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Although this does not fix Bug#27986, it is a step forward.
I plan to propose a more-significant patch later.
* lisp/files.el (directory-name-p): Move from here ...
* src/fileio.c (Fdirectory_name_p): ... to here.
(directory_like, cp_like_target): New static functions.
(Fcopy_file, Frename_file, Fadd_name_to_file)
(Fmake_symbolic_link):
Use them, to avoid directory-testing syscalls on file names that
must be directories if they exist.  Omit unnecessary
initializations and CHECK_STRING calls.
(Frename_file): Don't call file_name_case_insensitive_p
twice on the same file.  Compare both file names expanded, instead
of the old name expanded and the new one unexpanded.