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

From: Stefan Monnier <[email-masked]>
Date: Sun, 17 Dec 2023 10:24:49 -0500

>> Maybe I'm confused, but won't this change have at least _some_ effect
>> on Lisp programs?  For example, what about this fragment from the
>> ELisp manual, which describes the effect of
>> make-variable-buffer-local:
>>
>>      A peculiar wrinkle of this feature is that binding the variable
>>      (with ‘let’ or other binding constructs) does not create a
>>      buffer-local binding for it.  Only setting the variable (with ‘set’
>>      or ‘setq’), while the variable does not have a ‘let’-style binding
>>      that was made in the current buffer, does so.
>>
>> Will this case work the same after the change as it did before?
>
> I read this differently - when you have
>
>    (let ((var 'val))
>     (make-variable-buffer-local 'var))
>
> it will not set buffer-local value to 'val.

So, what the paragraph above says is that

    (setq var val)

will make `var` buffer-local, but

    (let ((var val))
      ...)

will not and neither will

    (let ((var val))
      ...
      (setq var val2)
      ...)

IOW, `setq` makes the var buffer-local only when the assignment affects
the global binding.

The same holds for DEFVAR_PER_BUFFER variables like `case-fold-search`:

    (with-temp-buffer
      (let ((case-fold-search nil))
        (list (local-variable-p 'case-fold-search)
              (setq case-fold-search t)
              (local-variable-p 'case-fold-search))))
    ==> (nil t nil)

-- Stefan