Problem with font selection / BigSur OS

["Nicolas P. Rougier" <nicolas.rougier@inria.fr>]

After upgrading to BigSur OS (Apple) and using EmacsMac 27.1, I'm 
unable to select some font weights. It worked with previous 
version (Catalina) and I think this is not related to any Emacs 
change. I would thus like to understand why it fails but I don't 
know where to look for the code responsible to select a font given 
a spec.

More precisely, for the following three lines, the last one 
evaluates to nil:

(find-font (font-spec :family "Roboto Mono" :weight 'regular))
(find-font (font-spec :family "Roboto Mono" :weight 'thin))
(find-font (font-spec :family "Roboto Mono" :weight 'light))

while the equivalent using fc-match on a terminal returns the 
correct filename:

fc-match "Roboto Mono:style=Regular"
fc-match "Roboto Mono:style=Thin"
fc-match "Roboto Mono:style=Light"

From this, I can deduce Emacs doesn't use fontconfig on mac but 
then I don't know which method is used instead.


Nicolas