Fix #1053: Added a page about the dyn keyword

src/SUMMARY.md

@@ -133,6 +133,7 @@
 
 - [Traits](trait.md)
     - [Derive](trait/derive.md)
+    - [Returning Traits with `dyn`](trait/dyn.md)
     - [Operator Overloading](trait/ops.md)
     - [Drop](trait/drop.md)
     - [Iterators](trait/iter.md)

src/trait/dyn.md

@@ -0,0 +1,47 @@
+# Returning Traits with `dyn`
+
+The Rust compiler needs to know how much space every function's return type requires. This means all your functions have to return a concrete type. Unlike other languages, if you have a trait like `Animal`, you can't write a function that returns `Animal`, because its different implementations will need different amounts of memory. 
+
+However, there's an easy workaround. Instead of returning a trait object directly, our functions return a `Box` which _contains_ some `Animal`. A `box` is just a reference to some memory in the heap. Because a reference has a statically-known size, and the compiler can guarantee it points to a heap-allocated `Animal`, we can return a trait from our function!
+
+Rust tries to be as explicit as possible whenever it allocates memory on the heap. So if your function returns a pointer-to-trait-on-heap in this way, you need to write the return type with the `dyn` keyword, e.g. `Box<dyn Animal>`.
+
+```rust,editable
+struct Sheep {}
+struct Cow {}
+
+trait Animal {
+    // Instance method signature
+    fn noise(&self) -> &'static str;
+}
+
+// Implement the `Animal` trait for `Sheep`.
+impl Animal for Sheep {
+    fn noise(&self) -> &'static str {
+        "baaaaah!"
+    }
+}
+
+// Implement the `Animal` trait for `Cow`.
+impl Animal for Cow {
+    fn noise(&self) -> &'static str {
+        "moooooo!"
+    }
+}
+
+// Returns some struct that implements Animal, but we don't know which one at compile time.
+fn random_animal(random_number: f64) -> Box<dyn Animal> {
+    if random_number < 0.5 {
+        Box::new(Sheep {})
+    } else {
+        Box::new(Cow {})
+    }
+}
+
+fn main() {
+    let random_number = 0.234;
+    let animal = random_animal(random_number);
+    println!("You've randomly chosen an animal, and it says {}", animal.noise());
+}
+
+```