(prefixExpr()))
+      if (followingIsScala3Vararg())
+        atPos(expr.pos.start) {
+          Typed(expr, atPos(in.skipToken()) { Ident(tpnme.WILDCARD_STAR) })
+        }
+      else expr
     }
 
     /** {{{
@@ -2080,7 +2095,12 @@ self =>
             if (isCloseDelim) atPos(top.pos.start, in.prev.offset)(Star(stripParens(top)))
             else EmptyTree
           )
-          case _ => EmptyTree
+          case Ident(name) if isSequenceOK && followingIsScala3Vararg() =>
+            atPos(top.pos.start) {
+              Bind(name, atPos(in.skipToken()) { Star(Ident(nme.WILDCARD)) })
+            }
+          case _ =>
+            EmptyTree
         }
         @tailrec
         def loop(top: Tree): Tree = reducePatternStack(base, top) match {

test/files/pos/varargs-future.scala

@@ -0,0 +1,22 @@
+// scalac: -Xsource:3
+//
+
+class Test {
+  def foo(xs: Int*): Seq[Int] = xs
+
+  val s: Seq[Int] = Seq(1, 2, 3)
+  foo(s*)
+
+  // not very useful, but supported by Scala 3 (and matches what works with `: _*` syntax)
+  foo(
+    s*,
+  )
+
+  s match {
+    case Seq(elems*) => println(elems)
+  }
+
+  s match {
+    case Seq(x, rest*) => println(rest)
+  }
+}