Rollup merge of #135080 - Enselic:debug-ptr-metadata, r=thomcc

core: Make `Debug` impl of raw pointers print metadata if present

Make Rust pointers appear less magic by including metadata information in their `Debug` output.

This does not break Rust stability guarantees because `Debug` impl are explicitly exempted from stability:
https://doc.rust-lang.org/std/fmt/trait.Debug.html#stability

> ## Stability
>
> Derived `Debug` formats are not stable, and so may change with future Rust versions. Additionally, `Debug` implementations of types provided by the standard library (`std`, `core`, `alloc`, etc.) are not stable, and may also change with future Rust versions.

Note that a regression test is added as a separate commit to make it clear what impact the last commit has on the output.

Closes #128684 because the output of that code now becomes:

```
thread 'main' panicked at src/main.rs:5:5:
assertion `left == right` failed
  left: Pointer { addr: 0x7ffd45c6fc6b, metadata: 5 }
 right: Pointer { addr: 0x7ffd45c6fc6b, metadata: 3 }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

library/core/src/fmt/mod.rs

@@ -2771,7 +2771,14 @@ impl Display for char {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Pointer for *const T {
     fn fmt(&self, f: &mut Formatter<'_>) -> Result {
-        pointer_fmt_inner(self.expose_provenance(), f)
+        if <<T as core::ptr::Pointee>::Metadata as core::unit::IsUnit>::is_unit() {
+            pointer_fmt_inner(self.expose_provenance(), f)
+        } else {
+            f.debug_struct("Pointer")
+                .field_with("addr", |f| pointer_fmt_inner(self.expose_provenance(), f))
+                .field("metadata", &core::ptr::metadata(*self))
+                .finish()
+        }
     }
 }
 

library/core/src/ptr/metadata.rs

@@ -61,6 +61,8 @@ pub trait Pointee {
     // NOTE: Keep trait bounds in `static_assert_expected_bounds_for_metadata`
     // in `library/core/src/ptr/metadata.rs`
     // in sync with those here:
+    // NOTE: The metadata of `dyn Trait + 'a` is `DynMetadata<dyn Trait + 'a>`
+    // so a `'static` bound must not be added.
     type Metadata: fmt::Debug + Copy + Send + Sync + Ord + Hash + Unpin + Freeze;
 }
 

library/core/src/unit.rs

@@ -17,3 +17,19 @@ impl FromIterator<()> for () {
         iter.into_iter().for_each(|()| {})
     }
 }
+
+pub(crate) trait IsUnit {
+    fn is_unit() -> bool;
+}
+
+impl<T: ?Sized> IsUnit for T {
+    default fn is_unit() -> bool {
+        false
+    }
+}
+
+impl IsUnit for () {
+    fn is_unit() -> bool {
+        true
+    }
+}