Nicer failures when an annotation is not resolvable

CHANGELOG.md

@@ -1,3 +1,14 @@
+## 0.6.0
+
+* **Breaking change**: `TypeChecker#annotationsOf|firstAnnotationOf` now
+  returns annotations that are _assignable_ to the `TypeChecker`'s type. As a
+  result we've added `#annotationsOfExact|firstAnnotationOfExact` which has the
+  old behavior for precise checks.
+
+* `TypeChecker#annotations...`-methods now throw a `StateError` if one or more
+  annotations on an element are not resolvable. This is usually a sign of a
+  mispelling, missing import, or missing dependency.
+
 ## 0.5.10+1
 
 * Update minimum `analyzer` package to `0.29.10`.

lib/src/type_checker.dart

@@ -38,7 +38,7 @@ abstract class TypeChecker {
   /// package like in the `dart:` SDK.
   const factory TypeChecker.fromUrl(dynamic url) = _UriTypeChecker;
 
-  /// Returns the first constant annotating [element] that is this type.
+  /// Returns the first constant annotating [element] assignable to this type.
   ///
   /// Otherwise returns `null`.
   DartObject firstAnnotationOf(Element element) {
@@ -49,18 +49,40 @@ abstract class TypeChecker {
     return results.isEmpty ? null : results.first;
   }
 
-  /// Returns every constant annotating [element] that is this type.
+  /// Returns the first constant annotating [element] that is exactly this type.
+  DartObject firstAnnotationOfExact(Element element) {
+    if (element.metadata.isEmpty) {
+      return null;
+    }
+    final results = annotationsOfExact(element);
+    return results.isEmpty ? null : results.first;
+  }
+
+  DartObject _checkedConstantValue(ElementAnnotation annotation) {
+    final result = annotation.computeConstantValue();
+    if (result == null) {
+      throw new StateError(
+          'Could not resolve $annotation. An import or dependency may be '
+          'missing or invalid.');
+    }
+    return result;
+  }
+
+  /// Returns annotating constants on [element] assignable to this type.
   Iterable<DartObject> annotationsOf(Element element) => element.metadata
-      .map((a) => a.computeConstantValue())
-      .where((a) => isExactlyType(a.type));
+      .map(_checkedConstantValue)
+      .where((a) => a?.type != null && isAssignableFromType(a.type));
+
+  /// Returns annotating constants on [element] of exactly this type.
+  Iterable<DartObject> annotationsOfExact(Element element) => element.metadata
+      .map(_checkedConstantValue)
+      .where((a) => a?.type != null && isExactlyType(a.type));
 
-  /// Returns `true` if the type of [element] can be assigned to the type
-  /// represented by `this`.
+  /// Returns `true` if the type of [element] can be assigned to this type.
   bool isAssignableFrom(Element element) =>
       isExactly(element) || _getAllSupertypes(element).any(isExactlyType);
 
-  /// Returns `true` if [staticType] can be assigned to the type represented
-  /// by `this`.
+  /// Returns `true` if [staticType] can be assigned to this type.
   bool isAssignableFromType(DartType staticType) =>
       isAssignableFrom(staticType.element);
 
@@ -97,7 +119,7 @@ abstract class TypeChecker {
   bool isSuperTypeOf(DartType staticType) => isSuperOf(staticType.element);
 }
 
-//TODO(kevmoo) Remove when bug with `ClassElement.allSupertypes` is fixed
+// TODO(kevmoo) Remove when bug with `ClassElement.allSupertypes` is fixed
 // https://github.com/dart-lang/sdk/issues/29767
 Iterable<InterfaceType> _getAllSupertypes(Element element) sync* {
   if (element is ClassElement) {

pubspec.yaml

@@ -1,5 +1,5 @@
 name: source_gen
-version: 0.5.10+1
+version: 0.6.0-dev
 author: Dart Team <[email protected]>
 description: Automated source code generation for Dart.
 homepage: https://github.com/dart-lang/source_gen

test/type_checker_test.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// Increase timeouts on this test which resolves source code and can be slow.
+@Timeout.factor(2.0)
 import 'dart:collection';
 
 import 'package:analyzer/dart/element/type.dart';
@@ -177,4 +179,19 @@ void main() {
         checkGeneratorForAnnotation: () => const TypeChecker.fromUrl(
             'package:source_gen/src/generator_for_annotation.dart#GeneratorForAnnotation'));
   });
+
+  test('should gracefully when something is not resolvable', () async {
+    final resolver = await resolveSource(r'''
+      library _test;
+
+      @depracated // Intentionally mispelled.
+      class X {}
+    ''');
+    final lib = resolver.getLibraryByName('_test');
+    final classX = lib.getType('X');
+    final $deprecated = const TypeChecker.fromRuntime(Deprecated);
+
+    expect(() => $deprecated.annotationsOf(classX), throwsStateError,
+        reason: 'deprecated was spelled wrong; no annotation can be resolved');
+  });
 }