Drop Check

We have seen how lifetimes provide us some fairly simple rules for ensuring that we never read dangling references. However up to this point we have only ever interacted with the outlives relationship in an inclusive manner. That is, when we talked about 'a: 'b, it was ok for 'a to live exactly as long as 'b. At first glance, this seems to be a meaningless distinction. Nothing ever gets dropped at the same time as another, right? This is why we used the following desugaring of let statements:

let x;
let y;
{
    let x;
    {
        let y;
    }
}

There are some more complex situations which are not possible to desugar using scopes, but the order is still defined ‒ variables are dropped in the reverse order of their definition, fields of structs and tuples in order of their definition. There are some more details about order of drop in RFC 1857.

Let's do this:

let tuple = (vec![], vec![]);

The left vector is dropped first. But does it mean the right one strictly outlives it in the eyes of the borrow checker? The answer to this question is no. The borrow checker could track fields of tuples separately, but it would still be unable to decide what outlives what in case of vector elements, which are dropped manually via pure-library code the borrow checker doesn't understand.

So why do we care? We care because if the type system isn't careful, it could accidentally make dangling pointers. Consider the following simple program:

struct Inspector<'a>(&'a u8);

struct World<'a> {
    inspector: Option<Inspector<'a>>,
    days: Box<u8>,
}

fn main() {
    let mut world = World {
        inspector: None,
        days: Box::new(1),
    };
    world.inspector = Some(Inspector(&world.days));
}

This program is totally sound and compiles today. The fact that days does not strictly outlive inspector doesn't matter. As long as the inspector is alive, so is days.

However if we add a destructor, the program will no longer compile!

struct Inspector<'a>(&'a u8);

impl<'a> Drop for Inspector<'a> {
    fn drop(&mut self) {
        println!("I was only {} days from retirement!", self.0);
    }
}

struct World<'a> {
    inspector: Option<Inspector<'a>>,
    days: Box<u8>,
}

fn main() {
    let mut world = World {
        inspector: None,
        days: Box::new(1),
    };
    world.inspector = Some(Inspector(&world.days));
    // Let's say `days` happens to get dropped first.
    // Then when Inspector is dropped, it will try to read free'd memory!
}
error[E0597]: `world.days` does not live long enough
  --> src/main.rs:20:39
   |
20 |     world.inspector = Some(Inspector(&world.days));
   |                                       ^^^^^^^^^^ borrowed value does not live long enough
...
23 | }
   | - `world.days` dropped here while still borrowed
   |
   = note: values in a scope are dropped in the opposite order they are created

You can try changing the order of fields or use a tuple instead of the struct, it'll still not compile.

Implementing Drop lets the Inspector execute some arbitrary code during its death. This means it can potentially observe that types that are supposed to live as long as it does actually were destroyed first.

Interestingly, only generic types need to worry about this. If they aren't generic, then the only lifetimes they can harbor are 'static, which will truly live forever. This is why this problem is referred to as sound generic drop. Sound generic drop is enforced by the drop checker. As of this writing, some of the finer details of how the drop checker validates types is totally up in the air. However The Big Rule is the subtlety that we have focused on this whole section:

For a generic type to soundly implement drop, its generics arguments must strictly outlive it.

Obeying this rule is (usually) necessary to satisfy the borrow checker; obeying it is sufficient but not necessary to be sound. That is, if your type obeys this rule then it's definitely sound to drop.

The reason that it is not always necessary to satisfy the above rule is that some Drop implementations will not access borrowed data even though their type gives them the capability for such access, or because we know the specific drop order and the borrowed data is still fine even if the borrow checker doesn't know that.

For example, this variant of the above Inspector example will never access borrowed data:

struct Inspector<'a>(&'a u8, &'static str);

impl<'a> Drop for Inspector<'a> {
    fn drop(&mut self) {
        println!("Inspector(_, {}) knows when *not* to inspect.", self.1);
    }
}

struct World<'a> {
    inspector: Option<Inspector<'a>>,
    days: Box<u8>,
}

fn main() {
    let mut world = World {
        inspector: None,
        days: Box::new(1),
    };
    world.inspector = Some(Inspector(&world.days, "gadget"));
    // Let's say `days` happens to get dropped first.
    // Even when Inspector is dropped, its destructor will not access the
    // borrowed `days`.
}

Likewise, this variant will also never access borrowed data:

struct Inspector<T>(T, &'static str);

impl<T> Drop for Inspector<T> {
    fn drop(&mut self) {
        println!("Inspector(_, {}) knows when *not* to inspect.", self.1);
    }
}

struct World<T> {
    inspector: Option<Inspector<T>>,
    days: Box<u8>,
}

fn main() {
    let mut world = World {
        inspector: None,
        days: Box::new(1),
    };
    world.inspector = Some(Inspector(&world.days, "gadget"));
    // Let's say `days` happens to get dropped first.
    // Even when Inspector is dropped, its destructor will not access the
    // borrowed `days`.
}

However, both of the above variants are rejected by the borrow checker during the analysis of fn main, saying that days does not live long enough.

The reason is that the borrow checking analysis of main does not know about the internals of each Inspector's Drop implementation. As far as the borrow checker knows while it is analyzing main, the body of an inspector's destructor might access that borrowed data.

Therefore, the drop checker forces all borrowed data in a value to strictly outlive that value.

An Escape Hatch

The precise rules that govern drop checking may be less restrictive in the future.

The current analysis is deliberately conservative and trivial; it forces all borrowed data in a value to outlive that value, which is certainly sound.

Future versions of the language may make the analysis more precise, to reduce the number of cases where sound code is rejected as unsafe. This would help address cases such as the two Inspectors above that know not to inspect during destruction.

In the meantime, there is an unstable attribute that one can use to assert (unsafely) that a generic type's destructor is guaranteed to not access any expired data, even if its type gives it the capability to do so.

That attribute is called may_dangle and was introduced in RFC 1327. To deploy it on the Inspector from above, we would write:

#![feature(dropck_eyepatch)]

struct Inspector<'a>(&'a u8, &'static str);

unsafe impl<#[may_dangle] 'a> Drop for Inspector<'a> {
    fn drop(&mut self) {
        println!("Inspector(_, {}) knows when *not* to inspect.", self.1);
    }
}

struct World<'a> {
    days: Box<u8>,
    inspector: Option<Inspector<'a>>,
}

fn main() {
    let mut world = World {
        inspector: None,
        days: Box::new(1),
    };
    world.inspector = Some(Inspector(&world.days, "gatget"));
}

Use of this attribute requires the Drop impl to be marked unsafe because the compiler is not checking the implicit assertion that no potentially expired data (e.g. self.0 above) is accessed.

The attribute can be applied to any number of lifetime and type parameters. In the following example, we assert that we access no data behind a reference of lifetime 'b and that the only uses of T will be moves or drops, but omit the attribute from 'a and U, because we do access data with that lifetime and that type:

use std::fmt::Display;

struct Inspector<'a, 'b, T, U: Display>(&'a u8, &'b u8, T, U);

unsafe impl<'a, #[may_dangle] 'b, #[may_dangle] T, U: Display> Drop for Inspector<'a, 'b, T, U> {
    fn drop(&mut self) {
        println!("Inspector({}, _, _, {})", self.0, self.3);
    }
}

It is sometimes obvious that no such access can occur, like the case above. However, when dealing with a generic type parameter, such access can occur indirectly. Examples of such indirect access are:

  • invoking a callback,
  • via a trait method call.

(Future changes to the language, such as impl specialization, may add other avenues for such indirect access.)

Here is an example of invoking a callback:

struct Inspector<T>(T, &'static str, Box<for <'r> fn(&'r T) -> String>);

impl<T> Drop for Inspector<T> {
    fn drop(&mut self) {
        // The `self.2` call could access a borrow e.g. if `T` is `&'a _`.
        println!("Inspector({}, {}) unwittingly inspects expired data.",
                 (self.2)(&self.0), self.1);
    }
}

Here is an example of a trait method call:

use std::fmt;

struct Inspector<T: fmt::Display>(T, &'static str);

impl<T: fmt::Display> Drop for Inspector<T> {
    fn drop(&mut self) {
        // There is a hidden call to `<T as Display>::fmt` below, which
        // could access a borrow e.g. if `T` is `&'a _`
        println!("Inspector({}, {}) unwittingly inspects expired data.",
                 self.0, self.1);
    }
}

And of course, all of these accesses could be further hidden within some other method invoked by the destructor, rather than being written directly within it.

In all of the above cases where the &'a u8 is accessed in the destructor, adding the #[may_dangle] attribute makes the type vulnerable to misuse that the borrower checker will not catch, inviting havoc. It is better to avoid adding the attribute.

A related side note about drop order

While the drop order of fields inside a struct is defined, relying on it is fragile and subtle. When the order matters, it is better to use the ManuallyDrop wrapper.

Is that all about drop checker?

It turns out that when writing unsafe code, we generally don't need to worry at all about doing the right thing for the drop checker. However there is one special case that you need to worry about, which we will look at in the next section.