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Rollup merge of #120354 - lukas-code:metadata-normalize, r=lcnr

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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 ✨ *safer* ✨, 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 = ();
}
```
This commit is contained in:
Matthias Krüger 2024-02-09 19:21:16 +01:00 • committed by GitHub
commit 99bafad6c2
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7 changed files with 148 additions and 59 deletions
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6
compiler/rustc_codegen_llvm/src/intrinsic.rs
View file

@ -1985,10 +1985,9 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
match in_elem.kind() {
ty::RawPtr(p) => {
let (metadata, check_sized) = p.ty.ptr_metadata_ty(bx.tcx, |ty| {
let metadata = p.ty.ptr_metadata_ty(bx.tcx, |ty| {
bx.tcx.normalize_erasing_regions(ty::ParamEnv::reveal_all(), ty)
});
assert!(!check_sized); // we are in codegen, so we shouldn't see these types
require!(
metadata.is_unit(),
InvalidMonomorphization::CastFatPointer { span, name, ty: in_elem }
@ -2000,10 +1999,9 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
}
match out_elem.kind() {
ty::RawPtr(p) => {
let (metadata, check_sized) = p.ty.ptr_metadata_ty(bx.tcx, |ty| {
let metadata = p.ty.ptr_metadata_ty(bx.tcx, |ty| {
bx.tcx.normalize_erasing_regions(ty::ParamEnv::reveal_all(), ty)
});
assert!(!check_sized); // we are in codegen, so we shouldn't see these types
require!(
metadata.is_unit(),
InvalidMonomorphization::CastFatPointer { span, name, ty: out_elem }