| use crate::marker::Unsize; |
| |
| /// Trait that indicates that this is a pointer or a wrapper for one, |
| /// where unsizing can be performed on the pointee. |
| /// |
| /// See the [DST coercion RFC][dst-coerce] and [the nomicon entry on coercion][nomicon-coerce] |
| /// for more details. |
| /// |
| /// For builtin pointer types, pointers to `T` will coerce to pointers to `U` if `T: Unsize<U>` |
| /// by converting from a thin pointer to a fat pointer. |
| /// |
| /// For custom types, the coercion here works by coercing `Foo<T>` to `Foo<U>` |
| /// provided an impl of `CoerceUnsized<Foo<U>> for Foo<T>` exists. |
| /// Such an impl can only be written if `Foo<T>` has only a single non-phantomdata |
| /// field involving `T`. If the type of that field is `Bar<T>`, an implementation |
| /// of `CoerceUnsized<Bar<U>> for Bar<T>` must exist. The coercion will work by |
| /// coercing the `Bar<T>` field into `Bar<U>` and filling in the rest of the fields |
| /// from `Foo<T>` to create a `Foo<U>`. This will effectively drill down to a pointer |
| /// field and coerce that. |
| /// |
| /// Generally, for smart pointers you will implement |
| /// `CoerceUnsized<Ptr<U>> for Ptr<T> where T: Unsize<U>, U: ?Sized`, with an |
| /// optional `?Sized` bound on `T` itself. For wrapper types that directly embed `T` |
| /// like `Cell<T>` and `RefCell<T>`, you |
| /// can directly implement `CoerceUnsized<Wrap<U>> for Wrap<T> where T: CoerceUnsized<U>`. |
| /// This will let coercions of types like `Cell<Box<T>>` work. |
| /// |
| /// [`Unsize`][unsize] is used to mark types which can be coerced to DSTs if behind |
| /// pointers. It is implemented automatically by the compiler. |
| /// |
| /// [dst-coerce]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md |
| /// [unsize]: crate::marker::Unsize |
| /// [nomicon-coerce]: ../../nomicon/coercions.html |
| #[unstable(feature = "coerce_unsized", issue = "18598")] |
| #[lang = "coerce_unsized"] |
| pub trait CoerceUnsized<T: ?Sized> { |
| // Empty. |
| } |
| |
| // &mut T -> &mut U |
| #[unstable(feature = "coerce_unsized", issue = "18598")] |
| impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a mut U> for &'a mut T {} |
| // &mut T -> &U |
| #[unstable(feature = "coerce_unsized", issue = "18598")] |
| impl<'a, 'b: 'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b mut T {} |
| // &mut T -> *mut U |
| #[unstable(feature = "coerce_unsized", issue = "18598")] |
| impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for &'a mut T {} |
| // &mut T -> *const U |
| #[unstable(feature = "coerce_unsized", issue = "18598")] |
| impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for &'a mut T {} |
| |
| // &T -> &U |
| #[unstable(feature = "coerce_unsized", issue = "18598")] |
| impl<'a, 'b: 'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b T {} |
| // &T -> *const U |
| #[unstable(feature = "coerce_unsized", issue = "18598")] |
| impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for &'a T {} |
| |
| // *mut T -> *mut U |
| #[unstable(feature = "coerce_unsized", issue = "18598")] |
| impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for *mut T {} |
| // *mut T -> *const U |
| #[unstable(feature = "coerce_unsized", issue = "18598")] |
| impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *mut T {} |
| |
| // *const T -> *const U |
| #[unstable(feature = "coerce_unsized", issue = "18598")] |
| impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {} |
| |
| /// `DispatchFromDyn` is used in the implementation of object safety checks (specifically allowing |
| /// arbitrary self types), to guarantee that a method's receiver type can be dispatched on. |
| /// |
| /// Note: `DispatchFromDyn` was briefly named `CoerceSized` (and had a slightly different |
| /// interpretation). |
| /// |
| /// Imagine we have a trait object `t` with type `&dyn Tr`, where `Tr` is some trait with a method |
| /// `m` defined as `fn m(&self);`. When calling `t.m()`, the receiver `t` is a wide pointer, but an |
| /// implementation of `m` will expect a narrow pointer as `&self` (a reference to the concrete |
| /// type). The compiler must generate an implicit conversion from the trait object/wide pointer to |
| /// the concrete reference/narrow pointer. Implementing `DispatchFromDyn` indicates that that |
| /// conversion is allowed and thus that the type implementing `DispatchFromDyn` is safe to use as |
| /// the self type in an object-safe method. (in the above example, the compiler will require |
| /// `DispatchFromDyn` is implemented for `&'a U`). |
| /// |
| /// `DispatchFromDyn` does not specify the conversion from wide pointer to narrow pointer; the |
| /// conversion is hard-wired into the compiler. For the conversion to work, the following |
| /// properties must hold (i.e., it is only safe to implement `DispatchFromDyn` for types which have |
| /// these properties, these are also checked by the compiler): |
| /// |
| /// * EITHER `Self` and `T` are either both references or both raw pointers; in either case, with |
| /// the same mutability. |
| /// * OR, all of the following hold |
| /// - `Self` and `T` must have the same type constructor, and only vary in a single type parameter |
| /// formal (the *coerced type*, e.g., `impl DispatchFromDyn<Rc<T>> for Rc<U>` is ok and the |
| /// single type parameter (instantiated with `T` or `U`) is the coerced type, |
| /// `impl DispatchFromDyn<Arc<T>> for Rc<U>` is not ok). |
| /// - The definition for `Self` must be a struct. |
| /// - The definition for `Self` must not be `#[repr(packed)]` or `#[repr(C)]`. |
| /// - Other than one-aligned, zero-sized fields, the definition for `Self` must have exactly one |
| /// field and that field's type must be the coerced type. Furthermore, `Self`'s field type must |
| /// implement `DispatchFromDyn<F>` where `F` is the type of `T`'s field type. |
| /// |
| /// An example implementation of the trait: |
| /// |
| /// ``` |
| /// # #![feature(dispatch_from_dyn, unsize)] |
| /// # use std::{ops::DispatchFromDyn, marker::Unsize}; |
| /// # struct Rc<T: ?Sized>(std::rc::Rc<T>); |
| /// impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> |
| /// where |
| /// T: Unsize<U>, |
| /// {} |
| /// ``` |
| #[unstable(feature = "dispatch_from_dyn", issue = "none")] |
| #[lang = "dispatch_from_dyn"] |
| pub trait DispatchFromDyn<T> { |
| // Empty. |
| } |
| |
| // &T -> &U |
| #[unstable(feature = "dispatch_from_dyn", issue = "none")] |
| impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<&'a U> for &'a T {} |
| // &mut T -> &mut U |
| #[unstable(feature = "dispatch_from_dyn", issue = "none")] |
| impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<&'a mut U> for &'a mut T {} |
| // *const T -> *const U |
| #[unstable(feature = "dispatch_from_dyn", issue = "none")] |
| impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<*const U> for *const T {} |
| // *mut T -> *mut U |
| #[unstable(feature = "dispatch_from_dyn", issue = "none")] |
| impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<*mut U> for *mut T {} |