Verify the old ABI

Author: glessard

Tests/FoundationEssentialsTests/DataLegacyABITests.swift

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#if FOUNDATION_FRAMEWORK
+@_spi(FoundationLegacyABI) @testable import Foundation
+import Testing
+
+@Test func validateLegacyABI() {
+    var data = Data()
+
+    data._legacy_withUnsafeBytes { _ in }
+    data._legacy_withUnsafeMutableBytes { _ in }
+}
+
+#endif // FOUNDATION_FRAMEWORK