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Rollup merge of rust-lang#120354 - lukas-code:metadata-normalize, r=lcnr improve normalization of `Pointee::Metadata` This PR makes it so that `<Wrapper<Tail> as Pointee>::Metadata` is normalized to `<Tail as Pointee>::Metadata` if we don't know `Wrapper<Tail>: Sized`. With that, the trait solver can prove projection predicates like `<Wrapper<Tail> as Pointee>::Metadata == <Tail as Pointee>::Metadata`, which makes it possible to use the metadata APIs to cast between the tail and the wrapper: ```rust #![feature(ptr_metadata)] use std::ptr::{self, Pointee}; fn cast_same_meta<T: ?Sized, U: ?Sized>(ptr: *const T) -> *const U where T: Pointee<Metadata = <U as Pointee>::Metadata>, { let (thin, meta) = ptr.to_raw_parts(); ptr::from_raw_parts(thin, meta) } struct Wrapper<T: ?Sized>(T); fn cast_to_wrapper<T: ?Sized>(ptr: *const T) -> *const Wrapper<T> { cast_same_meta(ptr) } ``` Previously, this failed to compile: ``` error[E0271]: type mismatch resolving `<Wrapper<T> as Pointee>::Metadata == <T as Pointee>::Metadata` --> src/lib.rs:16:5 | 15 | fn cast_to_wrapper<T: ?Sized>(ptr: *const T) -> *const Wrapper<T> { | - found this type parameter 16 | cast_same_meta(ptr) | ^^^^^^^^^^^^^^ expected `Wrapper<T>`, found type parameter `T` | = note: expected associated type `<Wrapper<T> as Pointee>::Metadata` found associated type `<T as Pointee>::Metadata` = note: an associated type was expected, but a different one was found ``` (Yes, you can already do this with `as` casts. But using functions is so much :sparkles: *safer* :sparkles:, because you can't change the metadata on accident.) --- This PR essentially changes the built-in impls of `Pointee` from this: ```rust // before impl Pointee for u8 { type Metadata = (); } impl Pointee for [u8] { type Metadata = usize; } // ... impl Pointee for Wrapper<u8> { type Metadata = (); } impl Pointee for Wrapper<[u8]> { type Metadata = usize; } // ... // This impl is only selected if `T` is a type parameter or unnormalizable projection or opaque type. fallback impl<T: ?Sized> Pointee for Wrapper<T> where Wrapper<T>: Sized { type Metadata = (); } // This impl is only selected if `T` is a type parameter or unnormalizable projection or opaque type. fallback impl<T /*: Sized */> Pointee for T { type Metadata = (); } ``` to this: ```rust // after impl Pointee for u8 { type Metadata = (); } impl Pointee for [u8] { type Metadata = usize; } // ... impl<T: ?Sized> Pointee for Wrapper<T> { // in the old solver this will instead project to the "deep" tail directly, // e.g. `Wrapper<Wrapper<T>>::Metadata = T::Metadata` type Metadata = <T as Pointee>::Metadata; } // ... // This impl is only selected if `T` is a type parameter or unnormalizable projection or opaque type. fallback impl<T /*: Sized */> Pointee for T { type Metadata = (); } ```
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,56 @@ | ||
| // check-pass | ||
| // revisions: old next | ||
| //[next] compile-flags: -Znext-solver | ||
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| #![feature(ptr_metadata)] | ||
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| use std::ptr::{self, Pointee}; | ||
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| fn cast_same_meta<T: ?Sized, U: ?Sized>(ptr: *const T) -> *const U | ||
| where | ||
| T: Pointee<Metadata = <U as Pointee>::Metadata>, | ||
| { | ||
| let (thin, meta) = ptr.to_raw_parts(); | ||
| ptr::from_raw_parts(thin, meta) | ||
| } | ||
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| struct Wrapper<T: ?Sized>(T); | ||
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| // normalize `Wrapper<T>::Metadata` -> `T::Metadata` | ||
| fn wrapper_to_tail<T: ?Sized>(ptr: *const T) -> *const Wrapper<T> { | ||
| cast_same_meta(ptr) | ||
| } | ||
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| // normalize `Wrapper<T>::Metadata` -> `T::Metadata` -> `()` | ||
| fn wrapper_to_unit<T>(ptr: *const ()) -> *const Wrapper<T> { | ||
| cast_same_meta(ptr) | ||
| } | ||
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| trait Project { | ||
| type Assoc: ?Sized; | ||
| } | ||
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| struct WrapperProject<T: ?Sized + Project>(T::Assoc); | ||
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| // normalize `WrapperProject<T>::Metadata` -> `T::Assoc::Metadata` | ||
| fn wrapper_project_tail<T: ?Sized + Project>(ptr: *const T::Assoc) -> *const WrapperProject<T> { | ||
| cast_same_meta(ptr) | ||
| } | ||
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| // normalize `WrapperProject<T>::Metadata` -> `T::Assoc::Metadata` -> `()` | ||
| fn wrapper_project_unit<T: ?Sized + Project>(ptr: *const ()) -> *const WrapperProject<T> | ||
| where | ||
| T::Assoc: Sized, | ||
| { | ||
| cast_same_meta(ptr) | ||
| } | ||
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| // normalize `<[T] as Pointee>::Metadata` -> `usize`, even if `[T]: Sized` | ||
| fn sized_slice<T>(ptr: *const [T]) -> *const str | ||
| where | ||
| [T]: Sized, | ||
| { | ||
| cast_same_meta(ptr) | ||
| } | ||
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| fn main() {} |