[libc++][modules] Modularize <cstddef> (llvm#107254)

Many headers include `<cstddef>` just for size_t, and pulling in
additional content (e.g. the traits used for std::byte) is unnecessary.
To solve this problem, this patch splits up `<cstddef>` into
subcomponents so that headers can include only the parts that they
actually require.

This has the added benefit of making the modules build a lot stricter
with respect to IWYU, and also providing a canonical location where we
define `std::size_t` and friends (which were previously defined in
multiple headers like `<cstddef>` and `<ctime>`).

After this patch, there's still many places in the codebase where we
include `<cstddef>` when `<__cstddef/size_t.h>` would be sufficient.
This patch focuses on removing `<cstddef>` includes from __type_traits
to make these headers non-circular with `<cstddef>`. Additional
refactorings can be tackled separately.

libcxx/include/CMakeLists.txt

@@ -324,6 +324,11 @@ set(files
   __coroutine/coroutine_traits.h
   __coroutine/noop_coroutine_handle.h
   __coroutine/trivial_awaitables.h
+  __cstddef/byte.h
+  __cstddef/max_align_t.h
+  __cstddef/nullptr_t.h
+  __cstddef/ptrdiff_t.h
+  __cstddef/size_t.h
   __debug_utils/randomize_range.h
   __debug_utils/sanitizers.h
   __debug_utils/strict_weak_ordering_check.h

libcxx/include/__algorithm/ranges_minmax.h

@@ -24,6 +24,7 @@
 #include <__ranges/access.h>
 #include <__ranges/concepts.h>
 #include <__type_traits/desugars_to.h>
+#include <__type_traits/is_integral.h>
 #include <__type_traits/is_reference.h>
 #include <__type_traits/is_trivially_copyable.h>
 #include <__type_traits/remove_cvref.h>

libcxx/include/__atomic/atomic.h

@@ -16,8 +16,10 @@
 #include <__config>
 #include <__functional/operations.h>
 #include <__memory/addressof.h>
+#include <__type_traits/enable_if.h>
 #include <__type_traits/is_floating_point.h>
 #include <__type_traits/is_function.h>
+#include <__type_traits/is_integral.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/remove_const.h>
 #include <__type_traits/remove_pointer.h>

libcxx/include/__charconv/to_chars_integral.h

@@ -21,6 +21,7 @@
 #include <__system_error/errc.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/integral_constant.h>
+#include <__type_traits/is_integral.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/make_32_64_or_128_bit.h>
 #include <__type_traits/make_unsigned.h>

libcxx/include/__cstddef/byte.h

@@ -0,0 +1,84 @@
+//===---------------------------------------------------------------------===//
+//
+// 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___CSTDDEF_BYTE_H
+#define _LIBCPP___CSTDDEF_BYTE_H
+
+#include <__config>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/is_integral.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 17
+namespace std { // purposefully not versioned
+
+enum class byte : unsigned char {};
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr byte operator|(byte __lhs, byte __rhs) noexcept {
+  return static_cast<byte>(
+      static_cast<unsigned char>(static_cast<unsigned int>(__lhs) | static_cast<unsigned int>(__rhs)));
+}
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr byte& operator|=(byte& __lhs, byte __rhs) noexcept {
+  return __lhs = __lhs | __rhs;
+}
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr byte operator&(byte __lhs, byte __rhs) noexcept {
+  return static_cast<byte>(
+      static_cast<unsigned char>(static_cast<unsigned int>(__lhs) & static_cast<unsigned int>(__rhs)));
+}
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr byte& operator&=(byte& __lhs, byte __rhs) noexcept {
+  return __lhs = __lhs & __rhs;
+}
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr byte operator^(byte __lhs, byte __rhs) noexcept {
+  return static_cast<byte>(
+      static_cast<unsigned char>(static_cast<unsigned int>(__lhs) ^ static_cast<unsigned int>(__rhs)));
+}
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr byte& operator^=(byte& __lhs, byte __rhs) noexcept {
+  return __lhs = __lhs ^ __rhs;
+}
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr byte operator~(byte __b) noexcept {
+  return static_cast<byte>(static_cast<unsigned char>(~static_cast<unsigned int>(__b)));
+}
+
+template <class _Integer, __enable_if_t<is_integral<_Integer>::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI constexpr byte& operator<<=(byte& __lhs, _Integer __shift) noexcept {
+  return __lhs = __lhs << __shift;
+}
+
+template <class _Integer, __enable_if_t<is_integral<_Integer>::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI constexpr byte operator<<(byte __lhs, _Integer __shift) noexcept {
+  return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(__lhs) << __shift));
+}
+
+template <class _Integer, __enable_if_t<is_integral<_Integer>::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI constexpr byte& operator>>=(byte& __lhs, _Integer __shift) noexcept {
+  return __lhs = __lhs >> __shift;
+}
+
+template <class _Integer, __enable_if_t<is_integral<_Integer>::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI constexpr byte operator>>(byte __lhs, _Integer __shift) noexcept {
+  return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(__lhs) >> __shift));
+}
+
+template <class _Integer, __enable_if_t<is_integral<_Integer>::value, int> = 0>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Integer to_integer(byte __b) noexcept {
+  return static_cast<_Integer>(__b);
+}
+
+} // namespace std
+#endif // _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___CSTDDEF_BYTE_H

libcxx/include/__cstddef/max_align_t.h

@@ -0,0 +1,27 @@
+//===---------------------------------------------------------------------===//
+//
+// 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___CSTDDEF_MAX_ALIGN_T_H
+#define _LIBCPP___CSTDDEF_MAX_ALIGN_T_H
+
+#include <__config>
+#include <stddef.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if !defined(_LIBCPP_CXX03_LANG)
+using ::max_align_t _LIBCPP_USING_IF_EXISTS;
+#endif
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CSTDDEF_MAX_ALIGN_T_H

libcxx/include/__cstddef/nullptr_t.h

@@ -0,0 +1,25 @@
+//===---------------------------------------------------------------------===//
+//
+// 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___CSTDDEF_NULLPTR_T_H
+#define _LIBCPP___CSTDDEF_NULLPTR_T_H
+
+#include <__config>
+#include <stddef.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+using ::nullptr_t;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CSTDDEF_NULLPTR_T_H

libcxx/include/__cstddef/ptrdiff_t.h

@@ -0,0 +1,25 @@
+//===---------------------------------------------------------------------===//
+//
+// 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___CSTDDEF_PTRDIFF_T_H
+#define _LIBCPP___CSTDDEF_PTRDIFF_T_H
+
+#include <__config>
+#include <stddef.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+using ::ptrdiff_t _LIBCPP_USING_IF_EXISTS;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CSTDDEF_PTRDIFF_T_H

libcxx/include/__cstddef/size_t.h

@@ -0,0 +1,25 @@
+//===---------------------------------------------------------------------===//
+//
+// 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___CSTDDEF_SIZE_T_H
+#define _LIBCPP___CSTDDEF_SIZE_T_H
+
+#include <__config>
+#include <stddef.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+using ::size_t _LIBCPP_USING_IF_EXISTS;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CSTDDEF_SIZE_T_H

libcxx/include/__exception/nested_exception.h

@@ -13,6 +13,8 @@
 #include <__exception/exception_ptr.h>
 #include <__memory/addressof.h>
 #include <__type_traits/decay.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
 #include <__type_traits/is_base_of.h>
 #include <__type_traits/is_class.h>
 #include <__type_traits/is_constructible.h>

libcxx/include/__fwd/array.h

@@ -10,7 +10,8 @@
 #define _LIBCPP___FWD_ARRAY_H
 
 #include <__config>
-#include <cstddef>
+#include <__cstddef/size_t.h>
+#include <__type_traits/integral_constant.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header

libcxx/include/__fwd/complex.h

@@ -10,7 +10,7 @@
 #define _LIBCPP___FWD_COMPLEX_H
 
 #include <__config>
-#include <cstddef>
+#include <__cstddef/size_t.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header

libcxx/include/__fwd/pair.h

@@ -10,8 +10,8 @@
 #define _LIBCPP___FWD_PAIR_H
 
 #include <__config>
+#include <__cstddef/size_t.h>
 #include <__fwd/tuple.h>
-#include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header

libcxx/include/__fwd/span.h

@@ -11,7 +11,7 @@
 #define _LIBCPP___FWD_SPAN_H
 
 #include <__config>
-#include <cstddef>
+#include <__cstddef/size_t.h>
 #include <limits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)

libcxx/include/__fwd/subrange.h

@@ -11,8 +11,8 @@
 
 #include <__concepts/copyable.h>
 #include <__config>
+#include <__cstddef/size_t.h>
 #include <__iterator/concepts.h>
-#include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header

libcxx/include/__fwd/tuple.h

@@ -10,7 +10,7 @@
 #define _LIBCPP___FWD_TUPLE_H
 
 #include <__config>
-#include <cstddef>
+#include <__cstddef/size_t.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header

libcxx/include/__iterator/concepts.h

@@ -34,6 +34,7 @@
 #include <__memory/pointer_traits.h>
 #include <__type_traits/add_pointer.h>
 #include <__type_traits/common_reference.h>
+#include <__type_traits/integral_constant.h>
 #include <__type_traits/is_pointer.h>
 #include <__type_traits/is_primary_template.h>
 #include <__type_traits/is_reference.h>

libcxx/include/__iterator/iterator_traits.h

@@ -24,6 +24,8 @@
 #include <__type_traits/common_reference.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/disjunction.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
 #include <__type_traits/is_convertible.h>
 #include <__type_traits/is_object.h>
 #include <__type_traits/is_primary_template.h>

libcxx/include/__iterator/wrap_iter.h

@@ -17,6 +17,7 @@
 #include <__memory/addressof.h>
 #include <__memory/pointer_traits.h>
 #include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
 #include <__type_traits/is_convertible.h>
 #include <cstddef>
 

libcxx/include/__mdspan/layout_stride.h

@@ -25,6 +25,7 @@
 #include <__type_traits/common_type.h>
 #include <__type_traits/is_constructible.h>
 #include <__type_traits/is_convertible.h>
+#include <__type_traits/is_integral.h>
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/is_same.h>
 #include <__utility/as_const.h>

libcxx/include/__memory/pointer_traits.h

@@ -15,6 +15,7 @@
 #include <__type_traits/conditional.h>
 #include <__type_traits/conjunction.h>
 #include <__type_traits/decay.h>
+#include <__type_traits/enable_if.h>
 #include <__type_traits/is_class.h>
 #include <__type_traits/is_function.h>
 #include <__type_traits/is_void.h>

libcxx/include/__memory/shared_ptr.h

@@ -34,6 +34,8 @@
 #include <__type_traits/conditional.h>
 #include <__type_traits/conjunction.h>
 #include <__type_traits/disjunction.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
 #include <__type_traits/is_array.h>
 #include <__type_traits/is_bounded_array.h>
 #include <__type_traits/is_constructible.h>

libcxx/include/__memory/unique_ptr.h

@@ -23,6 +23,7 @@
 #include <__type_traits/common_type.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/dependent_type.h>
+#include <__type_traits/enable_if.h>
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_array.h>
 #include <__type_traits/is_assignable.h>

libcxx/include/__memory/uses_allocator.h

@@ -11,6 +11,7 @@
 #define _LIBCPP___MEMORY_USES_ALLOCATOR_H
 
 #include <__config>
+#include <__type_traits/integral_constant.h>
 #include <__type_traits/is_convertible.h>
 #include <cstddef>
 

libcxx/include/__random/mersenne_twister_engine.h

@@ -13,6 +13,7 @@
 #include <__algorithm/min.h>
 #include <__config>
 #include <__random/is_seed_sequence.h>
+#include <__type_traits/enable_if.h>
 #include <cstddef>
 #include <cstdint>
 #include <iosfwd>

libcxx/include/__random/seed_seq.h

@@ -14,6 +14,8 @@
 #include <__algorithm/max.h>
 #include <__config>
 #include <__iterator/iterator_traits.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/is_integral.h>
 #include <__type_traits/is_unsigned.h>
 #include <cstdint>
 #include <initializer_list>

libcxx/include/__random/subtract_with_carry_engine.h

@@ -14,6 +14,7 @@
 #include <__config>
 #include <__random/is_seed_sequence.h>
 #include <__random/linear_congruential_engine.h>
+#include <__type_traits/enable_if.h>
 #include <cstddef>
 #include <cstdint>
 #include <iosfwd>

libcxx/include/__ranges/subrange.h

@@ -33,6 +33,7 @@
 #include <__tuple/tuple_size.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/decay.h>
+#include <__type_traits/integral_constant.h>
 #include <__type_traits/is_pointer.h>
 #include <__type_traits/is_reference.h>
 #include <__type_traits/make_unsigned.h>