revise std::vector uninit-mem hacks under libc++ (#1912)

Summary:
X-link: facebook/folly#1912
Closes: facebook/folly#1867

The `std::__vector_base` class in libc++ is removed in libc++ 15 (llvm/llvm-project@b82da8b). So the two current implementations of the hacks - the implementation for libc++ 14 and the implementation for earlier libc++ - both fail.

Even with libc++ 14, which retains `std::__vector_base`, Xcode 14 fails to compile the hacks:

```
In file included from folly/memory/test/UninitializedMemoryHacksODR.cpp:17:
folly/memory/UninitializedMemoryHacks.h:461:1: error: conversion from 'std::__vector_base<char, std::allocator<char>>::pointer std::__vector_base<char, std::allocator<char>>::*' to 'char *std::vector<char>::*' is not allowed in a converted constant expression
FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT(char)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
folly/memory/UninitializedMemoryHacks.h:327:7: note: expanded from macro 'FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT'
      &std::vector<TYPE>::__end_,                                    \
      ^~~~~~~~~~~~~~~~~~~~~~~~~~
...
```

Since libc++ is unlikely to change the internal layout of `std::vector` on a cadence which is too frequent for folly - any material change would be an ABI break -, we can use a layout struct rather than a template instance for the hacks and have it work for a range of versions of libc++ and Xcode.

Reviewed By: simpkins, swolchok

Differential Revision: D42459219

fbshipit-source-id: e23ef383869681b5c81e4301114020b9089c7461

third-party/folly/src/folly/memory/UninitializedMemoryHacks.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <cstddef>
 #include <string>
 #include <type_traits>
 #include <vector>
@@ -291,89 +292,47 @@ namespace detail {
   }                                                                      \
   }
 
-#if defined(_LIBCPP_VECTOR) && _LIBCPP_VERSION >= 14000
-// libc++ newer
-
-template <
-    typename Tag,
-    typename T,
-    typename A,
-    A Ptr__end_,
-    typename B,
-    B Ptr__annotate_contiguous_container_>
-struct MakeUnsafeVectorSetLargerSize {
-  friend void unsafeVectorSetLargerSizeImpl(std::vector<T>& v, std::size_t n) {
-    const auto old_size = v.size();
-    v.*Ptr__end_ += (n - v.size());
-
-    // libc++ contiguous containers use special annotation functions that help
-    // the address sanitizer to detect improper memory accesses. When ASAN is
-    // enabled we need to call the appropriate annotation functions in order to
-    // stop ASAN from reporting false positives. When ASAN is disabled, the
-    // annotation function is a no-op.
-    (v.*Ptr__annotate_contiguous_container_)(
-        v.data(),
-        v.data() + v.capacity(),
-        v.data() + old_size,
-        v.data() + v.size());
-  }
-};
-
-#define FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT(TYPE)               \
-  template struct folly::detail::MakeUnsafeVectorSetLargerSize<      \
-      FollyMemoryDetailTranslationUnitTag,                           \
-      TYPE,                                                          \
-      TYPE*(std::vector<TYPE, std::allocator<TYPE>>::*),             \
-      &std::vector<TYPE>::__end_,                                    \
-      void (std::vector<TYPE>::*)(                                   \
-          const void*, const void*, const void*, const void*) const, \
-      &std::vector<TYPE>::__annotate_contiguous_container>;          \
-  FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT_IMPL(TYPE)
-
-#elif defined(_LIBCPP_VECTOR)
+#if defined(_LIBCPP_VECTOR)
 // libc++
 
-template <
-    typename Tag,
-    typename T,
-    typename A,
-    A Ptr__end_,
-    typename B,
-    B Ptr__annotate_contiguous_container_>
-struct MakeUnsafeVectorSetLargerSize {
-  friend void unsafeVectorSetLargerSizeImpl(std::vector<T>& v, std::size_t n) {
-    // v.__end_ += (n - v.size());
-    using Base = std::__vector_base<T, std::allocator<T>>;
-    static_assert(
-        std::is_standard_layout<std::vector<T>>::value &&
-            sizeof(std::vector<T>) == sizeof(Base),
-        "reinterpret_cast safety conditions not met");
-    const auto old_size = v.size();
-    reinterpret_cast<Base&>(v).*Ptr__end_ += (n - v.size());
-
-    // libc++ contiguous containers use special annotation functions that help
-    // the address sanitizer to detect improper memory accesses. When ASAN is
-    // enabled we need to call the appropriate annotation functions in order to
-    // stop ASAN from reporting false positives. When ASAN is disabled, the
-    // annotation function is a no-op.
-    (v.*Ptr__annotate_contiguous_container_)(
-        v.data(),
-        v.data() + v.capacity(),
-        v.data() + old_size,
-        v.data() + v.size());
-  }
+template <typename T, typename Alloc = std::allocator<T>>
+struct __std_vector_layout {
+  static_assert(!std::is_same<T, bool>::value, "bad instance");
+  using allocator_type = Alloc;
+  using pointer = typename std::allocator_traits<allocator_type>::pointer;
+
+  pointer __begin_;
+  pointer __end_;
+  std::__compressed_pair<pointer, allocator_type> __end_cap_;
 };
 
-#define FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT(TYPE)               \
-  template struct folly::detail::MakeUnsafeVectorSetLargerSize<      \
-      FollyMemoryDetailTranslationUnitTag,                           \
-      TYPE,                                                          \
-      TYPE*(std::__vector_base<TYPE, std::allocator<TYPE>>::*),      \
-      &std::vector<TYPE>::__end_,                                    \
-      void (std::vector<TYPE>::*)(                                   \
-          const void*, const void*, const void*, const void*) const, \
-      &std::vector<TYPE>::__annotate_contiguous_container>;          \
-  FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT_IMPL(TYPE)
+template <typename T>
+void unsafeVectorSetLargerSize(std::vector<T>& v, std::size_t n) {
+  using real = std::vector<T>;
+  using fake = __std_vector_layout<T>;
+  using pointer = typename fake::pointer;
+  static_assert(sizeof(fake) == sizeof(real), "mismatch");
+  static_assert(alignof(fake) == alignof(real), "mismatch");
+
+  auto const l = reinterpret_cast<unsigned char*>(&v);
+
+  auto const s = v.size();
+
+  auto& e = *reinterpret_cast<pointer*>(l + offsetof(fake, __end_));
+  e += (n - s);
+
+  // libc++ contiguous containers use special annotation functions that help
+  // the address sanitizer to detect improper memory accesses. When ASAN is
+  // enabled we need to call the appropriate annotation functions in order to
+  // stop ASAN from reporting false positives. When ASAN is disabled, the
+  // annotation function is a no-op.
+#ifndef _LIBCPP_HAS_NO_ASAN
+  __sanitizer_annotate_contiguous_container(
+      v.data(), v.data() + v.capacity(), v.data() + s, v.data() + n);
+#endif
+}
+
+#define FOLLY_DECLARE_VECTOR_RESIZE_WITHOUT_INIT(TYPE)
 
 #elif defined(_GLIBCXX_VECTOR)
 // libstdc++