Function substitute 

pub fn substitute<'tcx, T>(
    tcx: TyCtxt<'tcx>,
    typing_env: TypingEnv<'tcx>,
    args: Option<GenericArgsRef<'tcx>>,
    x: T,
) -> T
where
    T: TypeFoldable<TyCtxt<'tcx>>,

Instantiate this value with the provided args. Use None if the args are known to be the identity args.