1. In the folder bin, create a file called drop.rs.

2. Add the following code and run it with cargo run --bin drop:

1   use std::fmt::Debug;
2 
3   struct CustomSmartPointer<D>
4   where
5   D: Debug,
6   {
7   data: D,
8   }
9 
10  impl<D> CustomSmartPointer<D>
11  where
12  D: Debug,
13  {
14    fn new(data: D) -> Self {
15    CustomSmartPointer { data }
16    }
17  }
18 
19  impl<D> Drop for CustomSmartPointer<D>
20  where
21  D: Debug,
22  {
23    // This will automatically be called when a variable is 
       dropped
24    // It cannot be called manually
25    fn drop(&mut self) {
26    println!("Dropping CustomSmartPointer with data `{:?}`", 
       self.data);
27    }
28  }
29 
30  fn main() {
31    let a = CustomSmartPointer::new("A");
32    let b = CustomSmartPointer::new("B");
33    let c = CustomSmartPointer::new("C");
34    let d = CustomSmartPointer::new("D");
35 
36    // The next line would cause a compiler error,
37    // as destructors cannot be explicitely called
38    // c.drop();
39 
40    // The correct way to drop variables early is the following:
41    std::mem::drop(c);
42  }