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

From: Stefan Monnier <[email-masked]>
Date: Mon, 01 Jan 2024 20:30:48 -0500

Ihor Radchenko [2024-01-01 18:02:30] wrote:

> Gerd Möllmann <[email-masked]> writes:
>
>>> (defvar-local let-tests-buffer-local-var 'value)
>>> (with-temp-buffer
>>>   (setq-local let-tests-buffer-local-var 'baz)
>>>   (let ((let-tests-buffer-local-var 'bar))
>>>     ;; This is failing.
>>>     (should (eq 'bar (default-value 'let-tests-buffer-local-var)))
>>
>> C-h f default-value explains this:
>>
>>   Return SYMBOL’s default value.
>>   This is the value that is seen in buffers that do not have their own values
>>   for this variable.
>
> Are you saying that `let' never changes `default-value' output?
> Unfortunately, no.

Indeed, see the doc that Eli quoted 10 posts "ago":

    [...] 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.

IOW, `let` changes the binding that is "current": if the variable is
buffer-local in the current buffer it changes that buffer-local value
and otherwise it changes the global value.
[ This for "automatically buffer-local variables".  ]

        Stefan