Function is_ty_must_use 

pub fn is_ty_must_use<'tcx>(
    cx: &LateContext<'tcx>,
    ty: Ty<'tcx>,
    expr: &Expr<'_>,
) -> IsTyMustUse

Returns Some(path) if ty should be considered as “must_use” in the context of expr (expr is used to get the parent module, which can affect which types are considered uninhabited).