Laziness

Author: Philippe-Cholet

CONTRIBUTING.md

@@ -29,6 +29,12 @@ This mostly applies to your new module, each import from it, and to your new `It
 - Mutably borrow when it consumes only part of the iterator, such as `find`, `all`, `try_collect`...
 - Immutably borrow when there is no change, such as `size_hint`.
 
+### Laziness
+Iterators are [lazy](https://doc.rust-lang.org/std/iter/index.html#laziness):
+
+- structs of iterator adaptors should have `#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]` ;
+- structs of iterators should have `#[must_use = "iterators are lazy and do nothing unless consumed"]`.
+
 ## Specialize `Iterator` methods
 It might be more performant to specialize some methods.
 However, each specialization should be thoroughly tested.