Give basic_memory_buffer allocator [[no_unique_address]]

This allows stateless allocators to take up no space while still avoiding the empty base class optimization.

include/fmt/core.h

@@ -87,6 +87,9 @@
 #define FMT_HAS_CPP17_ATTRIBUTE(attribute) \
   (FMT_CPLUSPLUS >= 201703L && FMT_HAS_CPP_ATTRIBUTE(attribute))
 
+#define FMT_HAS_CPP20_ATTRIBUTE(attribute) \
+  (FMT_CPLUSPLUS >= 202002L && FMT_HAS_CPP_ATTRIBUTE(attribute))
+
 // Check if relaxed C++14 constexpr is supported.
 // GCC doesn't allow throw in constexpr until version 6 (bug 67371).
 #ifndef FMT_USE_CONSTEXPR
@@ -154,6 +157,16 @@
 #  endif
 #endif
 
+#ifndef FMT_NO_UNIQUE_ADDRESS
+#  if FMT_HAS_CPP20_ATTRIBUTE(no_unique_address)
+#    define FMT_NO_UNIQUE_ADDRESS [[no_unique_address]]
+#  elif FMT_MSC_VERSION >= 1929 && FMT_CPLUSPLUS >= 202002L // VS2019 v16.10 and later
+#    define FMT_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
+#  else
+#    define FMT_NO_UNIQUE_ADDRESS
+#  endif
+#endif
+
 #ifndef FMT_INLINE
 #  if FMT_GCC_VERSION || FMT_CLANG_VERSION
 #    define FMT_INLINE inline __attribute__((always_inline))

include/fmt/format.h

@@ -920,7 +920,7 @@ class basic_memory_buffer final : public detail::buffer<T> {
   T store_[SIZE];
 
   // Don't inherit from Allocator avoid generating type_info for it.
-  Allocator alloc_;
+  FMT_NO_UNIQUE_ADDRESS Allocator alloc_;
 
   // Deallocate memory allocated by the buffer.
   FMT_CONSTEXPR20 void deallocate() {