[vm] Record toString()

TEST=language/records/simple/to_string_test

Issue: https://github.com/dart-lang/sdk/issues/49719
Change-Id: I7e55a2486b1d964b24a287ffcf87c40c20f9cfe9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/260480
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

sdk/lib/_internal/vm/lib/object_patch.dart

@@ -132,6 +132,28 @@ class _Record {
     return SystemHash.finish(hash);
   }
 
+  // Do not inline to avoid mixing _fieldAt with
+  // record field accesses.
+  @pragma("vm:never-inline")
+  String toString() {
+    StringBuffer buffer = StringBuffer("(");
+    final int numFields = _numFields;
+    final _List fieldNames = _fieldNames;
+    final int numPositionalFields = numFields - fieldNames.length;
+    for (int i = 0; i < numFields; ++i) {
+      if (i != 0) {
+        buffer.write(", ");
+      }
+      if (i >= numPositionalFields) {
+        buffer.write(unsafeCast<String>(fieldNames[i - numPositionalFields]));
+        buffer.write(": ");
+      }
+      buffer.write(_fieldAt(i).toString());
+    }
+    buffer.write(")");
+    return buffer.toString();
+  }
+
   @pragma("vm:recognized", "other")
   @pragma("vm:prefer-inline")
   external int get _numFields;

tests/language/records/simple/to_string_test.dart

@@ -0,0 +1,33 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code as governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// SharedOptions=--enable-experiment=records
+
+import "package:expect/expect.dart";
+
+class A {
+  final String name;
+  const A(this.name);
+
+  String toString() => name;
+}
+
+main() {
+  // Although the order of fields in toString() is unspecified,
+  // this test assumes that positional fields are printed first and
+  // named fields are sorted lexicographically.
+  // This test might need more sophisticated checks if there is
+  // an implementation which doesn't follow that order.
+
+  Expect.equals("(1, 2)", (1, 2).toString());
+  Expect.equals("(1, 2)", (const (1, 2)).toString());
+  Expect.equals("(3, 2, 1)", (3, 2, 1).toString());
+
+  Expect.equals("(1, foo: 2)", (1, foo: 2).toString());
+  Expect.equals("(1, foo: 2)", (foo: 2, 1).toString());
+
+  Expect.equals("(1, abc, bar: 3, foo: 2)", (1, foo: 2, "abc", bar: 3).toString());
+
+  Expect.equals("((A1, A2), (foo: A3), (A7, bar: A5, baz: A6, foo: A4))", ((A("A1"), A("A2")), const (foo: A("A3")), (foo: A("A4"), bar: A("A5"), baz: A("A6"), A("A7"))).toString());
+}