Deprecating 'lexical-binding: nil'

[Richard Stallman <rms@gnu.org>]

[[[ To any NSA and FBI agents reading my email: please consider    ]]]
[[[ whether defending the US Constitution against all enemies,     ]]]
[[[ foreign or domestic, requires you to follow Snowden's example. ]]]

I support making lexical binding the default, eventually, but instead
of proposing to make nonlexical binding mode obsolete (which implies
removing it), let's stop with deprecating it.

To delete a mode of usage that was formerly the default would make it
almost unfrasible to run old Lisp programs in newer Emacs versions.

The changes required to make those old programs function with lexical
binding are not simple or eash to write.  You can't find them by
searching for certain names, as you could for many other incompatible
changes.  In some cases, they require studying the code carefully.

To avoid that, let's keep the nonlexical mode working.

I like Stefan's proposal for the approach for doing that:

      > The plan for phasing out 'lexical-binding:nil' is as follows:

      > Emacs 28.1 - The byte-compiler warns if there is no 'lexical-binding'
      >              cookie.

      > Emacs 30.1 - The byte-compiler warns if there is no 'lexical-binding'
      >              cookie, or if the cookie is 'lexical-binding:nil'.

      > Emacs 32.1 - The warnings remain as before, and 'lexical-binding:t' is
      >              the default.

One good thing is that this plans for it to take many years.  It took
us 10 years to switch in Emacs, and we are to some extent organized.
The user community is not organized and will not be quick to switch.

Another is that there will be two stages of warnings, proposed here
for Emacs 28 and Emacs 30.  Each warning will convince some users to
switch, so two warning stages are better than one.

I have no opinion about whether to take the first step in Emacs 28 or
in Emacs 29.

-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)