mirror of https://github.com/rust-lang/rust
133 lines
6.6 KiB
Rust
133 lines
6.6 KiB
Rust
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 {}
|