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Improvements to PhantomData<T>'s docs 👻

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Fixes #22914
This commit is contained in:
Steve Klabnik 2015-03-31 20:18:32 -04:00
parent d754722a04
commit bfc2f5de85
1 changed files with 6 additions and 12 deletions
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18
src/libcore/marker.rs
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@ -346,17 +346,16 @@ impl<T:?Sized> MarkerTrait for T { }
#[stable(feature = "rust1", since = "1.0.0")]
pub trait PhantomFn<A:?Sized,R:?Sized=()> { }
/// `PhantomData` is a way to tell the compiler about fake fields.
/// Phantom data is required whenever type parameters are not used.
/// The idea is that if the compiler encounters a `PhantomData<T>`
/// instance, it will behave *as if* an instance of the type `T` were
/// present for the purpose of various automatic analyses.
/// `PhantomData<T>` allows you to describe that a type acts as if it stores a value of type `T`,
/// even though it does not. This allows you to inform the compiler about certain safety properties
/// of your code.
///
/// Though they both have scary names, `PhantomData<T>` and "phantom types" are unrelated. 👻👻👻
///
/// # Examples
///
/// When handling external resources over a foreign function interface, `PhantomData<T>` can
/// prevent mismatches by enforcing types in the method implementations, although the struct
/// doesn't actually contain values of the resource type.
/// prevent mismatches by enforcing types in the method implementations:
///
/// ```
/// # trait ResType { fn foo(&self); };
@ -397,11 +396,6 @@ pub trait PhantomFn<A:?Sized,R:?Sized=()> { }
/// commonly necessary if the structure is using an unsafe pointer
/// like `*mut T` whose referent may be dropped when the type is
/// dropped, as a `*mut T` is otherwise not treated as owned.
///
/// FIXME. Better documentation and examples of common patterns needed
/// here! For now, please see [RFC 738][738] for more information.
///
/// [738]: https://github.com/rust-lang/rfcs/blob/master/text/0738-variance.md
#[lang="phantom_data"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct PhantomData<T:?Sized>;