A patch generated by git format-patch is attached below, which simply
wraps `ucs-normalize-region` with `save-match-data`.

I'm a user of the Emacs Mac port by mituharu was investigating a bug
where dired fails to open a certain local directory on macOS.  The
error was raised at `replace-match` in the `insert-directory`
function:

```
		(when (re-search-forward "^ *\\(total\\)" nil t)
		  (let ((available (get-free-disk-space ".")))
		    (when available
		      ;; Replace "total" with "used", to avoid confusion.
		      (replace-match "total used in directory" nil nil nil 1)
```

And it turned out the match data changed after returning from
`get-free-disk-space` and that was why `replace-match` failed.

Inside of `get-free-disk-space` most platforms uses a generic method
to get the free space, and that part is fine because it is surrounded
by `save-match-data`.  However, the Mac port is one of the few
platforms that implements a native 'file-system-info` function, which
is called if it exists.  Then, the `file-system-info` in `src/mac.c`
calls ENCODE_FILE() on a given directory name, which in the end calls
`ucs-normalize-region` to normalize the filename, where the match data
is clobbered.

https://bitbucket.org/mituharu/emacs-mac/src/df827786d7a7fb0a0e2f27577af67e32d9a888a9/src/mac.c#lines-2337

ENCODE_FILE() is transparently called by many C functions, which means
`ucs-normalize-region` can be called at unpredictable timings, so I
think it should keep match data unchanged.

-- 
Akinori MUSHA / https://akinori.org/