Function handle_conversion 

pub(crate) fn handle_conversion(
    ident: &Ident,
    sexp_ident: &Ident,
    ty: &Type,
    arg: &ExternalArg,
    self_ty: Option<&Type>,
    err_arm: TokenStream,
) -> TokenStream

The statement converting the borrowed argument ident (of type arg) to its handle, given its R-facing failure arm (err_arm, an Err(e) => match arm). self_ty replaces Self.