feat: Add box1.rs exercise

Author: AlexandruGG

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);
+}

info.toml

@@ -614,6 +614,24 @@ hint = """
 
 # STANDARD LIBRARY TYPES
 
+[[exercises]]
+name = "box1"
+path = "exercises/standard_library_types/box1.rs"
+mode = "compile"
+hint = """
+Step 1
+The compiler's message should help: since we cannot store the value of the actual type
+when working with recursive types, we need to store a reference (pointer) to its value.
+We should therefore place our `List` inside a `Box`. More details in the book here:
+https://doc.rust-lang.org/book/ch15-01-box.html#enabling-recursive-types-with-boxes
+
+Step 2
+Creating an empty list should be fairly straightforward (hint: peek at the assertions).
+For a non-empty list keep in mind that wee want to use our Cons "list builder".
+Although the current list is one of integers (i32), feel free to change the definition
+and try other types!
+"""
+
 [[exercises]]
 name = "arc1"
 path = "exercises/standard_library_types/arc1.rs"