feat: Resource references should be strings.

docs/rules/0122/resource-reference-type.md

@@ -0,0 +1,82 @@
+---
+rule:
+  aip: 122
+  name: [core, '0122', resource-reference-type]
+  summary: All resource references must be strings.
+permalink: /122/resource-reference-type
+redirect_from:
+  - /0122/resource-reference-type
+---
+
+# Resource reference type
+
+This rule enforces that all fields with the `google.api.resource_reference`
+annotation are strings, as mandated in [AIP-122][].
+
+## Details
+
+This rule complains if it sees a field with a `google.api.resource_reference`
+that has a type other than `string`.
+
+## Examples
+
+**Incorrect** code for this rule:
+
+```proto
+// Incorrect.
+message Book {
+  string name = 1;
+
+  // Resource references should be strings.
+  Author author = 2 [(google.api.resource_reference) = {
+    type: "library.googleapis.com/Author"
+  }];
+}
+```
+
+**Correct** code for this rule:
+
+```proto
+// Correct.
+message Book {
+  string name = 1;
+
+  string author = 2 [(google.api.resource_reference) = {
+    type: "library.googleapis.com/Author"
+  }];
+}
+```
+
+```proto
+// Correct.
+message Book {
+  string name = 1;
+
+  // If "author" is not a first-class resource, then it may be a composite
+  // field within the book.
+  Author author = 2;
+}
+```
+
+## Disabling
+
+If you need to violate this rule, use a leading comment above the method.
+Remember to also include an [aip.dev/not-precedent][] comment explaining why.
+
+```proto
+message Book {
+  string name = 1;
+
+  // (-- api-linter: core::0122::resource-reference-type=disabled
+  //     aip.dev/not-precedent: We need to do this because reasons. --)
+  Author author = 2 [(google.api.resource_reference) = {
+    type: "library.googleapis.com/Author"
+  }];
+}
+```
+
+If you need to violate this rule for an entire file, place the comment at the
+top of the file.
+
+[aip-122]: https://aip.dev/122
+[aip.dev/not-precedent]: https://aip.dev/not-precedent

rules/aip0122/aip0122.go

@@ -26,5 +26,6 @@ func AddRules(r lint.RuleRegistry) error {
 		122,
 		httpURICase,
 		nameSuffix,
+		resourceReferenceType,
 	)
 }

rules/aip0122/resource_reference_type.go

@@ -0,0 +1,36 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package aip0122
+
+import (
+	"github.com/googleapis/api-linter/lint"
+	"github.com/googleapis/api-linter/locations"
+	"github.com/googleapis/api-linter/rules/internal/utils"
+	"github.com/jhump/protoreflect/desc"
+)
+
+var resourceReferenceType = &lint.FieldRule{
+	Name: lint.NewRuleName(122, "resource-reference-type"),
+	LintField: func(f *desc.FieldDescriptor) []lint.Problem {
+		if utils.GetResourceReference(f) != nil && utils.GetTypeName(f) != "string" {
+			return []lint.Problem{{
+				Message:    "The resource_reference annotation should only be used on string fields.",
+				Descriptor: f,
+				Location:   locations.FieldResourceReference(f),
+			}}
+		}
+		return nil
+	},
+}

rules/aip0122/resource_reference_type_test.go

@@ -0,0 +1,53 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package aip0122
+
+import (
+	"testing"
+
+	"github.com/googleapis/api-linter/rules/internal/testutils"
+)
+
+func TestResourceReferenceType(t *testing.T) {
+	ann := ` [(google.api.resource_reference) = {
+		type: "library.googleapis.com/Author"
+	}]`
+	for _, test := range []struct {
+		name       string
+		Type       string
+		Annotation string
+		problems   testutils.Problems
+	}{
+		{"ValidString", "string", ann, nil},
+		{"InvalidMessage", "Author", ann, testutils.Problems{{Message: "string fields"}}},
+		{"IrrelevantNoAnnotation", "Author", "", nil},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			f := testutils.ParseProto3Tmpl(t, `
+				import "google/api/resource.proto";
+
+				message Book {
+					{{.Type}} author = 1{{.Annotation}};
+
+				}
+				message Author {}
+			`, test)
+			field := f.GetMessageTypes()[0].GetFields()[0]
+			if diff := test.problems.SetDescriptor(field).Diff(resourceReferenceType.Lint(f)); diff != "" {
+				t.Errorf(diff)
+			}
+		})
+	}
+}