Function identity_helper_arg 

fn identity_helper_arg<'gcx>(
    gcx: Gcx<'gcx>,
    callee: &'gcx Expr<'gcx>,
    args: &'gcx CallArgs<'gcx>,
) -> Option<&'gcx Expr<'gcx>>

Argument returned verbatim (modulo casts) by an identity helper call id(x) / Lib.id(x).