fmt: Format fat pointers as (addr, meta) for better insight

Use the new ptr::metadata() fn to improve debug prints of fat pointers.
Pointee::Metadata gets an additional Debug bound, impls already implemented it anyway.

Author: vojtechkral

library/core/src/fmt/mod.rs

@@ -4,11 +4,13 @@
 
 use crate::cell::{Cell, Ref, RefCell, RefMut, UnsafeCell};
 use crate::char::EscapeDebugExtArgs;
+use crate::intrinsics::size_of;
 use crate::iter;
 use crate::marker::PhantomData;
 use crate::mem;
 use crate::num::flt2dec;
 use crate::ops::Deref;
+use crate::ptr;
 use crate::result;
 use crate::str;
 
@@ -2130,6 +2132,15 @@ 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 {
+        if size_of::<*const T>() > size_of::<*const ()>() {
+            // Fat pointer, format as (address, metadata)
+            let meta = ptr::metadata(*self);
+            return f.debug_tuple("")
+                .field(&(*self as *const ()))
+                .field(&meta)
+                .finish();
+        }
+
         let old_width = f.width;
         let old_flags = f.flags;
 

library/core/src/ptr/metadata.rs

@@ -1,6 +1,6 @@
 #![unstable(feature = "ptr_metadata", issue = "81513")]
 
-use crate::fmt;
+use crate::fmt::{self, Debug};
 use crate::hash::{Hash, Hasher};
 
 /// Provides the pointer metadata type of any pointed-to type.
@@ -56,7 +56,7 @@ 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:
-    type Metadata: Copy + Send + Sync + Ord + Hash + Unpin;
+    type Metadata: Copy + Send + Sync + Ord + Hash + Unpin + Debug;
 }
 
 /// Pointers to types implementing this trait alias are “thin”.

library/core/tests/fmt/mod.rs

@@ -2,6 +2,10 @@ mod builders;
 mod float;
 mod num;
 
+use core::any;
+use core::fmt;
+use core::ptr;
+
 #[test]
 fn test_format_flags() {
     // No residual flags left by pointer formatting
@@ -13,10 +17,20 @@ fn test_format_flags() {
 
 #[test]
 fn test_pointer_formats_data_pointer() {
-    let b: &[u8] = b"";
-    let s: &str = "";
-    assert_eq!(format!("{:p}", s), format!("{:p}", s.as_ptr()));
-    assert_eq!(format!("{:p}", b), format!("{:p}", b.as_ptr()));
+    // Thin ptr
+    let thinptr = &42 as *const i32;
+    assert_eq!(format!("{:p}", thinptr), format!("{:?}", thinptr as *const ()));
+
+    // Ptr with length
+    let b: &[u8] = b"hello";
+    let s: &str = "hello";
+    assert_eq!(format!("{:p}", b), format!("({:?}, 5)", b.as_ptr()));
+    assert_eq!(format!("{:p}", s), format!("({:?}, 5)", s.as_ptr()));
+
+    // Ptr with v-table
+    let mut any: Box<dyn any::Any> = Box::new(42);
+    let dyn_ptr = &mut *any as *mut dyn any::Any;
+    assert_eq!(format!("{:p}", dyn_ptr), format!("({:?}, {:?})", dyn_ptr as *const (), ptr::metadata(dyn_ptr)));
 }
 
 #[test]
@@ -33,8 +47,8 @@ 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)

library/core/tests/ptr.rs

@@ -500,7 +500,7 @@ fn ptr_metadata_bounds() {
     fn static_assert_expected_bounds_for_metadata<Meta>()
     where
         // Keep this in sync with the associated type in `library/core/src/ptr/metadata.rs`
-        Meta: Copy + Send + Sync + Ord + std::hash::Hash + Unpin,
+        Meta: Copy + Send + Sync + Ord + std::hash::Hash + Unpin + Debug,
     {
     }
 }