Validate listType usage on struct fields

pkg/generators/openapi.go

@@ -957,6 +957,14 @@ func (g openAPITypeWriter) generateProperty(m *types.Member, parent *types.Type)
 	if name == "" {
 		return nil
 	}
+	if listType, err := getSingleTagsValue(m.CommentLines, "listType"); err != nil {
+		return err
+	} else if listType != "" {
+		t := resolveAliasAndPtrType(m.Type)
+		if t.Kind != types.Slice && t.Kind != types.Array {
+			return fmt.Errorf("listType marker may only be used on slice or array fields, but was used on %v", m.Name)
+		}
+	}
 	validationSchema, err := ParseCommentTags(m.Type, m.CommentLines, markerPrefix)
 	if err != nil {
 		return err

pkg/generators/openapi_test.go

@@ -88,6 +88,78 @@ func assertEqual(t *testing.T, got, want string) {
 	}
 }
 
+func TestListTypeValidation(t *testing.T) {
+	tests := []struct {
+		name          string
+		inputFile     string
+		expectedError string
+	}{
+		{
+			name: "listType on slice",
+			inputFile: `
+				package foo
+
+				type Blah struct {
+					// +listType=atomic
+					Slice []string
+				}
+			`,
+			expectedError: "",
+		},
+		{
+			name: "listType on array",
+			inputFile: `
+				package foo
+
+				type Blah struct {
+					// +listType=atomic
+					Array [1]string
+				}
+			`,
+			expectedError: "",
+		},
+		{
+			name: "listType on string (fail)",
+			inputFile: `
+				package foo
+
+				type Blah struct {
+					// +listType=atomic
+					String string
+				}
+			`,
+			expectedError: "listType marker may only be used on slice or array fields",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			packagestest.TestAll(t, func(t *testing.T, exporter packagestest.Exporter) {
+				exported := packagestest.Export(t, exporter, []packagestest.Module{{
+					Name: "example.com/base",
+					Files: map[string]interface{}{
+						"foo/foo.go": test.inputFile,
+					},
+				}})
+				defer exported.Cleanup()
+
+				_, funcErr, _, _, _ := testOpenAPITypeWriter(t, exported.Config)
+				if test.expectedError == "" {
+					if funcErr != nil {
+						t.Errorf("Expected no error, got: %v", funcErr)
+					}
+				} else {
+					if funcErr == nil {
+						t.Errorf("Expected error containing %q, got nil", test.expectedError)
+					} else if !strings.Contains(funcErr.Error(), test.expectedError) {
+						t.Errorf("Expected error containing %q, got: %v", test.expectedError, funcErr)
+					}
+				}
+			})
+		})
+	}
+}
+
 func TestSimple(t *testing.T) {
 	inputFile := `
 		package foo