Impl `rect_area` for 3.1

rustbyexample/031/src/main.rs

@@ -2,17 +2,26 @@ struct Nil;
 
 struct Pair(i32, f32);
 
+#[derive(Debug)]
 struct Point {
     x: f32,
     y: f32,
 }
 
-#[allow(dead_code)]
+#[derive(Debug)]
 struct Rectangle {
     p1: Point,
     p2: Point,
 }
 
+fn rect_area(rect: Rectangle) -> f32 {
+    let Rectangle {
+        p1: Point { x: x1, y: y1 },
+        p2: Point { x: x2, y: y2 }
+    } = rect;
+    ((x1 - x2) * (y1 - y2)).abs()
+}
+
 fn main() {
     let point: Point = Point { x: 0.3, y: 0.4 };
 
@@ -34,4 +43,7 @@ fn main() {
     let Pair(integer, decimal) = pair;
 
     println!("pair contains {:?} and {:?}", integer, decimal);
+
+    println!("rect is {:?}", _rectangle);
+    println!("rect area is {}", rect_area(_rectangle));
 }