// 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 by replacing `todo!()`
//
// Note: the tests should not be changed
//
// Execute `rustlings hint box1` or use the `hint` watch subcommand for a hint.

// I AM NOT DONE

#[derive(PartialEq, Debug)]
pub enum List {
    // 用 Box 包装 List，将递归类型转为固定大小的指针
    Cons(i32, Box<List>),
    Nil,
}

fn main() {
    println!("This is an empty cons list: {:?}", create_empty_list());
    println!(
        "This is a non-empty cons list: {:?}",
        create_non_empty_list()
    );
}

pub fn create_empty_list() -> List {
    // 空列表直接返回 Nil 变体
    List::Nil
}

pub fn create_non_empty_list() -> List {
    // 非空列表：嵌套 Cons 结构，最后用 Box::new(Nil) 终止递归
    List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))))
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_create_empty_list() {
        assert_eq!(List::Nil, create_empty_list())
    }

    #[test]
    fn test_create_non_empty_list() {
        assert_ne!(create_empty_list(), create_non_empty_list())
    }
}

Author: unix-dot

exercises/smart_pointers/box1.rs

@@ -22,7 +22,8 @@
 
 #[derive(PartialEq, Debug)]
 pub enum List {
-    Cons(i32, List),
+    // 用 Box 包装 List，将递归类型转为固定大小的指针
+    Cons(i32, Box<List>),
     Nil,
 }
 
@@ -35,11 +36,13 @@ fn main() {
 }
 
 pub fn create_empty_list() -> List {
-    todo!()
+    // 空列表直接返回 Nil 变体
+    List::Nil
 }
 
 pub fn create_non_empty_list() -> List {
-    todo!()
+    // 非空列表：嵌套 Cons 结构，最后用 Box::new(Nil) 终止递归
+    List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))))
 }
 
 #[cfg(test)]