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

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

>> But it will only solve specific problem with `find-buffer-visiting' and
>> we can always go for it if we cannot find anything better.
> Which other popular functions need to loop through all the buffers in
> Lisp?

Not sure why that matters.  The performance problem comes from repeated
uses of `(let ((case-fold-search ..)) ...)` where the repetition can be
due to anything (not only enumerating buffers).

The "loop through buffers" happens when entering and leaving the `let`,
because it has to `set/unset` the corresponding filed of the `struct
buffer` of all the buffers whose `case-fold-search` is "global".

Making it a DEFVAR_LISP rather than a DEFVAR_PER_BUFFER will fix this
problem (at the cost of making it less efficient to `set-buffer` when
that variable has been made buffer-local).

DEFVAR_PER_BUFFER should be for variables which are usually
buffer-local, and apparently that's not the case of `case-fold-search`,
so I'm starting to think that changing it from DEFVAR_PER_BUFFER to
DEFVAR_LISP might be the better option.


        Stefan