[no_destruct] Improve ergonomics (#30564)

* [no_destruct] Improve ergonomics

* comment

* fix

* Automated change: Fix sanity tests

Co-authored-by: ctiller <ctiller@users.noreply.github.com>

src/core/lib/gprpp/no_destruct.h

@@ -58,25 +58,31 @@ class NoDestruct {
   NoDestruct& operator=(const NoDestruct&) = delete;
   ~NoDestruct() = default;
 
-  T* operator->() { return reinterpret_cast<T*>(&space_); }
-  const T* operator->() const { return *reinterpret_cast<const T*>(&space_); }
-  T& operator*() { return *reinterpret_cast<T*>(&space_); }
-  const T& operator*() const { return *reinterpret_cast<const T*>(&space_); }
+  T* operator->() { return get(); }
+  const T* operator->() const { return get(); }
+  T& operator*() { return *get(); }
+  const T& operator*() const { return *get(); }
+
+  T* get() { return reinterpret_cast<T*>(&space_); }
+  const T* get() const { return reinterpret_cast<const T*>(&space_); }
 
  private:
   typename std::aligned_storage<sizeof(T), alignof(T)>::type space_;
 };
 
-// Helper for when a program desires a single instance of a default constructed
-// T to be always available.
-// T is constructed eagerly at program startup, so it's essentially free to load
-// the pointer to the instance.
+// Helper for when a program desires a single *process wide* instance of a
+// default constructed T to be always available.
+// The instance is constructed eagerly at program startup, so it's essentially
+// free to load the pointer to the instance.
 template <typename T>
 class NoDestructSingleton {
  public:
   static T* Get() { return &*value_; }
 
  private:
+  NoDestructSingleton() = delete;
+  ~NoDestructSingleton() = delete;
+
   static NoDestruct<T> value_;
 };
 

test/core/gprpp/no_destruct_test.cc

@@ -41,6 +41,29 @@ TEST(NoDestruct, CrashOnDestructionIsAccessible) {
   g_test_crash_on_destruction->Exists();
 }
 
+bool g_thing_constructed = false;
+
+class Thing {
+ public:
+  Thing() {
+    EXPECT_FALSE(g_thing_constructed);
+    g_thing_constructed = true;
+  }
+
+  int Add(int i, int j) { return i + j; }
+
+ private:
+  ~Thing() = delete;
+};
+
+TEST(GlobalSingleton, Works) {
+  // Thing should be eagerly constructed, so we should not observe it being not
+  // constructed.
+  EXPECT_TRUE(g_thing_constructed);
+  // We should be able to fetch the global Thing and use it.
+  EXPECT_EQ(NoDestructSingleton<Thing>::Get()->Add(1, 2), 3);
+}
+
 }  // namespace
 }  // namespace testing
 }  // namespace grpc_core