+/// equivalence relation rather than a partial equivalence relation. Note that
+/// the `derive` strategy requires all fields are `PartialEq`, which isn't
+/// always desired.
+///
+/// ## How can I implement `Eq`?
+///
+/// If you cannot use the `derive` strategy, specify that your type implements
+/// `Eq`, which has no methods:
+///
+/// ```
+/// 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
+/// }
+/// }
+/// impl Eq for Book {}
+/// ```