add explanation of dbg! macro

Author: mikemorris

src/ch05-02-example-structs.md

@@ -252,6 +252,28 @@ rect1 is Rectangle {
 }
 ```
 
+Now that we're familiar with deriving the `Debug` trait for helpful `println!`
+formatting, let's take a look at a macro that builds on top of that functionality.
+The `dbg!` macro (available in Rust 1.32.0 and later) can accept any argument
+that implements the `Debug` trait, and will print helpful debugging context in
+the same style as `println!("{:?#}", rect1)`, plus the filename and line number.
+
+<span class="filename">Filename: src/main.rs</span>
+
+```rust
+#[derive(Debug)]
+struct Rectangle {
+    width: u32,
+    height: u32,
+}
+
+fn main() {
+    let rect1 = Rectangle { width: 30, height: 50 };
+
+    dbg!(rect1);
+}
+```
+
 Rust has provided a number of traits for us to use with the `derive` annotation
 that can add useful behavior to our custom types. Those traits and their
 behaviors are listed in Appendix C. We’ll cover how to implement these traits