DST custom coercions. #982
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| * If the impl is for a built-in pointer type, we check nothing, otherwise... | ||
| * The compiler checks that the `Self` type is a struct or tuple struct and that | ||
| the `Target` type is a simple substitution of type parameters from the `Self` |
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I don't quite know what "simple substitution of type parameters" means... but I think we want to check that both of them are applications of the same base type constructor. e.g. Foo<X> and Foo<Y> are ok, but not Foo<X> and Bar<Y>.
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I meant that there exists some Ts (where Ts = a sequence of T) and some Xs such that Target = [Ts/Xs]Self and Self is not in Xs. Which I think subsumes your requirement, but also precludes some pathalogical cases such as Target = Foo<T1, T2>, Self = Foo<X, X>.
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I read over the revised version -- it seems pretty good to me, I left a few comments. |
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| ``` | ||
| impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {} | ||
| impl<T: ?Sized+CoerceUnsized<U>, U: ?Sized> NonZero<U> for NonZero<T> {} |
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Is this supposed to be implementing CoerceUnsized<NonZero<U>>?
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I do, I had forgotten this was not merged yet. |
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Hear ye, hear ye. This RFC is now in final comment period until June 2nd. |
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This RFC has been accepted. It is not yet merged to allow @nrc to incorporate final comments reflecting the current design. (It has been partially implemented in the meantime.) -- Language design subteam |
Custom coercions allow smart pointers to fully participate in the DST system. In particular, they allow practical use of `Rc<T>` and `Arc<T>` where `T` is unsized. This RFC subsumes part of [RFC 401 coercions](https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md).
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@nikomatsakis updated |
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Merged. Tracking issue is rust-lang/rust#18598 |
Custom coercions allow smart pointers to fully participate in the DST system.
In particular, they allow practical use of
Rc<T>andArc<T>whereTis unsized.This RFC subsumes part of RFC 401 coercions.
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