Generic unit variants are handled incorrectly inside format args#332

The following is a valid format expression, but thiserror fails to compile it. ```rust use thiserror::Error; #[derive(Debug, Error)] #[error("{}", None::<i32>.is_some())] pub struct Error; fn main() { println!("{}", None::<i32>.is_some()); } ``` ```console error: expected one of `!`, `,`, `.`, `::`, `?`, `{`, or an operator, found `is_some` --> src/main.rs:4:27 | 4 | #[error("{}", None::<i32>.is_some())] | ^^^^^^^ expected one of 7 possible tokens ```