Function tupled_args_ty 

pub fn tupled_args_ty<'tcx>(
    s: &impl UnderOwnerState<'tcx>,
    sig: PolyFnSig<'tcx>,
) -> Binder<'tcx, Ty<'tcx>>

The arguments of the given signature, tupled as the Fn* traits take them, e.g. (A, B, C). The result binds the same variables as the signature it comes from, so that the two agree on the regions they use.