Rewrite/redirect '#<topic>' search expressions to 'topic:<topic>'

app/lib/search/backend.dart

@@ -19,16 +19,14 @@ import 'package:meta/meta.dart';
 // ignore: implementation_imports
 import 'package:pana/src/dartdoc/pub_dartdoc_data.dart';
 import 'package:pool/pool.dart';
-import 'package:pub_dev/publisher/backend.dart';
-
-import 'package:pub_dev/search/search_client.dart';
-import 'package:pub_dev/service/download_counts/backend.dart';
-import 'package:pub_dev/shared/popularity_storage.dart';
-import 'package:pub_dev/shared/redis_cache.dart';
-import 'package:pub_dev/shared/utils.dart';
 import 'package:retry/retry.dart';
 
+import '../../publisher/backend.dart';
+import '../../service/download_counts/backend.dart';
 import '../../service/topics/models.dart';
+import '../../shared/popularity_storage.dart';
+import '../../shared/redis_cache.dart';
+import '../../shared/utils.dart';
 import '../package/backend.dart';
 import '../package/model_properties.dart';
 import '../package/models.dart';
@@ -48,6 +46,7 @@ import 'dart_sdk_mem_index.dart';
 import 'flutter_sdk_mem_index.dart';
 import 'models.dart';
 import 'result_combiner.dart';
+import 'search_client.dart';
 import 'search_service.dart';
 import 'text_utils.dart';
 
@@ -549,9 +548,28 @@ SearchForm? canonicalizeSearchForm(SearchForm form) {
   }
   if (newTags != null) {
     return form.change(query: query.change(tagsPredicate: newTags).toString());
-  } else {
-    return null;
   }
+
+  final newQueryText = form.parsedQuery.text?.split(' ').map((p) {
+    if (p.startsWith('#') && p.length > 1) {
+      final topic = p.substring(1);
+      // Checking the surface format, and skipping the change if the
+      // text would be an invalid topic.
+      if (!isValidTopicFormat(topic)) {
+        return p;
+      }
+      // NOTE: We don't know if this topic exists or spelled correctly.
+      //       We should consider restricting the updates to existing
+      //       topics only (TBD).
+      return 'topic:$topic';
+    }
+    return p;
+  }).join(' ');
+  if (newQueryText != form.parsedQuery.text) {
+    return form.change(query: newQueryText);
+  }
+
+  return null;
 }
 
 /// Creates the index-related API data structure from the extracted dartdoc data.

app/lib/service/topics/models.dart

@@ -7,7 +7,6 @@ import 'dart:io';
 
 import 'package:json_annotation/json_annotation.dart';
 import 'package:logging/logging.dart';
-import 'package:meta/meta.dart';
 import 'package:path/path.dart' as p;
 import 'package:pub_dev/frontend/static_files.dart';
 import 'package:yaml/yaml.dart';
@@ -92,7 +91,6 @@ class CanonicalTopicFileContent {
 }
 
 /// True, if [topic] is formatted like a valid topic.
-@visibleForTesting
 bool isValidTopicFormat(String topic) =>
     RegExp(r'^[a-z0-9-]{2,32}$').hasMatch(topic) &&
     !topic.contains('--') &&

app/test/frontend/handlers/redirects_test.dart

@@ -117,5 +117,19 @@ void main() {
         'https://api.flutter.dev/',
       );
     });
+
+    testWithProfile('search canonicalization: topic name', fn: () async {
+      await expectRedirectResponse(
+        await issueGet('/packages?q=topic%3Awidgets'),
+        '/packages?q=topic%3Awidget',
+      );
+    });
+
+    testWithProfile('search canonicalization: topic shortcut', fn: () async {
+      await expectRedirectResponse(
+        await issueGet('/packages?q=%23hash'),
+        '/packages?q=topic%3Ahash',
+      );
+    });
   });
 }

app/test/search/backend_test.dart

@@ -69,5 +69,12 @@ void main() {
         'topic:widget abc',
       );
     });
+
+    test('query with topic shortcut `#`', () {
+      expect(
+        canonicalizeSearchForm(_parse('#widget #testing'))?.query,
+        'topic:widget topic:testing',
+      );
+    });
   });
 }