bug#66706: [PATCH] Automatic elisp dialect insertion

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> I thought we already decided on that: we should make lexical-binding
> the default at some point.

The question is how to get to that point.

My understanding is that we're waiting for enough files/packages to use
lexbind so that the change will not introduce too much breakage.

> Until then, I see no reason why the prominent indication on the mode
> line should not be enough.

It's much easier to write code directly in lexbind dialect than to go
back and change existing code to adapt to the lexbind dialect.

Since "we should make lexical-binding the default at some point",
all files which come without `lexical-binding:t` cookie will need to go
through the harder path, compared to those who started their life
with that cookie.
So inserting that cookie in new files would help our users take the
easier road.

For that same reason, it will cause less breakage when the default is
changed, and thus will let us make this change sooner rather than later.

Of course, maybe I'm not being bold enough and we shouldn't wait: let's
change the default `lexical-binding` to t right away and just tell
people to add a `lexical-binding:nil` cookie if they don't know how (or
don't want to) adjust their code to the new dialect.


        Stefan