Add spanForElement, for nicer debugging messages.

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.6.1
+
+* Added `spanForElement`; returns a `SourceSpan` for an analyzer `Element`.
+
 ## 0.6.0
 
 * **Breaking change**: `TypeChecker#annotationsOf|firstAnnotationOf` now

lib/source_gen.dart

@@ -10,4 +10,5 @@ export 'src/generator.dart';
 export 'src/generator_for_annotation.dart';
 export 'src/library.dart' show LibraryReader;
 export 'src/revive.dart' show Revivable;
+export 'src/span_for_element.dart' show spanForElement;
 export 'src/type_checker.dart' show TypeChecker;

lib/src/span_for_element.dart

@@ -0,0 +1,41 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+import 'package:analyzer/dart/element/element.dart';
+import 'package:source_span/source_span.dart';
+
+import 'utils.dart';
+
+/// Returns a source span that spans the location where [element] is defined.
+///
+/// May be used to emit user-friendly warning and error messages:
+/// ```dart
+/// void invalidClass(ClassElement class) {
+///   log.warning(spanForElement.message('Cannot implement "Secret"'));
+/// }
+/// ```
+///
+/// Not all results from the analyzer API may return source information as part
+/// of the element, so [file] may need to be manually provided in those cases.
+SourceSpan spanForElement(Element element, [SourceFile file]) {
+  final url = assetToPackageUrl(element.source.uri);
+  if (file == null) {
+    final contents = element?.source?.contents;
+    if (contents == null) {
+      return new SourceSpan(
+        new SourceLocation(
+          element.nameOffset,
+          sourceUrl: url,
+        ),
+        new SourceLocation(
+          element.nameOffset + element.nameLength,
+          sourceUrl: url,
+        ),
+        element.name,
+      );
+    }
+    file = new SourceFile.fromString(contents.data, url: url);
+  }
+  return file.span(element.nameOffset, element.nameOffset + element.nameLength);
+}

lib/src/utils.dart

@@ -105,13 +105,13 @@ Uri normalizeDartUrl(Uri url) => url.pathSegments.isNotEmpty
     ? url.replace(pathSegments: url.pathSegments.take(1))
     : url;
 
-/// Returns a `package:` URL into a `asset:` URL.
+/// Returns a `package:` URL converted to a `asset:` URL.
 ///
 /// This makes internal comparison logic much easier, but still allows users
 /// to define assets in terms of `package:`, which is something that makes more
 /// sense to most.
 ///
-/// For example this transforms `package:source_gen/source_gen.dart` into:
+/// For example, this transforms `package:source_gen/source_gen.dart` into:
 /// `asset:source_gen/lib/source_gen.dart`.
 Uri packageToAssetUrl(Uri url) => url.scheme == 'package'
     ? url.replace(
@@ -122,6 +122,24 @@ Uri packageToAssetUrl(Uri url) => url.scheme == 'package'
           ..addAll(url.pathSegments.skip(1)))
     : url;
 
+/// Returns a `asset:` URL converted to a `package:` URL.
+///
+/// For example, this transformers `asset:source_gen/lib/source_gen.dart' into:
+/// `package:source_gen/source_gen.dart`. Asset URLs that aren't pointing to a
+/// file in the 'lib' folder are not modified.
+///
+/// Asset URLs come from `package:build`, as they are able to describe URLs that
+/// are not describable using `package:...`, such as files in the `bin`, `tool`,
+/// `web`, or even root directory of a package - `asset:some_lib/web/main.dart`.
+Uri assetToPackageUrl(Uri url) => url.scheme == 'asset' &&
+        url.pathSegments.length >= 1 &&
+        url.pathSegments[1] == 'lib'
+    ? url.replace(
+        scheme: 'package',
+        pathSegments: [url.pathSegments.first]
+          ..addAll(url.pathSegments.skip(2)))
+    : url;
+
 /// Returns all of the declarations in [unit], including [unit] as the first
 /// item.
 Iterable<Element> getElementsFromLibraryElement(LibraryElement unit) sync* {

pubspec.yaml

@@ -1,5 +1,5 @@
 name: source_gen
-version: 0.6.0
+version: 0.6.1-dev
 author: Dart Team <misc@dartlang.org>
 description: Automated source code generation for Dart.
 homepage: https://github.com/dart-lang/source_gen
@@ -11,6 +11,7 @@ dependencies:
   collection: ^1.1.2
   dart_style: '>=0.1.7 <2.0.0'
   path: ^1.3.2
+  source_span: ^1.0.0
 dev_dependencies:
   build_runner: ^0.3.2
   build_test: ^0.6.0

test/span_for_element_test.dart

@@ -0,0 +1,31 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+import 'package:analyzer/dart/element/element.dart';
+import 'package:build/build.dart';
+import 'package:build_test/build_test.dart';
+import 'package:source_gen/source_gen.dart';
+import 'package:test/test.dart';
+
+void main() {
+  LibraryElement library;
+
+  setUpAll(() async {
+    final resolver = await resolveSource(r'''
+      library test_lib;
+      
+      abstract class Example implements List {}
+    ''', inputId: new AssetId('test_lib', 'lib/test_lib.dart'));
+    library = resolver.getLibraryByName('test_lib');
+  });
+
+  test('should highlight the use of "class Example"', () {
+    expect(
+        spanForElement(library.getType('Example')).message('Here it is'),
+        ''
+        'line 3, column 22 of package:test_lib/test_lib.dart: Here it is\n'
+        '      abstract class Example implements List {}\n'
+        '                     ^^^^^^^');
+  });
+}