# PhantomData

불안전한 코드와 작업을 하다 보면, 우리는 종종 타입이나 수명이 구조체와 논리적으로 연관되어 있지만, 실제로 그 필드의 일부분은 아닌 상황을 마주할 수 있습니다. 이런 상황은 보통 수명인 경우가 많은데요, 예를 들어 `&'a [T]`를 위한 `Iter`는 (거의) 다음과 같이 정의되어 있습니다:

```rust,compile_fail
struct Iter<'a, T: 'a> {
    ptr: *const T,
    end: *const T,
}
```

하지만 `'a`가 구조체의 본문에 쓰이지 않았기 때문에, 이 수명은 *무제한이* 됩니다. [이것이 역사적으로 초래해왔던 문제들 때문에][unused-param], 무제한 수명과 이를 사용하는 타입은 구조체 선언에서 *금지되었습니다*. 따라서 우리는 어떻게든 이 타입들을 구조체 안에서 참조해야 합니다. 
이것을 올바르게 하는 것은 올바른 변성과 해제 검사에 있어서 필수적입니다.

[unused-param]: https://rust-lang.github.io/rfcs/0738-variance.html#the-corner-case-unused-parameters-and-parameters-that-are-only-used-unsafely

우리는 이것을 특별한 표시 타입인 `PhantomData`를 통해서 합니다. `PhantomData`는 공간을 차지하지 않지만, 컴파일러의 분석을 위해 주어진 타입의 필드를 흉내냅니다. 이 방식은 우리가 원하는 변성을 직접 타입 시스템에 말하는 것보다 더 오류에 견고하다고 평가되었습니다. 또한 이 방식은 자동 트레잇과 해제 검사에 필요한 정보 등의 유용한 것들을 컴파일러에게 제공합니다.

`Iter`는 논리적으로 여러 개의 `&'a T`를 포함하므로, 바로 이렇게 우리는 `PhantomData`에게 흉내내라고 할 것입니다:

```rust
use std::marker;

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

이렇게만 하면 됩니다. 수명은 제한될 것이고, 반복자는 `'a`와 `T`에 대해서 공변할 것입니다. 모든 게 그냥 마법처럼 동작할 겁니다.

## 제네릭 매개변수와 해제 검사

이전에는 신경써야 할 다른 것이 있었습니다.

바로 이 문서는 이렇게 말했었지요:

> Another important example is Vec, which is (approximately) defined as follows:
>
> ```rust
> 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:
>
> ```rust
> use std::marker;
>
> struct Vec<T> {
>     data: *const T, // *const for variance!
>     len: usize,
>     cap: usize,
>     _owns_T: marker::PhantomData<T>,
> }
> ```

since [RFC 1238](https://rust-lang.github.io/rfcs/1238-nonparametric-dropck.html),
**this is no longer true nor necessary**.

If you were to write:

```rust
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.

<details><summary>Click here to see why</summary>

Consider the following example:

```rust
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].

[is denied]: https://rust.godbolt.org/z/ans15Kqz3

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]`:

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

or, more generally:

```rust ,ignore
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:

```rust
# #[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,
}
```

</details>

___

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`<br/>(or lack thereof)       | dangling `'a` or `T` in drop glue<br/>(_e.g._, `#[may_dangle] Drop`) |
|-----------------------------|:----------------:|:-----------------:|:-----------------------------------------:|:------------------------------------------------:|
| `PhantomData<T>`            | -                | **cov**ariant     | inherited                                 | disallowed ("owns `T`")                          |
| `PhantomData<&'a T>`        | **cov**ariant    | **cov**ariant     | `Send + Sync`<br/>requires<br/>`T : Sync` | allowed                                          |
| `PhantomData<&'a mut T>`    | **cov**ariant    | **inv**ariant     | inherited                                 | allowed                                          |
| `PhantomData<*const T>`     | -                | **cov**ariant     | `!Send + !Sync`                           | allowed                                          |
| `PhantomData<*mut T>`       | -                | **inv**ariant     | `!Send + !Sync`                           | allowed                                          |
| `PhantomData<fn(T)>`        | -                | **contra**variant | `Send + Sync`                             | allowed                                          |
| `PhantomData<fn() -> T>`    | -                | **cov**ariant     | `Send + Sync`                             | allowed                                          |
| `PhantomData<fn(T) -> T>`   | -                | **inv**ariant     | `Send + Sync`                             | allowed                                          |
| `PhantomData<Cell<&'a ()>>` | **inv**ariant    | -                 | `Send + !Sync`                            | allowed                                          |

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

[`PhantomPinned`]: ../core/marker/struct.PhantomPinned.html