Warn if the user did not specify a language on the fenced code block

lib/src/model/documentation_comment.dart

@@ -61,11 +61,15 @@ mixin DocumentationComment
   String processCommentWithoutTools(String documentationComment) {
     var docs = stripComments(documentationComment);
     if (!docs.contains('{@')) {
+      _analyzeCodeBlocks(docs);
       return docs;
     }
     docs = _injectExamples(docs);
     docs = _injectYouTube(docs);
     docs = _injectAnimations(docs);
+
+    _analyzeCodeBlocks(docs);
+
     // TODO(srawlins): Processing templates here causes #2281. But leaving them
     // unprocessed causes #2272.
     docs = _stripHtmlAndAddToIndex(docs);
@@ -79,6 +83,7 @@ mixin DocumentationComment
     // Must evaluate tools first, in case they insert any other directives.
     docs = await _evaluateTools(docs);
     docs = processCommentDirectives(docs);
+    _analyzeCodeBlocks(docs);
     return docs;
   }
 
@@ -673,4 +678,27 @@ mixin DocumentationComment
       return '$option${match[0]}';
     });
   }
+
+  static final _codeBlockPattern =
+      RegExp(r'^[ ]{0,3}(`{3,}|~{3,})(.*)$', multiLine: true);
+
+  /// Analyze fenced code blocks present in the documentation comment,
+  /// warning if there is no language specified.
+  void _analyzeCodeBlocks(String docs) {
+    final results = _codeBlockPattern.allMatches(docs).toList(growable: false);
+    final firstOfPair = <Match>[];
+    for (var i = 0; i < results.length; i++) {
+      if (i.isEven && i != results.length - 1) {
+        firstOfPair.add(results[i]);
+      }
+    }
+    firstOfPair.forEach((element) {
+      final result = element.group(2).trim();
+      if (result.isEmpty) {
+        warn(PackageWarning.missingCodeBlockLanguage,
+            message:
+                'A fenced code block in Markdown should have a language specified');
+      }
+    });
+  }
 }

lib/src/model/package_graph.dart

@@ -452,6 +452,9 @@ class PackageGraph {
       case PackageWarning.missingExampleFile:
         warningMessage = 'example file not found: $message';
         break;
+      case PackageWarning.missingCodeBlockLanguage:
+        warningMessage = 'missing code block language: $message';
+        break;
     }
 
     var messageParts = <String>[warningMessage];

lib/src/warnings.dart

@@ -256,6 +256,16 @@ const Map<PackageWarning, PackageWarningDefinition> packageWarningDefinitions =
       // Defaults to ignore as this doesn't impact the docs severely but is
       // useful for debugging package structure.
       defaultWarningMode: PackageWarningMode.ignore),
+  PackageWarning.missingCodeBlockLanguage: PackageWarningDefinition(
+      PackageWarning.missingCodeBlockLanguage,
+      'missing-code-block-language',
+      'A fenced code block is missing a specified language.',
+      longHelp: [
+        'To enable proper syntax highlighting of Markdown code blocks,',
+        'Dartdoc requires code blocks to specify the language used after',
+        'the initial declaration.  As an example, to specify Dart you would',
+        'specify ```dart or ~~~dart.'
+      ]),
 };
 
 /// Something that package warnings can be called on.  Optionally associated
@@ -306,6 +316,7 @@ enum PackageWarning {
   unresolvedExport,
   missingConstantConstructor,
   missingExampleFile,
+  missingCodeBlockLanguage,
 }
 
 /// Used to declare defaults for a particular package warning.

test/documentation_comment_test.dart

@@ -482,7 +482,7 @@ End text.'''));
 
     test('processes @example with file', () async {
       projectRoot.getChildAssumingFile('abc.md').writeAsStringSync('''
-```
+```plaintext
 Code snippet
 ```
 ''');
@@ -498,7 +498,7 @@ Code snippet
       expect(doc, equals('''
 Text.
 
-```
+```plaintext
 Code snippet
 ```
 
@@ -844,6 +844,67 @@ Text.
               'Supported YouTube URLs have the following format: '
               'https://www.youtube.com/watch?v=oHg5SJYRHA0.'));
     });
+
+    test('warns when fenced code block does not specify language', () async {
+      await libraryModel.processComment('''
+/// ```
+/// void main() {}
+/// ```
+''');
+
+      expect(
+          packageGraph.packageWarningCounter.hasWarning(
+              libraryModel,
+              PackageWarning.missingCodeBlockLanguage,
+              'A fenced code block in Markdown should have a language specified'),
+          isTrue);
+    });
+
+    test('warns when squiggly fenced code block does not specify language',
+        () async {
+      await libraryModel.processComment('''
+/// ~~~
+/// void main() {}
+/// ~~~
+''');
+
+      expect(
+          packageGraph.packageWarningCounter.hasWarning(
+              libraryModel,
+              PackageWarning.missingCodeBlockLanguage,
+              'A fenced code block in Markdown should have a language specified'),
+          isTrue);
+    });
+
+    test('does not warn when fenced code block does specify language',
+        () async {
+      await libraryModel.processComment('''
+/// ```dart
+/// void main() {}
+/// ```
+''');
+
+      expect(
+          packageGraph.packageWarningCounter.hasWarning(
+              libraryModel,
+              PackageWarning.missingCodeBlockLanguage,
+              'A fenced code block in Markdown should have a language specified'),
+          isFalse);
+    });
+
+    test('does not warn when fenced block is not closed', () async {
+      await libraryModel.processComment('''
+/// ```
+/// A not closed fenced code block
+''');
+
+      expect(
+          packageGraph.packageWarningCounter.hasWarning(
+              libraryModel,
+              PackageWarning.missingCodeBlockLanguage,
+              'A fenced code block in Markdown should have a language specified'),
+          isFalse);
+    });
   }, onPlatform: {
     'windows': Skip('These tests do not work on Windows (#2446)')
   });