enum BookFormat { Paperback, Hardback, Ebook, } struct Book { isbn: i32, format: BookFormat, } impl PartialEq for Book { fn eq(&self, other: &Self) -> bool { self.isbn == other.isbn } } let b1 = Book { isbn: 3, format: BookFormat::Paperback }; let b2 = Book { isbn: 3, format: BookFormat::Ebook }; let b3 = Book { isbn: 10, format: BookFormat::Paperback }; assert!(b1 == b2); assert!(b1 != b3);