[libc++] Add the __is_replaceable type trait

That type trait represents whether move-assigning an object is
equivalent to destroying it and then move-constructing a new one
from the same argument. This will be useful in a few places where
we may want to destroy + construct instead of doing an assignment,
in particular when implementing some container operations in terms
of relocation.

Eventually, the library "emulation" added by this patch can be
replaced once the compiler implements the trivial relocation
proposals.

Author: ldionne

libcxx/include/CMakeLists.txt

@@ -838,6 +838,7 @@ set(files
   __type_traits/is_reference.h
   __type_traits/is_reference_wrapper.h
   __type_traits/is_referenceable.h
+  __type_traits/is_replaceable.h
   __type_traits/is_same.h
   __type_traits/is_scalar.h
   __type_traits/is_signed.h

libcxx/include/__exception/exception_ptr.h

@@ -65,8 +65,11 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
   friend _LIBCPP_HIDE_FROM_ABI exception_ptr make_exception_ptr(_Ep) _NOEXCEPT;
 
 public:
-  // exception_ptr is basically a COW string.
+  // exception_ptr is basically a COW string so it is trivially relocatable.
+  // However, it's not replaceable because destroying and move-constructing could cause
+  // the underlying refcount to hit 0 if we're self-assigning.
   using __trivially_relocatable _LIBCPP_NODEBUG = exception_ptr;
+  using __replaceable _LIBCPP_NODEBUG           = void;
 
   _LIBCPP_HIDE_FROM_ABI exception_ptr() _NOEXCEPT : __ptr_() {}
   _LIBCPP_HIDE_FROM_ABI exception_ptr(nullptr_t) _NOEXCEPT : __ptr_() {}

libcxx/include/__expected/expected.h

@@ -29,6 +29,7 @@
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/is_reference.h>
+#include <__type_traits/is_replaceable.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_constructible.h>
@@ -470,6 +471,8 @@ class expected : private __expected_base<_Tp, _Err> {
       __conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value && __libcpp_is_trivially_relocatable<_Err>::value,
                       expected,
                       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable<_Tp>::value && __is_replaceable<_Err>::value, expected, void>;
 
   template <class _Up>
   using rebind = expected<_Up, error_type>;

libcxx/include/__locale

@@ -50,8 +50,11 @@ _LIBCPP_HIDE_FROM_ABI const _Facet& use_facet(const locale&);
 
 class _LIBCPP_EXPORTED_FROM_ABI locale {
 public:
-  // locale is essentially a shared_ptr that doesn't support weak_ptrs and never got a move constructor.
+  // locale is essentially a shared_ptr that doesn't support weak_ptrs and never got a move constructor,
+  // so it is trivially relocatable. However, it is not replaceable because self-assignment must prevent
+  // the refcount from hitting 0.
   using __trivially_relocatable _LIBCPP_NODEBUG = locale;
+  using __replaceable _LIBCPP_NODEBUG           = void;
 
   // types:
   class _LIBCPP_EXPORTED_FROM_ABI facet;

libcxx/include/__memory/shared_ptr.h

@@ -317,7 +317,11 @@ class _LIBCPP_SHARED_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS shared_ptr {
 
   // A shared_ptr contains only two raw pointers which point to the heap and move constructing already doesn't require
   // any bookkeeping, so it's always trivially relocatable.
+  //
+  // However, it's not replaceable because of self-assignment, which must prevent the refcount from
+  // hitting 0.
   using __trivially_relocatable _LIBCPP_NODEBUG = shared_ptr;
+  using __replaceable _LIBCPP_NODEBUG           = void;
 
 private:
   element_type* __ptr_;
@@ -1212,7 +1216,10 @@ class _LIBCPP_SHARED_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS weak_ptr {
 
   // A weak_ptr contains only two raw pointers which point to the heap and move constructing already doesn't require
   // any bookkeeping, so it's always trivially relocatable.
+  //
+  // However, it's not replaceable because we must preserve a non-zero refcount through self-assignment.
   using __trivially_relocatable _LIBCPP_NODEBUG = weak_ptr;
+  using __replaceable _LIBCPP_NODEBUG           = void;
 
 private:
   element_type* __ptr_;

libcxx/include/__memory/unique_ptr.h

@@ -39,6 +39,7 @@
 #include <__type_traits/is_function.h>
 #include <__type_traits/is_pointer.h>
 #include <__type_traits/is_reference.h>
+#include <__type_traits/is_replaceable.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_relocatable.h>
@@ -144,6 +145,8 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr {
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<deleter_type>::value,
       unique_ptr,
       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable<pointer>::value && __is_replaceable<deleter_type>::value, unique_ptr, void>;
 
 private:
   _LIBCPP_COMPRESSED_PAIR(pointer, __ptr_, deleter_type, __deleter_);
@@ -410,6 +413,8 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr<_Tp[], _Dp>
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<deleter_type>::value,
       unique_ptr,
       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable<pointer>::value && __is_replaceable<deleter_type>::value, unique_ptr, void>;
 
 private:
   template <class _Up, class _OtherDeleter>

libcxx/include/__split_buffer

@@ -28,6 +28,7 @@
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_replaceable.h>
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_destructible.h>
 #include <__type_traits/is_trivially_relocatable.h>
@@ -72,6 +73,10 @@ public:
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<allocator_type>::value,
       __split_buffer,
       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable<pointer>::value && __container_allocator_is_replaceable<__alloc_traits>::value,
+                      __split_buffer,
+                      void>;
 
   pointer __first_;
   pointer __begin_;

libcxx/include/__type_traits/is_replaceable.h

@@ -0,0 +1,59 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H
+#define _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H
+
+#include <__config>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_trivially_copyable.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// A type is replaceable if `x = std::move(y)` is equivalent to:
+//
+//  std::destroy_at(&x)
+//  std::construct_at(&x, std::move(y))
+//
+// This allows turning a move-assignment into a sequence of destroy + move-construct, which
+// is often more efficient. This is especially relevant when the move-construct is in fact
+// part of a trivial relocation from somewhere else, in which case there is a huge win.
+//
+// Note that this requires language support in order to be really effective, but we
+// currently emulate the base template with something very conservative.
+template <class _Tp, class = void>
+struct __is_replaceable : is_trivially_copyable<_Tp> {};
+
+template <class _Tp>
+struct __is_replaceable<_Tp, __enable_if_t<is_same<_Tp, typename _Tp::__replaceable>::value> > : true_type {};
+
+// Determines whether an allocator member of a container is replaceable.
+//
+// We take into account whether the allocator is propagated on assignments. If the allocator
+// always compares equal, then it doesn't matter whether we propagate it or not on assignments,
+// the result will be the same and we can just as much move-construct it instead.
+//
+// If the allocator does not always compare equal, we check whether it propagates on assignment
+// and it is replaceable.
+template <class _AllocatorTraits>
+struct __container_allocator_is_replaceable
+    : integral_constant<bool,
+                        _AllocatorTraits::is_always_equal::value ||
+                            (_AllocatorTraits::propagate_on_container_move_assignment::value &&
+                             _AllocatorTraits::propagate_on_container_copy_assignment::value &&
+                             __is_replaceable<typename _AllocatorTraits::allocator_type>::value)> {};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H

libcxx/include/__utility/pair.h

@@ -31,6 +31,8 @@
 #include <__type_traits/is_implicitly_default_constructible.h>
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_replaceable.h>
+#include <__type_traits/is_same.h>
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_relocatable.h>
 #include <__type_traits/nat.h>
@@ -72,6 +74,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
       __conditional_t<__libcpp_is_trivially_relocatable<_T1>::value && __libcpp_is_trivially_relocatable<_T2>::value,
                       pair,
                       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable<_T1>::value && __is_replaceable<_T2>::value, pair, void>;
 
   _LIBCPP_HIDE_FROM_ABI pair(pair const&) = default;
   _LIBCPP_HIDE_FROM_ABI pair(pair&&)      = default;

libcxx/include/__vector/vector.h

@@ -55,6 +55,7 @@
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/is_pointer.h>
+#include <__type_traits/is_replaceable.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_trivially_relocatable.h>
 #include <__type_traits/type_identity.h>
@@ -120,6 +121,10 @@ class _LIBCPP_TEMPLATE_VIS vector {
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<allocator_type>::value,
       vector,
       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable<pointer>::value && __container_allocator_is_replaceable<__alloc_traits>::value,
+                      vector,
+                      void>;
 
   static_assert(__check_valid_allocator<allocator_type>::value, "");
   static_assert(is_same<typename allocator_type::value_type, value_type>::value,