bug#50268: 28.0.50; Assertion warning during native compilation

[Stefan Monnier]

>> It's been a while, but looking at Bug#29165, the reason I added it is
>> that it was supported in Emacs 25 and earlier.  Stefan M had some
>> justifications there for &optional without variables, but I think they
>> don't apply for &rest.
> Stefan, can you please chime in?

With my type-systems-guy hat on, I'd say we want to detect those `&rest`
without a following variable and shame the author of the code.

But at the same time we could think of `&rest` as something similar to
`&optional` and decide that it can be handy for generated code to be
able to generate (x y &optional z &rest).

The main difference I see is that

    `(x y &optional ,@args)

makes a lot of sense and will naturally occasionally lead to `&optional`
not followed by anything, whereas `&rest` only expects 1 following
identifier so it seems much less likely that the code will sometimes put
0 vars in there.  After all, we do signal an error if there's more than
1 var after `&rest`, so I think it makes sense to follow my
types-systems-guy heart here.


        Stefan