Function protobuf::reflect::accessor::make_singular_message_accessor

pub fn make_singular_message_accessor<M: Message + 'static, F: Message + 'static>(
    name: &'static str, 
    has: fn(_: &M) -> bool, 
    get: fn'a(_: &'a M) -> &'a F
) -> Box<dyn FieldAccessor + 'static>