When encountering `-> Trait`, suggest `-> Box<dyn Trait>` (instead of `-> Box<Trait>`. If there's a single returned type within the `fn`, suggest `-> impl Trait`.
When encountering `-> Trait`, suggest `-> Box<dyn Trait>` (instead of `-> Box<Trait>`. If there's a single returned type within the `fn`, suggest `-> impl Trait`.