Function visit_fn_use

fn visit_fn_use<'tcx>(
    tcx: TyCtxt<'tcx>,
    ty: Ty<'tcx>,
    is_direct_call: bool,
    source: Span,
    output: &mut Vec<Spanned<MonoItem<'tcx>>>,
)

For every call of this function in the visitor, make sure there is a matching call in the mentioned_items pass!