[cfe] Record type fields can be wildcards.

Positional record type fields named `_` are wildcards. They are non-binding and will not collide with other wildcard fields.

Bug: https://github.com/dart-lang/sdk/issues/55655
Change-Id: Ic6985da5bdb32564402d084a3282fa448ff28cbe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/371040
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

pkg/front_end/lib/src/fasta/builder/record_type_builder.dart

@@ -149,6 +149,9 @@ abstract class RecordTypeBuilderImpl extends RecordTypeBuilder {
         positionalEntries.add(type);
         String? fieldName = field.name;
         if (fieldName != null) {
+          if (library is SourceLibraryBuilder && field.isWildcard) {
+            continue;
+          }
           if (fieldName.startsWith("_")) {
             library.addProblem(messageRecordFieldsCantBePrivate,
                 field.charOffset, fieldName.length, fileUri);
@@ -380,7 +383,10 @@ class RecordTypeFieldBuilder {
 
   final int charOffset;
 
-  RecordTypeFieldBuilder(this.metadata, this.type, this.name, this.charOffset);
+  final bool isWildcard;
+
+  RecordTypeFieldBuilder(this.metadata, this.type, this.name, this.charOffset,
+      {this.isWildcard = false});
 
   RecordTypeFieldBuilder clone(
       List<NamedTypeBuilder> newTypes,
@@ -392,7 +398,8 @@ class RecordTypeFieldBuilder {
         metadata,
         type.clone(newTypes, contextLibrary, contextDeclaration),
         name,
-        charOffset);
+        charOffset,
+        isWildcard: isWildcard);
   }
 
   @override

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

@@ -5213,13 +5213,17 @@ class BodyBuilder extends StackListenerImpl
     Object? type = pop();
     // TODO(johnniwinther): How should we handle annotations?
     pop(NullValues.Metadata); // Annotations.
+
+    String? fieldName = name is Identifier ? name.name : null;
     push(new RecordTypeFieldBuilder(
         [],
         type is ParserRecovery
             ? new InvalidTypeBuilderImpl(uri, type.charOffset)
             : type as TypeBuilder,
-        name is Identifier ? name.name : null,
-        name is Identifier ? name.nameOffset : TreeNode.noOffset));
+        fieldName,
+        name is Identifier ? name.nameOffset : TreeNode.noOffset,
+        isWildcard:
+            libraryFeatures.wildcardVariables.isEnabled && fieldName == '_'));
   }
 
   @override

pkg/front_end/lib/src/fasta/source/outline_builder.dart

@@ -3172,13 +3172,17 @@ class OutlineBuilder extends StackListenerImpl {
     Object? type = pop();
     List<MetadataBuilder>? metadata =
         pop(NullValues.Metadata) as List<MetadataBuilder>?;
+
+    String? fieldName = identifier is Identifier ? identifier.name : null;
     push(new RecordTypeFieldBuilder(
         metadata,
         type is ParserRecovery
             ? new InvalidTypeBuilderImpl(uri, type.charOffset)
             : type as TypeBuilder,
-        identifier is Identifier ? identifier.name : null,
-        identifier is Identifier ? identifier.nameOffset : -1));
+        fieldName,
+        identifier is Identifier ? identifier.nameOffset : -1,
+        isWildcard:
+            libraryFeatures.wildcardVariables.isEnabled && fieldName == '_'));
   }
 
   @override

pkg/front_end/testcases/wildcard_variables/record_type.dart

@@ -0,0 +1,12 @@
+// Copyright (c) 2024, 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.
+
+typedef R = (String _, String _);
+(int _, int _) record = (1, 2);
+
+main() {
+  (int _, int _) localRecord = (1, 2);
+  print(localRecord.$1);
+  print(record.$1);
+}

pkg/front_end/testcases/wildcard_variables/record_type.dart.strong.expect

@@ -0,0 +1,11 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+typedef R = (core::String, core::String);
+static field (core::int, core::int) record = (1, 2);
+static method main() → dynamic {
+  (core::int, core::int) localRecord = (1, 2);
+  core::print(localRecord.$1{core::int});
+  core::print(self::record.$1{core::int});
+}

pkg/front_end/testcases/wildcard_variables/record_type.dart.strong.modular.expect

@@ -0,0 +1,11 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+typedef R = (core::String, core::String);
+static field (core::int, core::int) record = (1, 2);
+static method main() → dynamic {
+  (core::int, core::int) localRecord = (1, 2);
+  core::print(localRecord.$1{core::int});
+  core::print(self::record.$1{core::int});
+}

pkg/front_end/testcases/wildcard_variables/record_type.dart.strong.outline.expect

@@ -0,0 +1,8 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+typedef R = (core::String, core::String);
+static field (core::int, core::int) record;
+static method main() → dynamic
+  ;

pkg/front_end/testcases/wildcard_variables/record_type.dart.strong.transformed.expect

@@ -0,0 +1,17 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+typedef R = (core::String, core::String);
+static field (core::int, core::int) record = (1, 2);
+static method main() → dynamic {
+  (core::int, core::int) localRecord = (1, 2);
+  core::print(localRecord.$1{core::int});
+  core::print(self::record.$1{core::int});
+}
+
+
+Extra constant evaluation status:
+Evaluated: RecordLiteral @ org-dartlang-testcase:///record_type.dart:9:32 -> RecordConstant(const (1, 2))
+Evaluated: RecordLiteral @ org-dartlang-testcase:///record_type.dart:6:25 -> RecordConstant(const (1, 2))
+Extra constant evaluation: evaluated: 8, effectively constant: 2

pkg/front_end/testcases/wildcard_variables/record_type.dart.textual_outline.expect

@@ -0,0 +1,5 @@
+typedef R = (String _, String _);
+
+(int _, int _) record = (1, 2);
+
+main() {}

pkg/front_end/testcases/wildcard_variables/record_type.dart.textual_outline_modelled.expect

@@ -0,0 +1,5 @@
+(int _, int _) record = (1, 2);
+
+main() {}
+
+typedef R = (String _, String _);