bug#50946: Emacs-28: Inadequate coding in hack-elisp-shorthands

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

In emacs -Q in the emacs-28 branch, create the following two line file,
foobar.el, and try to load it:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(defvar foo-baz "foobar-baz")
FOOBARELISP-SHORTHANDS: (("foo" . "foobar")))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

This will throw an error, but that isn't important.

What is important is that the symbol foobar-baz is created by the
elisp-shorthands facility.

This shouldn't happen since:
1/- There is no Local Variables section.
2/- There is no variable elisp-shorthands in that non-existent section.

The following errors are evident in hack-elisp-shorthands:
1/- The code doesn't check for a correctly formatted Local Variables
  section.
2/- The code, even if it did check, would only check the last 3000 bytes
  in the file.  The section can occur anywhere in the last 3000
  CHARACTERS.
3/- The code doesn't do a case-sensitive search for "elisp-shorthands".
4/- The code doesn't check for "elisp-shorthands" being a complete
  symbol.
5/- The code doesn't even check that "elisp-shorthands" is in a comment.

I would suggest that these errors be corrected.  I would also suggest
that the entire code and documentation for this new facility be
carefully reviewed by somebody who isn't the original author.

-- 
Alan Mackenzie (Nuremberg, Germany).