Fix an msan warning in cord_ringbuffer_test

Stop the absl::Cord destructor from running on the constinit cord in
CordTest.ConstinitConstructor. This allows inspecting the cord at any point
while the test is exiting, which is important for the semantics of the test.

PiperOrigin-RevId: 448327386
Change-Id: Icef9faa2b63f1f0ae60b3430dcf6184f5dead885

absl/strings/cord_test.cc

@@ -2241,12 +2241,34 @@ class AfterExitCordTester {
   absl::string_view expected_;
 };
 
+// Deliberately prevents the destructor for an absl::Cord from running. The cord
+// is accessible via the cord member during the lifetime of the CordLeaker.
+// After the CordLeaker is destroyed, pointers to the cord will remain valid
+// until the CordLeaker's memory is deallocated.
+struct CordLeaker {
+  union {
+    absl::Cord cord;
+  };
+
+  template <typename Str>
+  constexpr explicit CordLeaker(const Str& str) : cord(str) {}
+
+  ~CordLeaker() {
+    // Don't do anything, including running cord's destructor. (cord's
+    // destructor won't run automatically because cord is hidden inside a
+    // union.)
+  }
+};
+
 template <typename Str>
 void TestConstinitConstructor(Str) {
   const auto expected = Str::value;
   // Defined before `cord` to be destroyed after it.
   static AfterExitCordTester exit_tester;  // NOLINT
-  ABSL_CONST_INIT static absl::Cord cord(Str{});  // NOLINT
+  ABSL_CONST_INIT static CordLeaker cord_leaker(Str{});  // NOLINT
+  // cord_leaker is static, so this reference will remain valid through the end
+  // of program execution.
+  static absl::Cord& cord = cord_leaker.cord;
   static bool init_exit_tester = exit_tester.Set(&cord, expected);
   (void)init_exit_tester;