[base] Add NO_UNIQUE_ADDRESS attribute

This change adds the HAS_CPP_ATTRIBUTE and NO_UNIQUE_ADDRESS macros to
base/compiler_specific.h. This allows using the C++20 no_unique_address
attribute in prior dialects if the compiler supports it.

Bug: None
Change-Id: I6aff82e7b195f5eaf93551049c9dfa6a3fd1fbc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510396Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822768}

base/compiler_specific.h

@@ -11,6 +11,19 @@
 #error "Only clang-cl is supported on Windows, see https://crbug.com/988071"
 #endif
 
+// This is a wrapper around `__has_cpp_attribute`, which can be used to test for
+// the presence of an attribute. In case the compiler does not support this
+// macro it will simply evaluate to 0.
+//
+// References:
+// https://wg21.link/sd6#testing-for-the-presence-of-an-attribute-__has_cpp_attribute
+// https://wg21.link/cpp.cond#:__has_cpp_attribute
+#if defined(__has_cpp_attribute)
+#define HAS_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
+#else
+#define HAS_CPP_ATTRIBUTE(x) 0
+#endif
+
 // Annotate a variable indicating it's ok if the variable is not used.
 // (Typically used to silence a compiler warning when the assignment
 // is important for some other reason.)
@@ -99,6 +112,20 @@
 #define WARN_UNUSED_RESULT
 #endif
 
+// In case the compiler supports it NO_UNIQUE_ADDRESS evaluates to the C++20
+// attribute [[no_unique_address]]. This allows annotating data members so that
+// they need not have an address distinct from all other non-static data members
+// of its class.
+//
+// References:
+// * https://en.cppreference.com/w/cpp/language/attributes/no_unique_address
+// * https://wg21.link/dcl.attr.nouniqueaddr
+#if HAS_CPP_ATTRIBUTE(no_unique_address)
+#define NO_UNIQUE_ADDRESS [[no_unique_address]]
+#else
+#define NO_UNIQUE_ADDRESS
+#endif
+
 // Tell the compiler a function is using a printf-style format string.
 // |format_param| is the one-based index of the format string parameter;
 // |dots_param| is the one-based index of the "..." parameter.

base/ranges/algorithm.h

@@ -11,6 +11,7 @@
 #include <type_traits>
 #include <utility>
 
+#include "base/compiler_specific.h"
 #include "base/functional/identity.h"
 #include "base/functional/invoke.h"
 #include "base/ranges/functional.h"
@@ -150,14 +151,13 @@ using range_category_t = iterator_category_t<ranges::iterator_t<Range>>;
 
 namespace ranges {
 
-// C++14 implementation of std::ranges::in_fun_result. Note the because C++14
-// lacks the `no_unique_address` attribute it is commented out.
+// C++14 implementation of std::ranges::in_fun_result.
 //
 // Reference: https://wg21.link/algorithms.results#:~:text=in_fun_result
 template <typename I, typename F>
 struct in_fun_result {
-  /* [[no_unique_address]] */ I in;
-  /* [[no_unique_address]] */ F fun;
+  NO_UNIQUE_ADDRESS I in;
+  NO_UNIQUE_ADDRESS F fun;
 
   template <typename I2,
             typename F2,