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| -rw-r--r-- | src/doc/nomicon/src/phantom-data.md | 22 |
1 files changed, 20 insertions, 2 deletions
diff --git a/src/doc/nomicon/src/phantom-data.md b/src/doc/nomicon/src/phantom-data.md index 72fa2e2a777..32539c2d01f 100644 --- a/src/doc/nomicon/src/phantom-data.md +++ b/src/doc/nomicon/src/phantom-data.md @@ -82,5 +82,23 @@ 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 +* 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 | `'a` | `T` | +|-----------------------------|-----------|---------------------------| +| `PhantomData<T>` | - | variant (with drop check) | +| `PhantomData<&'a T>` | variant | variant | +| `PhantomData<&'a mut T>` | variant | invariant | +| `PhantomData<*const T>` | - | variant | +| `PhantomData<*mut T>` | - | invariant | +| `PhantomData<fn(T)>` | - | contravariant (*) | +| `PhantomData<fn() -> T>` | - | variant | +| `PhantomData<fn(T) -> T>` | - | invariant | +| `PhantomData<Cell<&'a ()>>` | invariant | - | + +(*) If contravariance gets scrapped, this would be invariant. |