Re: master 431f8ff1e38: * lisp/imenu.el: Support more values for imenu-flatten (bug#70846)

[Stefan Monnier]

> because when `completing-read-default` calls `read-from-minibuffer`,
> it uses the value of `minibuffer-allow-text-properties` from
> the original buffer:
>
>   val = read_minibuf (keymap, initial_contents, prompt,
>                     !NILP (read),
>                     histvar, histpos, default_value,
>                     minibuffer_allow_text_properties,
>                     !NILP (inherit_input_method));

Damn!  This is too bad: a buffer-local setting of
`minibuffer_allow_text_properties` can basically never be used then,
because it's read from the wrong buffer.

> Then `read_minibuf` uses its argument `allow_props`:
>
>   if (allow_props)
>     val = Fminibuffer_contents ();
>   else
>     val = Fminibuffer_contents_no_properties ();
>
> Maybe it could use `minibuffer-allow-text-properties` directly here?

Indeed: since a buffer-local setting can't work, we know that all
callers must use a plain let-binding so the binding will be active
during the whole minibuffer session, so we may as well read it at the
end (in the minibuffer) rather than at the beginning (in the
`minibuffer--original-buffer`).


        Stefan


PS: Git shows that `minibuffer-allow-text-properties` was introduced
eons ago (and basically never touched since then, even the docstring is
mostly unchanged).  And Grep shows it's not used very often (and
several of those uses are around `completing-read`).
Funnily enough, one of the few uses in our tree binds it to nil,
I wonder why that was needed.