AK: Add Retained<T>, like RetainPtr, but never null.

Also use some Clang attribute wizardry to get a warning for use-after-move.

AK/Retained.h

@@ -0,0 +1,123 @@
+#pragma once
+
+#include <AK/Types.h>
+
+#ifdef __clang__
+#define CONSUMABLE(initial_state) __attribute__((consumable(initial_state)))
+#define CALLABLE_WHEN(state) __attribute__((callable_when(state)))
+#define SET_TYPESTATE(state) __attribute__((set_typestate(state)))
+#define RETURN_TYPESTATE(state) __attribute__((return_typestate(state)))
+#else
+#define CONSUMABLE(initial_state)
+#define CALLABLE_WHEN(state)
+#define SET_TYPESTATE(state)
+#define RETURN_TYPESTATE(state)
+#endif
+
+namespace AK {
+
+template<typename T>
+inline void retain_if_not_null(T* ptr)
+{
+    if (ptr)
+        ptr->retain();
+}
+
+template<typename T>
+inline void release_if_not_null(T* ptr)
+{
+    if (ptr)
+        ptr->release();
+}
+
+template<typename T>
+class CONSUMABLE(unconsumed) Retained {
+public:
+    enum AdoptTag { Adopt };
+
+    RETURN_TYPESTATE(unconsumed) Retained(T& object) : m_ptr(&object) { m_ptr->retain(); }
+    RETURN_TYPESTATE(unconsumed) Retained(AdoptTag, T& object) : m_ptr(&object) { }
+    RETURN_TYPESTATE(unconsumed) Retained(Retained& other) : m_ptr(&other.copy_ref().leak_ref()) { }
+    RETURN_TYPESTATE(unconsumed) Retained(Retained&& other) : m_ptr(&other.leak_ref()) { }
+    template<typename U> RETURN_TYPESTATE(unconsumed) Retained(Retained<U>&& other) : m_ptr(static_cast<T*>(&other.leak_ref())) { }
+    RETURN_TYPESTATE(unconsumed) Retained(const Retained& other) : m_ptr(&const_cast<Retained&>(other).copy_ref().leak_ref()) { }
+    template<typename U> RETURN_TYPESTATE(unconsumed) Retained(const Retained<U>& other) : m_ptr(&const_cast<Retained<U>&>(other).copy_ref().leak_ref()) { }
+    ~Retained()
+    {
+        release_if_not_null(m_ptr);
+        m_ptr = nullptr;
+#ifdef SANITIZE_PTRS
+        if constexpr(sizeof(T*) == 8)
+            m_ptr = (T*)(0xb0b0b0b0b0b0b0b0);
+        else
+            m_ptr = (T*)(0xb0b0b0b0);
+#endif
+    }
+
+    CALLABLE_WHEN(unconsumed) Retained& operator=(Retained&& other)
+    {
+        if (this != &other) {
+            release_if_not_null(m_ptr);
+            m_ptr = &other.leak_ref();
+        }
+        return *this;
+    }
+
+    template<typename U>
+    CALLABLE_WHEN(unconsumed) Retained& operator=(Retained<U>&& other)
+    {
+        if (this != static_cast<void*>(&other)) {
+            release_if_not_null(m_ptr);
+            m_ptr = &other.leak_ref();
+        }
+        return *this;
+    }
+
+    CALLABLE_WHEN(unconsumed) Retained& operator=(T& object)
+    {
+        if (m_ptr != &object)
+            release_if_not_null(m_ptr);
+        m_ptr = &object;
+        m_ptr->retain();
+        return *this;
+    }
+
+    CALLABLE_WHEN(unconsumed) Retained copy_ref() const
+    {
+        return Retained(*m_ptr);
+    }
+
+    CALLABLE_WHEN(unconsumed) SET_TYPESTATE(consumed)
+    T& leak_ref()
+    {
+        ASSERT(m_ptr);
+        T* leakedPtr = m_ptr;
+        m_ptr = nullptr;
+        return *leakedPtr;
+    }
+
+    CALLABLE_WHEN(unconsumed) T* ptr() { ASSERT(m_ptr); return m_ptr; }
+    CALLABLE_WHEN(unconsumed) const T* ptr() const { ASSERT(m_ptr); return m_ptr; }
+
+    CALLABLE_WHEN(unconsumed) T* operator->() { ASSERT(m_ptr); return m_ptr; }
+    CALLABLE_WHEN(unconsumed) const T* operator->() const { ASSERT(m_ptr); return m_ptr; }
+
+    CALLABLE_WHEN(unconsumed) T& operator*() { ASSERT(m_ptr); return *m_ptr; }
+    CALLABLE_WHEN(unconsumed) const T& operator*() const { ASSERT(m_ptr); return *m_ptr; }
+
+private:
+    Retained() { }
+
+    T* m_ptr { nullptr };
+};
+
+template<typename T>
+inline Retained<T> adopt(T& object)
+{
+    return Retained<T>(Retained<T>::Adopt, object);
+}
+
+}
+
+using AK::Retained;
+using AK::adopt;