100-exercises-to-learn-rust/book/src/04_traits/12_drop.md

The Drop trait

When we introduced destructors, we mentioned that the drop function:

1. reclaims the memory occupied by the type (i.e. std::mem::size_of bytes)
2. cleans up any additional resources that the value might be managing (e.g. the heap buffer of a String)

Step 2. is where the Drop trait comes in.

pub trait Drop {
    fn drop(&mut self);
}

The Drop trait is a mechanism for you to define additional cleanup logic for your types, beyond what the compiler does for you automatically.
Whatever you put in the drop method will be executed when the value goes out of scope.

Drop and Copy

When talking about the Copy trait, we said that a type can't implement Copy if it manages additional resources beyond the std::mem::size_of bytes that it occupies in memory.

You might wonder: how does the compiler know if a type manages additional resources? That's right: Drop trait implementations!
If your type has an explicit Drop implementation, the compiler will assume that your type has additional resources attached to it and won't allow you to implement Copy.

// This is a unit struct, i.e. a struct with no fields.
#[derive(Clone, Copy)]
struct MyType;

impl Drop for MyType {
    fn drop(&mut self) {
       // We don't need to do anything here,
       // it's enough to have an "empty" Drop implementation
    }
}

The compiler will complain with this error message:

error[E0184]: the trait `Copy` cannot be implemented for this type; the type has a destructor
 --> src/lib.rs:2:17
  |
2 | #[derive(Clone, Copy)]
  |                 ^^^^ `Copy` not allowed on types with destructors

References

• The exercise for this section is located in exercises/04_traits/12_drop