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Diffstat (limited to '')
-rw-r--r-- | library/core/src/ops/unsize.rs | 132 |
1 files changed, 132 insertions, 0 deletions
diff --git a/library/core/src/ops/unsize.rs b/library/core/src/ops/unsize.rs new file mode 100644 index 000000000..a920b9165 --- /dev/null +++ b/library/core/src/ops/unsize.rs @@ -0,0 +1,132 @@ +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 = "27732")] +#[lang = "coerce_unsized"] +pub trait CoerceUnsized<T: ?Sized> { + // Empty. +} + +// &mut T -> &mut U +#[unstable(feature = "coerce_unsized", issue = "27732")] +impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a mut U> for &'a mut T {} +// &mut T -> &U +#[unstable(feature = "coerce_unsized", issue = "27732")] +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 = "27732")] +impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for &'a mut T {} +// &mut T -> *const U +#[unstable(feature = "coerce_unsized", issue = "27732")] +impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for &'a mut T {} + +// &T -> &U +#[unstable(feature = "coerce_unsized", issue = "27732")] +impl<'a, 'b: 'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b T {} +// &T -> *const U +#[unstable(feature = "coerce_unsized", issue = "27732")] +impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for &'a T {} + +// *mut T -> *mut U +#[unstable(feature = "coerce_unsized", issue = "27732")] +impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for *mut T {} +// *mut T -> *const U +#[unstable(feature = "coerce_unsized", issue = "27732")] +impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *mut T {} + +// *const T -> *const U +#[unstable(feature = "coerce_unsized", issue = "27732")] +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 {} |