Add helpers and checks for partial names.

CHANGELOG.md

@@ -4,21 +4,35 @@
 * Deprecated `builder.dart`: import `source_gen.dart` instead.
 * Added `TypeChecker`, a high-level API for performing static type checks:
 
-```dart
-import 'package:analyzer/dart/element/type.dart';
-import 'package:source_gen/source_gen.dart';
-
-void checkType(DartType dartType) {
-  // Checks compared to runtime type `SomeClass`.
-  print(const TypeChecker.forRuntime(SomeClass).isExactlyType(dartType));
+  ```dart
+  import 'package:analyzer/dart/element/type.dart';
+  import 'package:source_gen/source_gen.dart';
+
+  void checkType(DartType dartType) {
+    // Checks compared to runtime type `SomeClass`.
+    print(const TypeChecker.forRuntime(SomeClass).isExactlyType(dartType));
+
+    // Checks compared to a known Url/Symbol:
+    const TypeChecker.forUrl('package:foo/foo.dart#SomeClass');
+
+    // Checks compared to another resolved `DartType`:
+    const TypeChecker.forStatic(anotherDartType);
+  }
+  ```
+
+* Failing to add a `library` directive to a library that is being used as a
+  generator target that generates partial files (`part of`) is now an explicit
+  error that gives a hint on how to name and fix your library:
 
-  // Checks compared to a known Url/Symbol:
-  const TypeChecker.forUrl('package:foo/foo.dart#SomeClass');
+  ```bash
+  > Could not find library identifier so a "part of" cannot be built.
+  >
+  > Consider adding the following to your source file:
+  >
+  > "library foo.bar;"
+  ```
 
-  // Checks compared to another resolved `DartType`:
-  const TypeChecker.forStatic(anotherDartType);
-}
-```
+  In Dart SDK `>=1.25.0` this can be relaxed as `part of` can refer to a path.
 
 ## 0.5.8
 

lib/src/builder.dart

@@ -82,16 +82,26 @@ class GeneratorBuilder extends Builder {
     var generatedOutputs =
         await _generate(library, generators, buildStep).toList();
 
-    // Don't outputs useless files.
-    if (generatedOutputs.isEmpty) return;
-
     var contentBuffer = new StringBuffer();
 
     if (!isStandalone) {
-      contentBuffer.writeln('part of ${library.name};');
+      var asset = buildStep.inputId;
+      var name = nameOfPartial(library, asset);
+      if (name == null) {
+        var suggest = suggestLibraryName(asset);
+        throw new InvalidGenerationSourceError(
+            'Could not find library identifier so a "part of" cannot be built.',
+            todo: ''
+                'Consider adding the following to your source file:\n\n'
+                'library $suggest;');
+      }
+      contentBuffer.writeln('part of $name;');
       contentBuffer.writeln();
     }
 
+    // Don't output useless files.
+    if (generatedOutputs.isEmpty) return;
+
     for (GeneratedOutput output in generatedOutputs) {
       contentBuffer.writeln('');
       contentBuffer.writeln(_headerLine);

lib/src/utils.dart

@@ -5,6 +5,7 @@
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:analyzer/dart/element/element.dart';
+import 'package:build/build.dart';
 
 String friendlyNameForElement(Element element) {
   var friendlyName = element.displayName;
@@ -39,6 +40,40 @@ String friendlyNameForElement(Element element) {
   return names.join(' ');
 }
 
+/// Returns a name suitable for `part of "..."` when pointing to [element].
+///
+/// Returns `null` if [element] is missing identifier.
+///
+/// Starting in `1.25.0`, setting [allowUnnamedPartials] will fallback
+/// (actually, preferred) to `'part of "package:foo/path.dart'`, and null will
+/// never be returned.
+String nameOfPartial(
+  LibraryElement element,
+  AssetId source, {
+  bool allowUnnamedPartials: false,
+}) {
+  if (element.name != null && element.name.isNotEmpty) {
+    return element.name;
+  }
+  if (allowUnnamedPartials) {
+    return '\'package:${source.package}/${source.path}\'';
+  }
+  return null;
+}
+
+/// Returns a suggested library identifier based on [source] path and package.
+String suggestLibraryName(AssetId source) {
+  // lib/test.dart --> [lib/test.dart]
+  var parts = source.path.split('/');
+  // [lib/test.dart] --> [lib/test]
+  parts[parts.length - 1] = parts.last.split('.').first;
+  // [lib/test] --> [test]
+  if (parts.first == 'lib') {
+    parts = parts.skip(1);
+  }
+  return '${source.package}.${parts.join('.')}';
+}
+
 /// Returns all of the declarations in [unit], including [unit] as the first
 /// item.
 Iterable<Element> getElementsFromLibraryElement(LibraryElement unit) sync* {

test/builder_test.dart

@@ -67,6 +67,15 @@ void main() {
         returnsNormally);
   });
 
+  test('Throws an exception when no library identifier is found', () async {
+    var sources = _createPackageStub(pkgName, testLibContent: 'class A {}');
+    var builder = new GeneratorBuilder([const CommentGenerator()]);
+    expect(
+        testBuilder(builder, sources,
+            outputs: {'$pkgName|lib/test_lib.g.dart': ''}),
+        throwsA(const isInstanceOf<InvalidGenerationSourceError>()));
+  });
+
   test(
       'Simple Generator test for library',
       () => _generateTest(