bug#41200: Displaying a tooltip with x-show-tip gets very slow as more faces are defined

[Noam Postavsky]

Eli Zaretskii <eliz@gnu.org> writes:

>> Done for frame-face-alist.  But how can one do a read-only variable?
>> Using the new variable watchers facility?
>
> At the very least mention in the doc string.  I don't think using
> watchable symbols is needed here, it sounds gross.  If there's an
> outcry that this breaks too much code out there, we could revisit
> this.

You can use make_symbol_constant to make a variable read-only.  You
could also simulate it with a variable watcher that signals an error,
but there's no sense in doing that when make_symbol_constant exists.

Neither of these makes the list object itself read-only though.