Added Clang nullability checks for basic_string methods

Author: igaztanaga

doc/container.qbk

@@ -1459,9 +1459,10 @@ use [*Boost.Container]? There are several reasons for that:
    * If available, uses C++17's utilities under the `__cpp_aligned_new` feature.
    * Uses alternative aligned allocation functions (`posix_memalign`, `aligned_alloc`, `_aligned_malloc`...) otherwise.
 * Implemented overaligned allocation support for `adaptive_pool`and `node_allocator`
-* Updated `basic_string` to the latest standard API:
+* Updated `basic_string`:
    * Added missing `string_view` members and updated `operator[]` to be able to return the terminating null.
    * Added C++20 `starts_with`/`ends_with` and C++23 `contains` overloads.
+   * Added Clang `nullability` checks for `basic_string` methods.
 * Fixed bugs/issues:
    * [@https://github.com/boostorg/container/issues/323 GitHub #323: ['"flat_tree::try_emplace UB"]].
    * [@https://github.com/boostorg/container/issues/328 GitHub #328: ['"boost::container::deque stores a redundant copy of the allocator, increasing size"]].

include/boost/container/detail/workaround.hpp

@@ -252,4 +252,18 @@ namespace boost {
     #define BOOST_CONTAINER_NOVTABLE
 #endif
 
+#if defined(__has_feature)
+#  if __has_feature(nullability)
+#     define BOOST_CONTAINER_NONNULL_SUPPORTED
+#     define BOOST_CONTAINER_NONNULL  _Nonnull
+#     define BOOST_CONTAINER_NULLABLE _Nullable
+#  endif
+#endif
+
+#if !defined(BOOST_CONTAINER_NONNULL_SUPPORTED)
+#  define BOOST_CONTAINER_NONNULL
+#  define BOOST_CONTAINER_NULLABLE
+#  undef BOOST_CONTAINER_NONNULL_SUPPORTED
+#endif
+
 #endif   //#ifndef BOOST_CONTAINER_DETAIL_WORKAROUND_HPP