nomicon/destructors.md

% Destructors

What the language does provide is full-blown automatic destructors through the Drop trait, which provides the following method:

fn drop(&mut self);

This method gives the type time to somehow finish what it was doing.

After drop is run, Rust will recursively try to drop all of the fields of self.

This is a convenience feature so that you don't have to write "destructor boilerplate" to drop children. If a struct has no special logic for being dropped other than dropping its children, then it means Drop doesn't need to be implemented at all!

There is no stable way to prevent this behavior in Rust 1.0.

Note that taking &mut self means that even if you could suppress recursive Drop, Rust will prevent you from e.g. moving fields out of self. For most types, this is totally fine.

For instance, a custom implementation of Box might write Drop like this:

#![feature(alloc, heap_api, drop_in_place, unique)]

extern crate alloc;

use std::ptr::{drop_in_place, Unique};
use std::mem;

use alloc::heap;

struct Box<T>{ ptr: Unique<T> }

impl<T> Drop for Box<T> {
    fn drop(&mut self) {
        unsafe {
            drop_in_place(*self.ptr);
            heap::deallocate((*self.ptr) as *mut u8,
                             mem::size_of::<T>(),
                             mem::align_of::<T>());
        }
    }
}
# fn main() {}

and this works fine because when Rust goes to drop the ptr field it just sees a Unique that has no actual Drop implementation. Similarly nothing can use-after-free the ptr because when drop exits, it becomes inaccessible.

However this wouldn't work:

#![feature(alloc, heap_api, drop_in_place, unique)]

extern crate alloc;

use std::ptr::{drop_in_place, Unique};
use std::mem;

use alloc::heap;

struct Box<T>{ ptr: Unique<T> }

impl<T> Drop for Box<T> {
    fn drop(&mut self) {
        unsafe {
            drop_in_place(*self.ptr);
            heap::deallocate((*self.ptr) as *mut u8,
                             mem::size_of::<T>(),
                             mem::align_of::<T>());
        }
    }
}

struct SuperBox<T> { my_box: Box<T> }

impl<T> Drop for SuperBox<T> {
    fn drop(&mut self) {
        unsafe {
            // Hyper-optimized: deallocate the box's contents for it
            // without `drop`ing the contents
            heap::deallocate((*self.my_box.ptr) as *mut u8,
                             mem::size_of::<T>(),
                             mem::align_of::<T>());
        }
    }
}
# fn main() {}

After we deallocate the box's ptr in SuperBox's destructor, Rust will happily proceed to tell the box to Drop itself and everything will blow up with use-after-frees and double-frees.

Note that the recursive drop behavior applies to all structs and enums regardless of whether they implement Drop. Therefore something like

struct Boxy<T> {
    data1: Box<T>,
    data2: Box<T>,
    info: u32,
}

will have its data1 and data2's fields destructors whenever it "would" be dropped, even though it itself doesn't implement Drop. We say that such a type needs Drop, even though it is not itself Drop.

Similarly,

enum Link {
    Next(Box<Link>),
    None,
}

will have its inner Box field dropped if and only if an instance stores the Next variant.

In general this works really nicely because you don't need to worry about adding/removing drops when you refactor your data layout. Still there's certainly many valid usecases for needing to do trickier things with destructors.

The classic safe solution to overriding recursive drop and allowing moving out of Self during drop is to use an Option:

#![feature(alloc, heap_api, drop_in_place, unique)]

extern crate alloc;

use std::ptr::{drop_in_place, Unique};
use std::mem;

use alloc::heap;

struct Box<T>{ ptr: Unique<T> }

impl<T> Drop for Box<T> {
    fn drop(&mut self) {
        unsafe {
            drop_in_place(*self.ptr);
            heap::deallocate((*self.ptr) as *mut u8,
                             mem::size_of::<T>(),
                             mem::align_of::<T>());
        }
    }
}

struct SuperBox<T> { my_box: Option<Box<T>> }

impl<T> Drop for SuperBox<T> {
    fn drop(&mut self) {
        unsafe {
            // Hyper-optimized: deallocate the box's contents for it
            // without `drop`ing the contents. Need to set the `box`
            // field as `None` to prevent Rust from trying to Drop it.
            let my_box = self.my_box.take().unwrap();
            heap::deallocate((*my_box.ptr) as *mut u8,
                             mem::size_of::<T>(),
                             mem::align_of::<T>());
            mem::forget(my_box);
        }
    }
}
# fn main() {}

However this has fairly odd semantics: you're saying that a field that should always be Some may be None, just because that happens in the destructor. Of course this conversely makes a lot of sense: you can call arbitrary methods on self during the destructor, and this should prevent you from ever doing so after deinitializing the field. Not that it will prevent you from producing any other arbitrarily invalid state in there.

On balance this is an ok choice. Certainly what you should reach for by default. However, in the future we expect there to be a first-class way to announce that a field shouldn't be automatically dropped.