bug: conditions should be parsed using `Expr::parse_without_eager_brace`

[mkovaxx]

The current implementation uses Expr::parse, which may behave incorrectly in certain edge cases.

For more details, see:
https://github.com/dtolnay/syn/blob/2c7392d2f5cf9b399043b11619728a269ac40ace/src/expr.rs#L772-L789

[mkovaxx]

First step here should be to find inputs for which the current implementation is incorrect.