Function notmuch_message_properties_key 

pub unsafe extern "C" fn notmuch_message_properties_key(
    properties: *mut notmuch_message_properties_t,
) -> *const c_char

Return the key from the current (key,value) pair.

this could be useful if iterating for a prefix

@since libnotmuch 4.4 (notmuch 0.23)