Does not compile on 1.12.0-nightly (1bf5fa326 2016-08-16)

[dtolnay]

error: no associated item named `None` found for type `syntax::ast::FunctionRetTy` in the current scope
  --> src/fn_decl.rs:98:22
   |
98 |         let ret_ty = ast::FunctionRetTy::None(self.span);
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^

[dtolnay]

FunctionRetTy::None was removed in rust-lang/rust#35162.