Rollup merge of #59390 - czipperz:ptr_eq_smart_pointer, r=Centril,steveklabnik

Make `ptr::eq` documentation mention fat-pointer behavior

Resolves #59214

Author: cuviper

src/libcore/ptr.rs

@@ -2495,11 +2495,57 @@ impl<T: ?Sized> Eq for *mut T {}
 /// let other_five_ref = &other_five;
 ///
 /// assert!(five_ref == same_five_ref);
-/// assert!(five_ref == other_five_ref);
-///
 /// assert!(ptr::eq(five_ref, same_five_ref));
+///
+/// assert!(five_ref == other_five_ref);
 /// assert!(!ptr::eq(five_ref, other_five_ref));
 /// ```
+///
+/// Slices are also compared by their length (fat pointers):
+///
+/// ```
+/// let a = [1, 2, 3];
+/// assert!(std::ptr::eq(&a[..3], &a[..3]));
+/// assert!(!std::ptr::eq(&a[..2], &a[..3]));
+/// assert!(!std::ptr::eq(&a[0..2], &a[1..3]));
+/// ```
+///
+/// Traits are also compared by their implementation:
+///
+/// ```
+/// #[repr(transparent)]
+/// struct Wrapper { member: i32 }
+///
+/// trait Trait {}
+/// impl Trait for Wrapper {}
+/// impl Trait for i32 {}
+///
+/// fn main() {
+///     let wrapper = Wrapper { member: 10 };
+///
+///     // Pointers have equal addresses.
+///     assert!(std::ptr::eq(
+///         &wrapper as *const Wrapper as *const u8,
+///         &wrapper.member as *const i32 as *const u8
+///     ));
+///
+///     // Objects have equal addresses, but `Trait` has different implementations.
+///     assert!(!std::ptr::eq(
+///         &wrapper as &dyn Trait,
+///         &wrapper.member as &dyn Trait,
+///     ));
+///     assert!(!std::ptr::eq(
+///         &wrapper as &dyn Trait as *const dyn Trait,
+///         &wrapper.member as &dyn Trait as *const dyn Trait,
+///     ));
+///
+///     // Converting the reference to a `*const u8` compares by address.
+///     assert!(std::ptr::eq(
+///         &wrapper as &dyn Trait as *const dyn Trait as *const u8,
+///         &wrapper.member as &dyn Trait as *const dyn Trait as *const u8,
+///     ));
+/// }
+/// ```
 #[stable(feature = "ptr_eq", since = "1.17.0")]
 #[inline]
 pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {