About impl Clone

Author: Philippe-Cholet

CONTRIBUTING.md

@@ -131,6 +131,14 @@ impl<I: fmt::Debug, F> fmt::Debug for Name4<I, F> {
 ```
 </details>
 
+### When/How to implement [`Clone`](https://doc.rust-lang.org/std/clone/trait.Clone.html)
+All our iterators should implement `Clone` when possible.
+
+Note that a mutable reference is never clonable so `struct Name<'a, I: 'a> { iter: &'a mut I }` can not implement `Clone`.
+
+Derive `Clone` on a generic struct adds the bound `Clone` on each generic parameter.
+It might be an issue in which case you should manually implement it with our helper macro `clone_fields` (it defines the `clone` function calling `clone` on each field) and be careful about the bounds.
+
 ### When to implement [`std::iter::FusedIterator`](https://doc.rust-lang.org/std/iter/trait.FusedIterator.html)
 This trait should be implemented _by all iterators that always return `None` after returning `None` once_, because it allows to optimize `Iterator::fuse()`.