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phantom-data: Add Send and Sync columns #411

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Jun 4, 2023
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phantom-data: Add Send and Sync columns #411

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24 changes: 12 additions & 12 deletions src/phantom-data.md
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Expand Up @@ -24,7 +24,7 @@ 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 the information needed by drop check.
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:
Expand Down Expand Up @@ -234,14 +234,14 @@ 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` |
|-----------------------------|-----------|---------------------------|
| `PhantomData<T>` | - | covariant (with drop check) |
| `PhantomData<&'a T>` | covariant | covariant |
| `PhantomData<&'a mut T>` | covariant | invariant |
| `PhantomData<*const T>` | - | covariant |
| `PhantomData<*mut T>` | - | invariant |
| `PhantomData<fn(T)>` | - | contravariant |
| `PhantomData<fn() -> T>` | - | covariant |
| `PhantomData<fn(T) -> T>` | - | invariant |
| `PhantomData<Cell<&'a ()>>` | invariant | - |
| 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` | - |