rectangles: implement area method

rectangles/src/main.rs

@@ -4,6 +4,12 @@ struct Rectangle {
     height: u32,
 }
 
+impl Rectangle {
+    fn area(&self) -> u32 {
+        self.width * self.height
+    }
+}
+
 fn main() {
     let width1 = 30;
     let height1 = 50;
@@ -29,6 +35,11 @@ fn main() {
         "The area of the rectangle is {} square pixels.",
         area_struct(&rect)
     );
+
+    println!(
+        "The area of the rectangle is {} square pixels.",
+        rect.area()
+    );
 }
 
 fn area(width: u32, height: u32) -> u32 {