Go: Fix RPC serialization for varargs

rewrite-go/rewrite/pkg/parser/go_parser.go

@@ -669,9 +669,15 @@ func (ctx *parseContext) mapFieldListAsParams(fl *ast.FieldList) tree.Container[
 		if len(field.Names) == 0 {
 			// Unnamed parameter: just a type expression (e.g., `int` in `func(int)`)
 			typeExpr := ctx.mapTypeExpr(field.Type)
+			var varargs *tree.Space
+			if u, ok := typeExpr.(*tree.Unary); ok && u.Operator.Element == tree.Spread {
+				varargs = &u.Prefix
+				typeExpr = u.Operand
+			}
 			vd := &tree.VariableDeclarations{
 				ID:       uuid.New(),
 				TypeExpr: typeExpr,
+				Varargs:  varargs,
 				Variables: []tree.RightPadded[*tree.VariableDeclarator]{
 					{Element: &tree.VariableDeclarator{ID: uuid.New(), Name: &tree.Identifier{ID: uuid.New()}}},
 				},
@@ -706,9 +712,15 @@ func (ctx *parseContext) mapFieldListAsParams(fl *ast.FieldList) tree.Container[
 			}
 
 			typeExpr := ctx.mapTypeExpr(field.Type)
+			var varargs *tree.Space
+			if u, ok := typeExpr.(*tree.Unary); ok && u.Operator.Element == tree.Spread {
+				varargs = &u.Prefix
+				typeExpr = u.Operand
+			}
 			vd := &tree.VariableDeclarations{
 				ID:        uuid.New(),
 				TypeExpr:  typeExpr,
+				Varargs:   varargs,
 				Variables: vars,
 			}
 

rewrite-go/rewrite/pkg/printer/go_printer.go

@@ -323,7 +323,11 @@ func (p *GoPrinter) VisitVariableDeclarations(vd *tree.VariableDeclarations, par
 			out.Append(",")
 		}
 	}
-	// Then type expression
+	// Then varargs + type expression
+	if vd.Varargs != nil {
+		p.visitSpace(*vd.Varargs, out)
+		out.Append("...")
+	}
 	if vd.TypeExpr != nil {
 		p.Visit(vd.TypeExpr, out)
 	}

rewrite-go/rewrite/pkg/rpc/java_receiver.go

@@ -356,7 +356,15 @@ func (r *JavaReceiver) receiveVariableDeclarations(vd *tree.VariableDeclarations
 		vd.TypeExpr = result.(tree.Expression)
 	}
 	// varargs
-	q.Receive(nil, nil)
+	var currentVarargs any
+	if vd.Varargs != nil {
+		currentVarargs = *vd.Varargs
+	}
+	varargsResult := q.Receive(currentVarargs, func(v any) any { return receiveSpace(v.(tree.Space), q) })
+	if varargsResult != nil {
+		sp := varargsResult.(tree.Space)
+		vd.Varargs = &sp
+	}
 	// variables
 	beforeVars := make([]any, len(vd.Variables))
 	for i, v := range vd.Variables {

rewrite-go/rewrite/pkg/rpc/java_sender.go

@@ -473,8 +473,14 @@ func (s *JavaSender) sendVariableDeclarations(vd *tree.VariableDeclarations, q *
 	// typeExpression
 	q.GetAndSend(vd, func(v any) any { return v.(*tree.VariableDeclarations).TypeExpr },
 		func(v any) { s.parent.Visit(v, q) })
-	// varargs (nil for Go)
-	q.GetAndSend(vd, func(_ any) any { return nil }, nil)
+	// varargs
+	q.GetAndSend(vd, func(v any) any {
+		va := v.(*tree.VariableDeclarations).Varargs
+		if va != nil {
+			return *va
+		}
+		return nil
+	}, func(v any) { sendSpace(v.(tree.Space), q) })
 	// variables (list of right-padded NamedVariable)
 	q.GetAndSendList(vd,
 		func(v any) []any {

rewrite-go/rewrite/pkg/tree/j.go

@@ -1076,6 +1076,7 @@ type VariableDeclarations struct {
 	Prefix    Space
 	Markers   Markers
 	TypeExpr  Expression                        // the declared type (nil if inferred)
+	Varargs   *Space                            // non-nil for variadic params (`...T`); holds prefix of `...`
 	Variables []RightPadded[*VariableDeclarator] // the declared variables
 	Specs     *Container[Statement]             // non-nil for grouped `var ( ... )`; Before = space before `(`
 }

rewrite-go/src/integTest/java/org/openrewrite/golang/rpc/GolangParserIntegTest.java

@@ -451,4 +451,18 @@ void literalWithNonPrimitiveType() {
                 )
         );
     }
+
+    @Test
+    void variadicParameter() {
+        rewriteRun(
+                go(
+                        """
+                                package main
+
+                                func foo(args ...string) {
+                                }
+                                """
+                )
+        );
+    }
 }