Fix parsing of a trailing comma in argument list (#7273)

Author: greg-at-moderne

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

@@ -736,19 +736,39 @@ func (ctx *parseContext) mapFieldListAsParams(fl *ast.FieldList) tree.Container[
 		}
 	}
 
-	closeParen := ctx.prefix(fl.Closing)
-	ctx.skip(1) // ")"
-
+	var markers tree.Markers
 	if len(elements) > 0 {
-		elements[len(elements)-1].After = closeParen
-	} else if len(closeParen.Comments) > 0 {
-		elements = append(elements, tree.RightPadded[tree.Statement]{
-			Element: &tree.Empty{ID: uuid.New()},
-			After:   closeParen,
-		})
+		trailingCommaOff := ctx.findNextBefore(',', int(fl.Closing)-ctx.file.Base())
+		if trailingCommaOff >= 0 {
+			commaBefore := ctx.prefix(ctx.file.Pos(trailingCommaOff))
+			ctx.skip(1) // ","
+			commaAfter := ctx.prefix(fl.Closing)
+			ctx.skip(1) // ")"
+			markers = tree.Markers{
+				ID: uuid.New(),
+				Entries: []tree.Marker{tree.TrailingComma{
+					Ident:  uuid.New(),
+					Before: commaBefore,
+					After:  commaAfter,
+				}},
+			}
+		} else {
+			closePrefix := ctx.prefix(fl.Closing)
+			ctx.skip(1) // ")"
+			elements[len(elements)-1].After = closePrefix
+		}
+	} else {
+		closeParen := ctx.prefix(fl.Closing)
+		ctx.skip(1) // ")"
+		if len(closeParen.Comments) > 0 {
+			elements = append(elements, tree.RightPadded[tree.Statement]{
+				Element: &tree.Empty{ID: uuid.New()},
+				After:   closeParen,
+			})
+		}
 	}
 
-	return tree.Container[tree.Statement]{Before: before, Elements: elements}
+	return tree.Container[tree.Statement]{Before: before, Elements: elements, Markers: markers}
 }
 
 // mapBlockStmt maps a block statement.

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

@@ -230,11 +230,16 @@ func (p *GoPrinter) VisitMethodDeclaration(md *tree.MethodDeclaration, param any
 func (p *GoPrinter) printParamList(params tree.Container[tree.Statement], out *PrintOutputCapture) {
 	p.visitSpace(params.Before, out)
 	out.Append("(")
+	tc := tree.FindMarker[tree.TrailingComma](params.Markers)
 	for i, rp := range params.Elements {
 		p.Visit(rp.Element, out)
 		if i < len(params.Elements)-1 {
 			p.visitSpace(rp.After, out)
 			out.Append(",")
+		} else if tc != nil {
+			p.visitSpace(tc.Before, out)
+			out.Append(",")
+			p.visitSpace(tc.After, out)
 		} else {
 			p.visitSpace(rp.After, out)
 		}
@@ -792,16 +797,7 @@ func (p *GoPrinter) VisitFuncType(ft *tree.FuncType, param any) tree.J {
 	out := param.(*PrintOutputCapture)
 	p.beforeSyntax(ft.Prefix, ft.Markers, out)
 	out.Append("func")
-	p.visitSpace(ft.Parameters.Before, out)
-	out.Append("(")
-	for i, rp := range ft.Parameters.Elements {
-		p.Visit(rp.Element, out)
-		if i < len(ft.Parameters.Elements)-1 {
-			p.visitSpace(rp.After, out)
-			out.Append(",")
-		}
-	}
-	out.Append(")")
+	p.printParamList(ft.Parameters, out)
 	if ft.ReturnType != nil {
 		p.Visit(ft.ReturnType, out)
 	}

rewrite-go/rewrite/test/trailing_comma_test.go

@@ -149,6 +149,18 @@ func TestParseTrailingCommaAnonymousStruct(t *testing.T) {
 		`))
 }
 
+func TestParseTrailingCommaFuncTypeParams(t *testing.T) {
+	NewRecipeSpec().RewriteRun(t,
+		Golang(`
+			package main
+
+			var f = map[string]func(
+				a int,
+				b int,
+			) error{}
+		`))
+}
+
 func TestParseTrailingCommaMapOfSlices(t *testing.T) {
 	NewRecipeSpec().RewriteRun(t,
 		Golang(`