Quoting Ebassi https://www.bassi.io/articles/2023/02/20/bindable-api-2023/:
Whenever you’re describing a function that takes a callback, you
should always annotate the callback argument with the argument that
contains the user data using the (closure argument) annotation
You should not annotate the data argument with a unary (closure).
The unary (closure) is meant to be used when annotating the callback
type
Recently gobject-introspection became a bit more strict with this and
that generated some warnings:
Warning: St: invalid "closure" annotation: only valid on callback
parameters
This commit fix all the closure annotations.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3535>
411fe9c42f