Cover Supertraits, Generic Traits (#1854)

src/methods-and-traits/traits.md

@@ -1,5 +1,5 @@
 ---
-minutes: 10
+minutes: 15
 ---
 
 # Traits

src/methods-and-traits/traits/associated-types.md

@@ -1,6 +1,6 @@
 # Associated Types
 
-Associated types are placeholder types which are filled in by the trait
+Associated types are placeholder types which are supplied by the trait
 implementation.
 
 ```rust,editable

src/methods-and-traits/traits/supertraits.md

@@ -0,0 +1,41 @@
+# Supertraits
+
+A trait can require that types implementing it also implement other traits,
+called _supertraits_. Here, any type implementing `Pet` must implement `Animal`.
+
+```rust,editable
+trait Animal {
+    fn leg_count(&self) -> u32;
+}
+
+trait Pet: Animal {
+    fn name(&self) -> String;
+}
+
+struct Dog(String);
+
+impl Animal for Dog {
+    fn leg_count(&self) -> u32 {
+        4
+    }
+}
+
+impl Pet for Dog {
+    fn name(&self) -> String {
+        self.0.clone()
+    }
+}
+
+fn main() {
+    let puppy = Dog(String::from("Rex"));
+    println!("{} has {} legs", puppy.name(), puppy.leg_count());
+}
+```
+
+<details>
+
+This is sometimes called "trait inheritance" but students should not expect this
+to behave like OO inheritance. It just specifies an additional requirement on
+implementations of a trait.
+
+<details>