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 ✨ *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 = ();
}
```

compiler/rustc_middle/src/ty/sty.rs

@@ -2279,12 +2279,12 @@ impl<'tcx> Ty<'tcx> {
     }
 
     /// Returns the type of metadata for (potentially fat) pointers to this type,
-    /// and a boolean signifying if this is conditional on this type being `Sized`.
-    pub fn ptr_metadata_ty(
+    /// or the struct tail if the metadata type cannot be determined.
+    pub fn ptr_metadata_ty_or_tail(
         self,
         tcx: TyCtxt<'tcx>,
         normalize: impl FnMut(Ty<'tcx>) -> Ty<'tcx>,
-    ) -> (Ty<'tcx>, bool) {
+    ) -> Result<Ty<'tcx>, Ty<'tcx>> {
         let tail = tcx.struct_tail_with_normalize(self, normalize, || {});
         match tail.kind() {
             // Sized types
@@ -2307,31 +2307,47 @@ impl<'tcx> Ty<'tcx> {
             | ty::Error(_)
             // Extern types have metadata = ().
             | ty::Foreign(..)
-            // `dyn*` has no metadata
+            // `dyn*` has metadata = ().
             | ty::Dynamic(_, _, ty::DynStar)
-            // If returned by `struct_tail_without_normalization` this is a unit struct
+            // If returned by `struct_tail_with_normalize` this is a unit struct
             // without any fields, or not a struct, and therefore is Sized.
             | ty::Adt(..)
-            // If returned by `struct_tail_without_normalization` this is the empty tuple,
+            // If returned by `struct_tail_with_normalize` this is the empty tuple,
             // a.k.a. unit type, which is Sized
-            | ty::Tuple(..) => (tcx.types.unit, false),
+            | ty::Tuple(..) => Ok(tcx.types.unit),
+
+            ty::Str | ty::Slice(_) => Ok(tcx.types.usize),
 
-            ty::Str | ty::Slice(_) => (tcx.types.usize, false),
             ty::Dynamic(_, _, ty::Dyn) => {
                 let dyn_metadata = tcx.require_lang_item(LangItem::DynMetadata, None);
-                (tcx.type_of(dyn_metadata).instantiate(tcx, &[tail.into()]), false)
-            },
+                Ok(tcx.type_of(dyn_metadata).instantiate(tcx, &[tail.into()]))
+            }
 
-            // type parameters only have unit metadata if they're sized, so return true
-            // to make sure we double check this during confirmation
-            ty::Param(_) |  ty::Alias(..) => (tcx.types.unit, true),
+            // We don't know the metadata of `self`, but it must be equal to the
+            // metadata of `tail`.
+            ty::Param(_) | ty::Alias(..) => Err(tail),
 
             ty::Infer(ty::TyVar(_))
             | ty::Bound(..)
             | ty::Placeholder(..)
-            | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
-                bug!("`ptr_metadata_ty` applied to unexpected type: {:?} (tail = {:?})", self, tail)
-            }
+            | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => bug!(
+                "`ptr_metadata_ty_or_tail` applied to unexpected type: {self:?} (tail = {tail:?})"
+            ),
+        }
+    }
+
+    /// Returns the type of metadata for (potentially fat) pointers to this type.
+    /// Causes an ICE if the metadata type cannot be determined.
+    pub fn ptr_metadata_ty(
+        self,
+        tcx: TyCtxt<'tcx>,
+        normalize: impl FnMut(Ty<'tcx>) -> Ty<'tcx>,
+    ) -> Ty<'tcx> {
+        match self.ptr_metadata_ty_or_tail(tcx, normalize) {
+            Ok(metadata) => metadata,
+            Err(tail) => bug!(
+                "`ptr_metadata_ty` failed to get metadata for type: {self:?} (tail = {tail:?})"
+            ),
         }
     }