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Improve the `PhantomData` table

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- simplify and clarify the auto-traits (_c.f._, #411)
      - No more `-` in the auto-traits part
  - move `dropck` to its own dedicated column
  - Name the `'a` and `T` columns with "variance"
  - Emphasize the `cov` _vs._ `inv` _vs._ `contra` distinctions
  - Add a mention to the `PhantomPinned` type.
pull/417/head
Daniel Henry-Mantilla 2 years ago committed by GitHub
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src/phantom-data.md
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@ -234,14 +234,18 @@ standard library made a utility for itself called `Unique<T>` which:
Here’s a table of all the wonderful ways `PhantomData` could be used:
| Phantom type | `'a` | `T` | `Send` | `Sync` |
|-----------------------------|-----------|-----------------------------|-----------|-----------|
| `PhantomData<T>` | - | covariant (with drop check) | `T: Send` | `T: Sync` |
| `PhantomData<&'a T>` | covariant | covariant | `T: Sync` | `T: Sync` |
| `PhantomData<&'a mut T>` | covariant | invariant | `T: Send` | `T: Sync` |
| `PhantomData<*const T>` | - | covariant | - | - |
| `PhantomData<*mut T>` | - | invariant | - | - |
| `PhantomData<fn(T)>` | - | contravariant | `Send` | `Sync` |
| `PhantomData<fn() -> T>` | - | covariant | `Send` | `Sync` |
| `PhantomData<fn(T) -> T>` | - | invariant | `Send` | `Sync` |
| `PhantomData<Cell<&'a ()>>` | invariant | - | `Send` | - |
| 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

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