The code of closure_sig is copy-pasted from fn_sig_for_fn_abi in compiler/rustc_middle/src/ty/layout.rs. We may have to do something similar for generators (#416), so it would be good if we could use the rustc function fn_abi_of_instance directly (as the comment on fn_sig_for_fn_abi suggests. This would prevent problems with keeping in sync with rustc.