[libc++] Call basic_string_view's assume-valid constructor from basic_string operations (#105863)

`basic_string` frequently calls `basic_string_view(data(), size())`,
which accounts for ~15% of the observed overhead when hardening is
enabled. This commit removes unnecessary checks when `basic_string` is
known to already have valid data, by bypassing the public constructor,
so that we eliminate that overhead.

Author: cjdb

libcxx/include/string

@@ -1213,7 +1213,7 @@ public:
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 operator __self_view() const _NOEXCEPT {
-    return __self_view(data(), size());
+    return __self_view(typename __self_view::__assume_valid(), data(), size());
   }
 
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS basic_string&
@@ -1822,7 +1822,7 @@ public:
 
 #if _LIBCPP_STD_VER >= 20
   constexpr _LIBCPP_HIDE_FROM_ABI bool starts_with(__self_view __sv) const noexcept {
-    return __self_view(data(), size()).starts_with(__sv);
+    return __self_view(typename __self_view::__assume_valid(), data(), size()).starts_with(__sv);
   }
 
   constexpr _LIBCPP_HIDE_FROM_ABI bool starts_with(value_type __c) const noexcept {
@@ -1834,7 +1834,7 @@ public:
   }
 
   constexpr _LIBCPP_HIDE_FROM_ABI bool ends_with(__self_view __sv) const noexcept {
-    return __self_view(data(), size()).ends_with(__sv);
+    return __self_view(typename __self_view::__assume_valid(), data(), size()).ends_with(__sv);
   }
 
   constexpr _LIBCPP_HIDE_FROM_ABI bool ends_with(value_type __c) const noexcept {
@@ -1848,15 +1848,15 @@ public:
 
 #if _LIBCPP_STD_VER >= 23
   constexpr _LIBCPP_HIDE_FROM_ABI bool contains(__self_view __sv) const noexcept {
-    return __self_view(data(), size()).contains(__sv);
+    return __self_view(typename __self_view::__assume_valid(), data(), size()).contains(__sv);
   }
 
   constexpr _LIBCPP_HIDE_FROM_ABI bool contains(value_type __c) const noexcept {
-    return __self_view(data(), size()).contains(__c);
+    return __self_view(typename __self_view::__assume_valid(), data(), size()).contains(__c);
   }
 
   constexpr _LIBCPP_HIDE_FROM_ABI bool contains(const value_type* __s) const {
-    return __self_view(data(), size()).contains(__s);
+    return __self_view(typename __self_view::__assume_valid(), data(), size()).contains(__s);
   }
 #endif
 

libcxx/include/string_view

@@ -211,6 +211,7 @@ namespace std {
 #include <__functional/hash.h>
 #include <__functional/unary_function.h>
 #include <__fwd/ostream.h>
+#include <__fwd/string.h>
 #include <__fwd/string_view.h>
 #include <__iterator/bounded_iter.h>
 #include <__iterator/concepts.h>
@@ -689,6 +690,9 @@ private:
 
   const value_type* __data_;
   size_type __size_;
+
+  template <class, class, class>
+  friend class basic_string;
 };
 _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(basic_string_view);