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

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

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

Author: Enselic

library/core/src/fmt/mod.rs

@@ -2767,7 +2767,15 @@ 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 core::mem::size_of::<<T as core::ptr::Pointee>::Metadata>() == 0 {
+            pointer_fmt_inner(self.expose_provenance(), f)
+        } else {
+            f.write_str("Pointer { addr: ")?;
+            pointer_fmt_inner(self.expose_provenance(), f)?;
+            f.write_str(", metadata: ")?;
+            Debug::fmt(&core::ptr::metadata(*self), f)?;
+            f.write_str(" }")
+        }
     }
 }
 

tests/ui/fmt/ptr-metadata.run.stdout

@@ -1,6 +1,6 @@
 *mut i32: $HEX
 *const i32: $HEX
-*mut [i32]: $HEX
-*const [i32]: $HEX
-*mut dyn Display: $HEX
-*const dyn Display: $HEX
+*mut [i32]: Pointer { addr: $HEX, metadata: 3 }
+*const [i32]: Pointer { addr: $HEX, metadata: 3 }
+*mut dyn Display: Pointer { addr: $HEX, metadata: DynMetadata($HEX) }
+*const dyn Display: Pointer { addr: $HEX, metadata: DynMetadata($HEX) }