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Rollup merge of #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 | ||
|
|
||
| #![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); | ||
|
|
||
| // normalize `Wrapper<T>::Metadata` -> `T::Metadata` | ||
| fn wrapper_to_tail<T: ?Sized>(ptr: *const T) -> *const Wrapper<T> { | ||
| cast_same_meta(ptr) | ||
| } | ||
|
|
||
| // normalize `Wrapper<T>::Metadata` -> `T::Metadata` -> `()` | ||
| fn wrapper_to_unit<T>(ptr: *const ()) -> *const Wrapper<T> { | ||
| cast_same_meta(ptr) | ||
| } | ||
|
|
||
| trait Project { | ||
| type Assoc: ?Sized; | ||
| } | ||
|
|
||
| struct WrapperProject<T: ?Sized + Project>(T::Assoc); | ||
|
|
||
| // 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) | ||
| } | ||
|
|
||
| // 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) | ||
| } | ||
|
|
||
| // 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) | ||
| } | ||
|
|
||
| fn main() {} |