[libcxx] Avoid intermediate string objects for substrings in windows operator/=

[mstorsjo]

Summary

Check that appends with a path object doesn't do allocations, even
on windows.

Suggested by Marek in D98398. The patch might apply without D98398
(depending on how much of the diff context has to match), but doesn't
make much sense until after that patch has landed.

[• Quuxplusone]

I think this makes plenty of sense to land before D98412!

[curdeius]

LGTM.

[mstorsjo]

Sorry, I forgot to amend out the bit about this not making sense without/before the other one...

libcxx/include/filesystem

path& operator/=(const path& __p) {
     if (__p.is_absolute() ||
         (!__p_root_name.empty() && __p_root_name != root_name())) {
       __pn_ = __p.__pn_;
       return *this;
     }
     if (__p.has_root_directory()) {
       path __root_name_str = root_name();
       __pn_ = __root_name_str.native();
-      __pn_ += __p.__pn_.substr(__p_root_name_size);
+      __pn_ += __string_view(__p.__pn_).substr(__p_root_name_size);
       return *this;
     }
     if (has_filename() || (!has_root_directory() && is_absolute()))
       __pn_ += preferred_separator;
-    __pn_ += __p.__pn_.substr(__p_root_name_size);
+    __pn_ += __string_view(__p.__pn_).substr(__p_root_name_size);
     return *this;
   }
   template <class _Source>
   _LIBCPP_INLINE_VISIBILITY _EnableIfPathable<_Source>
   operator/=(const _Source& __src) {
     return operator/=(path(__src));
   }
 

libcxx/test/std/input.output/filesystems/class.path/path.member/path.append.pass.cpp

// CharT*
     path LHS(L); PathReserve(LHS, ReserveSize);
     Ptr RHS(R);
     {
       DisableAllocationGuard g;
       LHS.append(RHS, StrEnd(RHS));
     }
     assert(PathEq(LHS, E));
   }
+  {
+    path LHS(L); PathReserve(LHS, ReserveSize);
+    path RHS(R);
+    {
+      DisableAllocationGuard g;
+      LHS /= RHS;
+    }
+    assert(PathEq(LHS, E));
+  }
   // input iterator - For non-native char types, appends needs to copy the
   // iterator range into a contiguous block of memory before it can perform the
   // code_cvt conversions.
   // For "char" no allocations will be performed because no conversion is
   // required.
   bool DisableAllocations = std::is_same<CharT, char>::value;
   {
     path LHS(L); PathReserve(LHS, ReserveSize);