bug#66546: 30.0.50; save-buffer to write-protected file without backup fails

[Eli Zaretskii <eliz@gnu.org>]

> From: Jens Schmidt <jschmidt4gnu@vodafonemail.de>
> Cc: 66546@debbugs.gnu.org
> Date: Wed, 01 Nov 2023 20:06:36 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Sure, addition to the test suite are always welcome.
> 
> Please review attached test.  I hope I haven't been overperforming.

Thanks, a few comments below:

> +    (cl-letf* (;; Define convenience functions.
> +               ((symbol-function 'file-contents)
> +                (lambda (file)
> +                  (if (file-exists-p file)
> +                      (condition-case err
> +                          (with-temp-buffer
> +                            (insert-file-contents file)
> +                            (buffer-string))
> +                        ((error err)))
> +                    'missing)))
> [...]
> +        (should     (equal (file-contents file) "foo\n"))

If you want to make sure the file's contents is _exactly_ some text,
you need to write the buffer text to the file with no encoding
conversions, and you need then to visit the file with
insert-file-contents-literally, to avoid decoding conversions.
Otherwise you might get false positives and false negatives due to
encoding/decoding of text and of EOLs.

Also, compiling the new test I get byte-compiler warnings:

  In toplevel form:
  lisp/files-tests.el:1748:39: Warning: Unused lexical variable `err'
  lisp/files-tests.el:1763:17: Warning: `local-write-file-hooks' is an obsolete variable (as of 22.1); use `write-file-functions' instead.

  In end of data:
  lisp/files-tests.el:1802:29: Warning: the function `file-contents' is not known to be defined.