Fix build for:

```
gen.rs:509:16: 509:26 error: quote_expr is not stable enough for use and
are subject to change
gen.rs:509     let expr = quote_expr!(
                          ^~~~~~~~~~
gen.rs:509:16: 509:26 note: add #[feature(quote)] to the crate
attributes to enable
gen.rs:509     let expr = quote_expr!(
                          ^~~~~~~~~~
```