Add a slide introducing dyn Trait in Generics (#2108)

I've been thinking it'd be simpler to introduce `dyn Trait` via `&dyn
Trait` rather than waiting for the smart pointers section and `Box<dyn
Trait>`. This PR adds a slide to the Generics section that introduces
`&dyn Trait` and compares it to `&impl Trait`, juxtaposing
monomorphization and static dispatch against type-erasure and dynamic
dispatch. I've then updated the existing trait object slide to call back
to the earlier introduction, and call out that using `Box<dyn Trait>`
gives you an owned trait object rather than a borrowed one.

src/SUMMARY.md

@@ -100,6 +100,7 @@
   - [Generic Traits](generics/generic-traits.md)
   - [Trait Bounds](generics/trait-bounds.md)
   - [`impl Trait`](generics/impl-trait.md)
+  - [`dyn Trait`](generics/dyn-trait.md)
   - [Exercise: Generic `min`](generics/exercise.md)
     - [Solution](generics/solution.md)
 - [Standard Library Types](std-types.md)
@@ -141,7 +142,7 @@
 - [Smart Pointers](smart-pointers.md)
   - [`Box<T>`](smart-pointers/box.md)
   - [`Rc`](smart-pointers/rc.md)
-  - [Trait Objects](smart-pointers/trait-objects.md)
+  - [Owned Trait Objects](smart-pointers/trait-objects.md)
   - [Exercise: Binary Tree](smart-pointers/exercise.md)
     - [Solution](smart-pointers/solution.md)
 

src/generics/dyn-trait.md

@@ -0,0 +1,87 @@
+---
+minutes: 5
+---
+
+# `dyn Trait`
+
+In addition to using traits for static dispatch via generics, Rust also supports
+using them for type-erased, dynamic dispatch via trait objects:
+
+```rust,editable
+struct Dog {
+    name: String,
+    age: i8,
+}
+struct Cat {
+    lives: i8,
+}
+
+trait Pet {
+    fn talk(&self) -> String;
+}
+
+impl Pet for Dog {
+    fn talk(&self) -> String {
+        format!("Woof, my name is {}!", self.name)
+    }
+}
+
+impl Pet for Cat {
+    fn talk(&self) -> String {
+        String::from("Miau!")
+    }
+}
+
+// Uses generics and static dispatch.
+fn generic(pet: &impl Pet) {
+    println!("Hello, who are you? {}", pet.talk());
+}
+
+// Uses type-erasure and dynamic dispatch.
+fn dynamic(pet: &dyn Pet) {
+    println!("Hello, who are you? {}", pet.talk());
+}
+
+fn main() {
+    let cat = Cat { lives: 9 };
+    let dog = Dog { name: String::from("Fido"), age: 5 };
+
+    generic(&cat);
+    generic(&dog);
+
+    dynamic(&cat);
+    dynamic(&dog);
+}
+```
+
+<details>
+
+- Generics, including `impl Trait`, use monomorphization to create a specialized
+  instance of the function for each different type that the generic is
+  instantiated with. This means that calling a trait method from within a
+  generic function still uses static dispatch, as the compiler has full type
+  information and can resolve which type's trait implementation to use.
+
+- When using `dyn Trait`, it instead uses dynamic dispatch through a
+  [virtual method table][vtable] (vtable). This means that there's a single
+  version of `fn dynamic` that is used regardless of what type of `Pet` is
+  passed in.
+
+- When using `dyn Trait`, the trait object needs to be behind some kind of
+  indirection. In this case it's a reference, though smart pointer types like
+  `Box` can also be used (this will be demonstrated on day 3).
+
+- At runtime, a `&dyn Pet` is represented as a "fat pointer", i.e. a pair of two
+  pointers: One pointer points to the concrete object that implements `Pet`, and
+  the other points to the vtable for the trait implementation for that type.
+  When calling the `talk` method on `&dyn Pet` the compiler looks up the
+  function pointer for `talk` in the vtable and then invokes the function,
+  passing the pointer to the `Dog` or `Cat` into that function. The compiler
+  doesn't need to know the concrete type of the `Pet` in order to do this.
+
+- A `dyn Trait` is considered to be "type-erased", because we no longer have
+  compile-time knowledge of what the concrete type is.
+
+[vtable]: https://en.wikipedia.org/wiki/Virtual_method_table
+
+</details>

src/smart-pointers/trait-objects.md

@@ -2,9 +2,11 @@
 minutes: 10
 ---
 
-# Trait Objects
+# Owned Trait Objects
 
-Trait objects allow for values of different types, for instance in a collection:
+We previously saw how trait objects can be used with references, e.g `&dyn Pet`.
+However, we can also use trait objects with smart pointers like `Box` to create
+an owned trait object: `Box<dyn Pet>`.
 
 ```rust,editable
 struct Dog {