103 lines
4.3 KiB
Rust
103 lines
4.3 KiB
Rust
use marker::Unsize;
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/// Trait that indicates that this is a pointer or a wrapper for one,
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/// where unsizing can be performed on the pointee.
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///
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/// See the [DST coercion RFC][dst-coerce] and [the nomicon entry on coercion][nomicon-coerce]
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/// for more details.
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///
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/// For builtin pointer types, pointers to `T` will coerce to pointers to `U` if `T: Unsize<U>`
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/// by converting from a thin pointer to a fat pointer.
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///
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/// For custom types, the coercion here works by coercing `Foo<T>` to `Foo<U>`
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/// provided an impl of `CoerceUnsized<Foo<U>> for Foo<T>` exists.
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/// Such an impl can only be written if `Foo<T>` has only a single non-phantomdata
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/// field involving `T`. If the type of that field is `Bar<T>`, an implementation
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/// of `CoerceUnsized<Bar<U>> for Bar<T>` must exist. The coercion will work by
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/// coercing the `Bar<T>` field into `Bar<U>` and filling in the rest of the fields
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/// from `Foo<T>` to create a `Foo<U>`. This will effectively drill down to a pointer
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/// field and coerce that.
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///
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/// Generally, for smart pointers you will implement
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/// `CoerceUnsized<Ptr<U>> for Ptr<T> where T: Unsize<U>, U: ?Sized`, with an
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/// optional `?Sized` bound on `T` itself. For wrapper types that directly embed `T`
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/// like `Cell<T>` and `RefCell<T>`, you
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/// can directly implement `CoerceUnsized<Wrap<U>> for Wrap<T> where T: CoerceUnsized<U>`.
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/// This will let coercions of types like `Cell<Box<T>>` work.
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///
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/// [`Unsize`][unsize] is used to mark types which can be coerced to DSTs if behind
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/// pointers. It is implemented automatically by the compiler.
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///
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/// [dst-coerce]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
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/// [unsize]: ../marker/trait.Unsize.html
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/// [nomicon-coerce]: ../../nomicon/coercions.html
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#[unstable(feature = "coerce_unsized", issue = "27732")]
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#[lang = "coerce_unsized"]
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pub trait CoerceUnsized<T: ?Sized> {
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// Empty.
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}
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// &mut T -> &mut U
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#[unstable(feature = "coerce_unsized", issue = "27732")]
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impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<&'a mut U> for &'a mut T {}
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// &mut T -> &U
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#[unstable(feature = "coerce_unsized", issue = "27732")]
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impl<'a, 'b: 'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b mut T {}
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// &mut T -> *mut U
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#[unstable(feature = "coerce_unsized", issue = "27732")]
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impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for &'a mut T {}
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// &mut T -> *const U
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#[unstable(feature = "coerce_unsized", issue = "27732")]
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impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for &'a mut T {}
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// &T -> &U
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#[unstable(feature = "coerce_unsized", issue = "27732")]
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impl<'a, 'b: 'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b T {}
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// &T -> *const U
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#[unstable(feature = "coerce_unsized", issue = "27732")]
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impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for &'a T {}
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// *mut T -> *mut U
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#[unstable(feature = "coerce_unsized", issue = "27732")]
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impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for *mut T {}
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// *mut T -> *const U
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#[unstable(feature = "coerce_unsized", issue = "27732")]
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impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *mut T {}
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// *const T -> *const U
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#[unstable(feature = "coerce_unsized", issue = "27732")]
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impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {}
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/// This is used for object safety, to check that a method's receiver type can be dispatched on.
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///
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/// example impl:
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///
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/// ```
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/// # #![feature(dispatch_from_dyn, unsize)]
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/// # use std::{ops::DispatchFromDyn, marker::Unsize};
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/// # struct Rc<T: ?Sized>(::std::rc::Rc<T>);
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/// impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T>
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/// where
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/// T: Unsize<U>,
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/// {}
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/// ```
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#[unstable(feature = "dispatch_from_dyn", issue = "0")]
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#[lang = "dispatch_from_dyn"]
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pub trait DispatchFromDyn<T> {
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// Empty.
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}
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// &T -> &U
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#[unstable(feature = "dispatch_from_dyn", issue = "0")]
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impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<&'a U> for &'a T {}
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// &mut T -> &mut U
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#[unstable(feature = "dispatch_from_dyn", issue = "0")]
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impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<&'a mut U> for &'a mut T {}
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// *const T -> *const U
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#[unstable(feature = "dispatch_from_dyn", issue = "0")]
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impl<T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<*const U> for *const T {}
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// *mut T -> *mut U
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#[unstable(feature = "dispatch_from_dyn", issue = "0")]
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impl<T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<*mut U> for *mut T {}
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