fmt: Debug-format fat pointers with their metadata for better insight

Use ptr::metadata() fn to improve debug prints of fat pointers.

Co-authored-by: Mara Bos <m-ou.se@m-ou.se>

library/core/src/fmt/mod.rs

@@ -6,6 +6,7 @@ use crate::cell::{Cell, Ref, RefCell, RefMut, UnsafeCell};
 use crate::char::EscapeDebugExtArgs;
 use crate::marker::PhantomData;
 use crate::mem;
+use crate::ptr::{self, DynMetadata};
 use crate::num::fmt as numfmt;
 use crate::ops::Deref;
 use crate::result;
@@ -2281,16 +2282,51 @@ impl<T: ?Sized> Pointer for &mut T {
 
 // Implementation of Display/Debug for various core types
 
+/// A local trait for pointer Debug impl so that we can have
+/// min_specialization-compliant specializations for two types of fat ptrs.
+trait PtrMetadataFmt {
+    fn fmt_ptr(&self, ptr: *const (), f: &mut Formatter<'_>) -> Result;
+}
+
+// Regular pointer / default impl
+impl<T> PtrMetadataFmt for T {
+    #[inline]
+    default fn fmt_ptr(&self, ptr: *const (), f: &mut Formatter<'_>) -> Result {
+        Pointer::fmt(&ptr, f)
+    }
+}
+
+// Pointer + length
+impl PtrMetadataFmt for usize {
+    #[inline]
+    fn fmt_ptr(&self, ptr: *const (), f: &mut Formatter<'_>) -> Result {
+        write!(f, "({:p}, {})", ptr, *self)
+    }
+}
+
+// Pointer + vtable
+impl<Dyn: ?Sized> PtrMetadataFmt for DynMetadata<Dyn> {
+    #[inline]
+    fn fmt_ptr(&self, ptr: *const (), f: &mut Formatter<'_>) -> Result {
+        f.debug_tuple("")
+            .field(&ptr)
+            .field(self)
+            .finish()
+   }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Debug for *const T {
     fn fmt(&self, f: &mut Formatter<'_>) -> Result {
-        Pointer::fmt(self, f)
+        let meta = ptr::metadata(*self);
+        meta.fmt_ptr(*self as *const (), f)
     }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Debug for *mut T {
     fn fmt(&self, f: &mut Formatter<'_>) -> Result {
-        Pointer::fmt(self, f)
+        let ptr: *const T = *self;
+        Debug::fmt(&ptr, f)
     }
 }
 

library/core/tests/fmt/mod.rs

@@ -2,6 +2,10 @@ mod builders;
 mod float;
 mod num;
 
+use core::any::Any;
+use core::fmt;
+use core::ptr;
+
 #[test]
 fn test_format_flags() {
     // No residual flags left by pointer formatting
@@ -19,6 +23,36 @@ fn test_pointer_formats_data_pointer() {
     assert_eq!(format!("{:p}", b), format!("{:p}", b.as_ptr()));
 }
 
+#[test]
+fn test_pointer_formats_debug_thin() {
+    let thinptr = &42 as *const i32;
+    assert_eq!(format!("{:?}", thinptr as *const ()), format!("{:p}", thinptr));
+}
+
+#[test]
+fn test_pointer_formats_debug_slice() {
+    let b: &[u8] = b"hello";
+    let s: &str = "hello";
+    let b_ptr = &*b as *const _;
+    let s_ptr = &*s as *const _;
+    assert_eq!(format!("{:?}", b_ptr), format!("({:?}, 5)", b.as_ptr()));
+    assert_eq!(format!("{:?}", s_ptr), format!("({:?}, 5)", s.as_ptr()));
+
+    // :p should format as a thin pointer / without metadata
+    assert_eq!(format!("{:p}", b_ptr), format!("{:p}", b.as_ptr()));
+    assert_eq!(format!("{:p}", s_ptr), format!("{:p}", s.as_ptr()));
+}
+
+#[test]
+fn test_pointer_formats_debug_trait_object() {
+    let mut any: Box<dyn Any> = Box::new(42);
+    let dyn_ptr = &mut *any as *mut dyn Any;
+    assert_eq!(format!("{:?}", dyn_ptr), format!("({:?}, {:?})", dyn_ptr as *const (), ptr::metadata(dyn_ptr)));
+
+    // :p should format as a thin pointer / without metadata
+    assert_eq!(format!("{:p}", dyn_ptr), format!("{:p}", dyn_ptr as *const ()));
+}
+
 #[test]
 fn test_estimated_capacity() {
     assert_eq!(format_args!("").estimated_capacity(), 0);
@@ -33,8 +67,8 @@ fn test_estimated_capacity() {
 fn pad_integral_resets() {
     struct Bar;
 
-    impl core::fmt::Display for Bar {
-        fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+    impl fmt::Display for Bar {
+        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
             "1".fmt(f)?;
             f.pad_integral(true, "", "5")?;
             "1".fmt(f)