PhantomData - The Rustonomicon
Description: The Dark Arts of Advanced and Unsafe Rust Programming

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# PhantomData

When working with unsafe code, we can often end up in a situation where types or lifetimes are logically associated with a struct, but not actually part of a field. This most commonly occurs with lifetimes. For instance, the `Iter` for `&'a [T]` is (approximately) defined as follows:

    #![allow(unused)]
    fn main() {
    struct Iter<'a, T: 'a> {
        ptr: *const T,
        end: *const T,
    }
    }

However because `'a` is unused within the struct’s body, it’s unbounded. Because of the troubles this has historically caused, unbounded lifetimes and types are forbidden in struct definitions. Therefore we must somehow refer to these types in the body. Correctly doing this is necessary to have correct variance and drop checking.

We do this using `PhantomData`, which is a special marker type. `PhantomData` consumes no space, but simulates a field of the given type for the purpose of static analysis. This was deemed to be less error-prone than explicitly telling the type-system the kind of variance that you want, while also providing other useful things such as auto traits and the information needed by drop check.

Iter logically contains a bunch of `&'a T`s, so this is exactly what we tell the `PhantomData` to simulate:

    #![allow(unused)]
    fn main() {
    use std::marker;

    struct Iter<'a, T: 'a> {
        ptr: *const T,
        end: *const T,
        _marker: marker::PhantomData<&'a T>,
    }
    }

and that’s it. The lifetime will be bounded, and your iterator will be covariant over `'a` and `T`. Everything Just Works.

## Generic parameters and drop-checking

In the past, there used to be another thing to take into consideration.

This very documentation used to say:

> Another important example is Vec, which is (approximately) defined as follows:

>     #![allow(unused)]
>     fn main() {
>     struct Vec<T> {
>         data: *const T, // *const for variance!
>         len: usize,
>         cap: usize,
>     }
>     }

> Unlike the previous example, it appears that everything is exactly as we want. Every generic argument to Vec shows up in at least one field. Good to go!

> Nope.

> The drop checker will generously determine that `Vec<T>` does not own any values of type T. This will in turn make it conclude that it doesn’t need to worry about Vec dropping any T’s in its destructor for determining drop check soundness. This will in turn allow people to create unsoundness using Vec’s destructor.

> In order to tell the drop checker that we do own values of type T, and therefore may drop some T’s when we drop, we must add an extra `PhantomData` saying exactly that:

>     #![allow(unused)]
>     fn main() {
>     use std::marker;

>     struct Vec<T> {
>         data: *const T, // *const for variance!
>         len: usize,
>         cap: usize,
>         _owns_T: marker::PhantomData<T>,
>     }
>     }

But ever since RFC 1238, this is no longer true nor necessary.

If you were to write:

    #![allow(unused)]
    fn main() {
    struct Vec<T> {
        data: *const T, // `*const` for variance!
        len: usize,
        cap: usize,
    }

    #[cfg(any())]
    impl<T> Drop for Vec<T> { /* … */ }
    }

then the existence of that `impl<T> Drop for Vec<T>` makes it so Rust will consider that that `Vec<T>` owns values of type `T` (more precisely: may use values of type `T` in its `Drop` implementation), and Rust will thus not allow them to dangle should a `Vec<T>` be dropped.

When a type already has a `Drop impl`, adding an extra `_owns_T: PhantomData<T>` field is thus superfluous and accomplishes nothing, dropck-wise (it still affects variance and auto-traits).

- (advanced edge case: if the type containing the `PhantomData` has no `Drop` impl at all, but still has drop glue (by having another field with drop glue), then the dropck/`#[may_dangle]` considerations mentioned herein do apply as well: a `PhantomData<T>` field will then require `T` to be droppable whenever the containing type goes out of scope).

But this situation can sometimes lead to overly restrictive code. That’s why the standard library uses an unstable and `unsafe` attribute to opt back into the old “unchecked” drop-checking behavior, that this very documentation warned about: the `#[may_dangle]` attribute.

### An exception: the special case of the standard library and its unstable `#[may_dangle]`

This section can be skipped if you are only writing your own library code; but if you are curious about what the standard library does with the actual `Vec` definition, you’ll notice that it still needs to use a `_owns_T: PhantomData<T>` field for soundness.

Click here to see why

Consider the following example:

    fn main() {
        let mut v: Vec<&str> = Vec::new();
        let s: String = "Short-lived".into();
        v.push(&s);
        drop(s);
    } // <- `v` is dropped here

with a classical `impl<T> Drop for Vec<T> {` definition, the above is denied.

Indeed, in this case we have a `Vec</* T = */ &'s str>` vector of `'s`-lived references to `str`ings, but in the case of `let s: String`, it is dropped before the `Vec` is, and thus `'s` is expired by the time the `Vec` is dropped, and the `impl<'s> Drop for Vec<&'s str> {` is used.

This means that if such `Drop` were to be used, it would be dealing with an expired, or dangling lifetime `'s`. But this is contrary to Rust principles, where by default all Rust references involved in a function signature are non-dangling and valid to dereference.

Hence why Rust has to conservatively deny this snippet.

And yet, in the case of the real `Vec`, the `Drop` impl does not care about `&'s str`, since it has no drop glue of its own: it only wants to deallocate the backing buffer.

In other words, it would be nice if the above snippet was somehow accepted, by special casing `Vec`, or by relying on some special property of `Vec`: `Vec` could try to promise not to use the `&'s str`s it holds when being dropped.

This is the kind of `unsafe` promise that can be expressed with `#[may_dangle]`:

    unsafe impl<#[may_dangle] 's> Drop for Vec<&'s str> { /* … */ }

or, more generally:

    unsafe impl<#[may_dangle] T> Drop for Vec<T> { /* … */ }

is the `unsafe` way to opt out of this conservative assumption that Rust’s drop checker makes about type parameters of a dropped instance not being allowed to dangle.

And when this is done, such as in the standard library, we need to be careful in the case where `T` has drop glue of its own. In this instance, imagine replacing the `&'s str`s with a `struct PrintOnDrop<'s> /* = */ (&'s str);` which would have a `Drop` impl wherein the inner `&'s str` would be dereferenced and printed to the screen.

Indeed, `Drop for Vec<T> {`, before deallocating the backing buffer, does have to transitively drop each `T` item when it has drop glue; in the case of `PrintOnDrop<'s>`, it means that `Drop for Vec<PrintOnDrop<'s>>` has to transitively drop the `PrintOnDrop<'s>`s elements before deallocating the backing buffer.

So when we said that `'s` `#[may_dangle]`, it was an excessively loose statement. We’d rather want to say: “`'s` may dangle provided it not be involved in some transitive drop glue”. Or, more generally, “`T` may dangle provided it not be involved in some transitive drop glue”. This “exception to the exception” is a pervasive situation whenever we own a `T`. That’s why Rust’s `#[may_dangle]` is smart enough to know of this opt-out, and will thus be disabled when the generic parameter is held in an owned fashion by the fields of the struct.

Hence why the standard library ends up with:

    #![allow(unused)]
    fn main() {
    #[cfg(any())]
    // we pinky-swear not to use `T` when dropping a `Vec`…
    unsafe impl<#[may_dangle] T> Drop for Vec<T> {
        fn drop(&mut self) {
            unsafe {
                if mem::needs_drop::<T>() {
                    /* … except here, that is, … */
                    ptr::drop_in_place::<[T]>(/* … */);
                }
                // …
                dealloc(/* … */)
                // …
            }
        }
    }

    struct Vec<T> {
        // … except for the fact that a `Vec` owns `T` items and
        // may thus be dropping `T` items on drop!
        _owns_T: core::marker::PhantomData<T>,

        ptr: *const T, // `*const` for variance (but this does not express ownership of a `T` *per se*)
        len: usize,
        cap: usize,
    }
    }

Raw pointers that own an allocation is such a pervasive pattern that the standard library made a utility for itself called `Unique<T>` which:

- wraps a `*const T` for variance
- includes a `PhantomData<T>`
- auto-derives `Send`/`Sync` as if T was contained
- marks the pointer as `NonZero` for the null-pointer optimization

## Table of `PhantomData` patterns

Here’s a table of all the wonderful ways `PhantomData` could be used:

Phantom type | variance of `'a` | variance of `T` | `Send`/`Sync` (or lack thereof) | dangling `'a` or `T` in drop glue (e.g., `#[may_dangle] Drop`)
--- | --- | --- | --- | ---
`PhantomData<T>` | - | covariant | inherited | disallowed (“owns `T`”)
`PhantomData<&'a T>` | covariant | covariant | `Send + Sync` requires `T : Sync` | allowed
`PhantomData<&'a mut T>` | covariant | invariant | inherited | allowed
`PhantomData<*const T>` | - | covariant | `!Send + !Sync` | allowed
`PhantomData<*mut T>` | - | invariant | `!Send + !Sync` | allowed
`PhantomData<fn(T)>` | - | contravariant | `Send + Sync` | allowed
`PhantomData<fn() -> T>` | - | covariant | `Send + Sync` | allowed
`PhantomData<fn(T) -> T>` | - | invariant | `Send + Sync` | allowed
`PhantomData<Cell<&'a ()>>` | invariant | - | `Send + !Sync` | allowed

- Note: opting out of the `Unpin` auto-trait requires the dedicated `PhantomPinned` type instead.