bug#70137: 30.0.50; pp-macroexpand-last-sexp buffers should turn lexical binding on

[Stefan Monnier]

> If you call pp-macroexpand-last-sexp in a buffer with lexical binding,
> the resulting *Pp Macroexpand Output* buffer has lexical binding turned off.
>
> The problem with this is that if you now go the *Pp Macroexpand Output*
> buffer and call pp-macroexpand-last-sexp again, this second macro
> expansion step may not reflect what would happen in the original buffer;
> I don't recall an example, but some macros expand differently in the two
> variable scoping modes.

Indeed.  `dolist` and `dotimes` were such examples (tho nowadays they
return the same result).  Most other examples I can think of signal
an error when used without lexical binding.  There are a few more subtle
examples where the macro tests `macroexp--dynamic-variable-p`.

Do you happen to have a patch to propagate the value of
`lexical-binding` to the new buffer?


        Stefan