feat: add structs4.rs exercise

exercises/structs/structs4.rs

@@ -0,0 +1,62 @@
+// structs4.rs
+// Structs can have methods and the first parameter is always self. In this exercise
+// we have defined the Planet struct and we want to test some logic attached to it,
+// make the code compile and the tests pass! If you have issues execute `rustlings hint structs4`
+
+// I AM NOT DONE
+
+#[derive(Debug)]
+struct Planet {
+    has_life: bool,
+    radius : u32
+}
+
+impl Planet {
+    fn new(radius: u32) -> Planet {
+        // Something goes here...
+    }
+
+    fn has_life(???) -> bool {
+        // Something goes here...
+    }
+
+    fn change_radius(???) {
+        // Something goes here...
+    }
+
+    fn create_life(???) {
+        // Something goes here... 
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn create_planet() {
+
+        let planet = Planet::new(1000);
+
+        assert_eq!(planet.radius, 1000);
+        assert_eq!(planet.has_life(), false);
+    }
+
+    #[test]
+    fn add_life_to_planet() {
+        let planet = Planet::new(1000);
+
+        planet.create_life();
+        
+        assert_eq!(planet.has_life(), true);
+    }
+
+    #[test]
+    fn change_radius_of_planet() {
+        let mut planet = Planet::new(1000);
+
+        planet.change_radius(2000);
+        
+        assert_eq!(planet.radius, 2000);
+    }
+}

info.toml

@@ -394,6 +394,16 @@ For calculate_transport_fees: Bigger is more expensive usually, we don't have si
 
 Have a look in The Book, to find out more about method implementations: https://doc.rust-lang.org/book/ch05-03-method-syntax.html"""
 
+[[exercises]]
+name = "structs4"
+path = "exercises/structs/structs4.rs"
+mode = "test"
+hint = """
+The signature of methods must be different depending on the fact that attributes of an instance are
+read or written. How do we do that in Rust?
+
+Have a look in The Book, to find out more about method implementations: https://doc.rust-lang.org/book/ch05-03-method-syntax.html"""
+
 # ENUMS
 
 [[exercises]]