Tolerate missing '=' value in compact options (bufbuild#219)

While still producing an error, see:
bufbuild#200

ast/options.go

@@ -100,13 +100,18 @@ func NewCompactOptionNode(name *OptionNameNode, equals *RuneNode, val ValueNode)
 	if name == nil {
 		panic("name is nil")
 	}
-	if equals == nil {
-		panic("equals is nil")
+	if equals == nil && val != nil {
+		panic("equals is nil but val is not")
 	}
-	if val == nil {
-		panic("val is nil")
+	if val == nil && equals != nil {
+		panic("val is nil but equals is not")
+	}
+	var children []Node
+	if equals == nil && val == nil {
+		children = []Node{name}
+	} else {
+		children = []Node{name, equals, val}
 	}
-	children := []Node{name, equals, val}
 	return &OptionNode{
 		compositeNode: compositeNode{
 			children: children,

parser/parser_test.go

@@ -408,6 +408,47 @@ func TestLenientParse_EmptyCompactOptions(t *testing.T) {
 	}
 }
 
+func TestLenientParse_EmptyCompactValue(t *testing.T) {
+	t.Parallel()
+	inputs := map[string]struct {
+		Error   string
+		NoError string
+	}{
+		"field-options": {
+			Error: `syntax = "proto2";
+							message Foo {
+								optional int32 bar = 1 [deprecated=true, default];
+							}`,
+			NoError: `syntax = "proto3";
+								message Foo {
+									optional int32 bar = 1 [deprecated=true, default=1];
+								}`,
+		},
+		"enum-options": {
+			Error: `syntax = "proto3";
+							enum Foo {
+								FOO = 0 [deprecated];
+							}`,
+			NoError: `syntax = "proto3";
+								enum Foo {
+									FOO = 0 [deprecated=true];
+								}`,
+		},
+	}
+	for name, input := range inputs {
+		name, input := name, input
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			errHandler := reporter.NewHandler(nil)
+			protoName := fmt.Sprintf("%s.proto", name)
+			_, err := Parse(protoName, strings.NewReader(input.NoError), errHandler)
+			require.NoError(t, err)
+			_, err = Parse(protoName, strings.NewReader(input.Error), errHandler)
+			require.ErrorContains(t, err, "compact option must have a value")
+		})
+	}
+}
+
 func TestSimpleParse(t *testing.T) {
 	t.Parallel()
 	protos := map[string]Result{}

parser/proto.y

@@ -616,6 +616,11 @@ compactOptionDecls : compactOption {
 compactOption: optionName '=' optionValue {
 		optName := ast.NewOptionNameNode($1.refs, $1.dots)
 		$$ = ast.NewCompactOptionNode(optName, $2, $3)
+	} |
+	optionName {
+		optName := ast.NewOptionNameNode($1.refs, $1.dots)
+		protolex.(*protoLex).Error("compact option must have a value")
+		$$ = ast.NewCompactOptionNode(optName, nil, nil)
 	}
 
 groupDecl : fieldCardinality _GROUP identifier '=' _INT_LIT '{' messageBody '}' {