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Rollup merge of #40069 - Rufflewind:master, r=steveklabnik

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Add Gankro's table to nomicon/src/phantom-data.md

Original: https://github.com/rust-lang/rust/issues/30069#issuecomment-159928136

Testing confirms that:

  - `PhantomData<fn() -> T>` does not actually enable drop checking.
  - `PhantomData<fn(T) -> T>` is neither variant nor contravariant.
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Eduard-Mihai Burtescu 2017-02-25 14:13:42 +02:00 • committed by GitHub
commit 49c8d58662
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22
src/doc/nomicon/src/phantom-data.md
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@ -82,5 +82,23 @@ standard library made a utility for itself called `Unique<T>` which:
* wraps a `*const T` for variance * wraps a `*const T` for variance
* includes a `PhantomData<T>` * includes a `PhantomData<T>`
* auto-derives Send/Sync as if T was contained * auto-derives `Send`/`Sync` as if T was contained
* marks the pointer as NonZero for the null-pointer optimization * 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.