feat: Add box1.rs exercise

exercises/standard_library_types/README.md

@@ -1,3 +1,5 @@
+For the Box exercise check out the chapter [Using Box to Point to Data on the Heap](https://doc.rust-lang.org/book/ch15-01-box.html).
+
 For the Arc exercise check out the chapter [Shared-State Concurrency](https://doc.rust-lang.org/book/ch16-03-shared-state.html) of the Rust Book.
 
 For the Iterator exercise check out the chapters [Iterator](https://doc.rust-lang.org/book/ch13-02-iterators.html) of the Rust Book and the [Iterator documentation](https://doc.rust-lang.org/stable/std/iter/).

exercises/standard_library_types/box1.rs

@@ -0,0 +1,35 @@
+// box1.rs
+//
+// At compile time, Rust needs to know how much space a type takes up. This becomes problematic
+// for recursive types, where a value can have as part of itself another value of the same type.
+// To get around the issue, we can use a `Box` - a smart pointer used to store data on the heap,
+// which also allows us to wrap a recursive type.
+//
+// The recursive type we're implementing in this exercise is the `cons list` - a data structure
+// frequently found in functional programming languages. Each item in a cons list contains two
+// elements: the value of the current item and the next item. The last item is a value called `Nil`.
+//
+// Step 1: use a `Box` in the enum definition to make the code compile
+// Step 2: create both empty and non-empty cons lists of by replacing `unimplemented!()`
+//
+// Execute `rustlings hint box1` for hints :)
+
+// I AM NOT DONE
+
+#[derive(PartialEq, Debug)]
+enum List {
+    Cons(i32, List),
+    Nil,
+}
+
+fn main() {
+    let empty_list = unimplemented!();
+    println!("This is an empty cons list: {:?}", empty_list);
+
+    let non_empty_list = unimplemented!();
+    println!("This is a non-empty cons list: {:?}", non_empty_list);
+
+    // Do not change these
+    assert_eq!(List::Nil, empty_list);
+    assert_ne!(empty_list, non_empty_list);
+}