bug#66117: 30.0.50; `find-buffer-visiting' is slow when opening large number of buffers

[Stefan Monnier]

> - What I propose is to avoid this loop in `do_specbind' altogether.
>   Instead of having to loop through all the buffers to set temporary
>   buffer-local value, I propose to introduce special buffer object
>   `buffer_overrides' that will hold such temporary bindings.
>   Then, we can change BVAR to something like
>
>   #define BVAR(buf, field) (buffer_overrides->field ## _ == UNSET ?\
>                               (buf)->field ## _ :\
>                              buffer_overrides->field ## _)

If we're willing to pay the cost of such a test, then we don't need
a new `buffer_overrides`, we can just set `buf->field` to a special value
(e.g. `Qunbound`, tho we'll probably need to use a new similar special
value) and then do:

    #define BVAR(buf, field) ((buf)->field ## _ != Qunbound ?\
                                (buf)->field ## _ :\
                               buffer_defaults->field ## _)

This said, in the case of `case-fold-search` I think there's a simpler
solution: make it "buffer-local only", meaning that let-binding it
affects only the current buffer (rather than affecting all the buffers
which have not made it buffer-local yet).


        Stefan