Auto merge of rust-lang#35992 - SimonSapin:rc-arc-ptr-eq, r=alexcrichton

Add `pub fn ptr_eq(this: &Self, other: &Self) -> bool` to Rc and Arc

Servo and Kuchiki have had helper functions doing this for some time.

Author: bors

src/liballoc/arc.rs

@@ -331,6 +331,33 @@ impl<T: ?Sized> Arc<T> {
             deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
         }
     }
+
+    #[inline]
+    #[unstable(feature = "ptr_eq",
+               reason = "newly added",
+               issue = "36497")]
+    /// Return whether two `Arc` references point to the same value
+    /// (not just values that compare equal).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ptr_eq)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let five = Arc::new(5);
+    /// let same_five = five.clone();
+    /// let other_five = Arc::new(5);
+    ///
+    /// assert!(Arc::ptr_eq(&five, &same_five));
+    /// assert!(!Arc::ptr_eq(&five, &other_five));
+    /// ```
+    pub fn ptr_eq(this: &Self, other: &Self) -> bool {
+        let this_ptr: *const ArcInner<T> = *this.ptr;
+        let other_ptr: *const ArcInner<T> = *other.ptr;
+        this_ptr == other_ptr
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1200,6 +1227,16 @@ mod tests {
         let foo: Weak<usize> = Weak::new();
         assert!(foo.upgrade().is_none());
     }
+
+    #[test]
+    fn test_ptr_eq() {
+        let five = Arc::new(5);
+        let same_five = five.clone();
+        let other_five = Arc::new(5);
+
+        assert!(Arc::ptr_eq(&five, &same_five));
+        assert!(!Arc::ptr_eq(&five, &other_five));
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/liballoc/rc.rs

@@ -376,6 +376,33 @@ impl<T: ?Sized> Rc<T> {
             None
         }
     }
+
+    #[inline]
+    #[unstable(feature = "ptr_eq",
+               reason = "newly added",
+               issue = "36497")]
+    /// Return whether two `Rc` references point to the same value
+    /// (not just values that compare equal).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ptr_eq)]
+    ///
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    /// let same_five = five.clone();
+    /// let other_five = Rc::new(5);
+    ///
+    /// assert!(Rc::ptr_eq(&five, &same_five));
+    /// assert!(!Rc::ptr_eq(&five, &other_five));
+    /// ```
+    pub fn ptr_eq(this: &Self, other: &Self) -> bool {
+        let this_ptr: *const RcBox<T> = *this.ptr;
+        let other_ptr: *const RcBox<T> = *other.ptr;
+        this_ptr == other_ptr
+    }
 }
 
 impl<T: Clone> Rc<T> {
@@ -1174,6 +1201,16 @@ mod tests {
         let foo: Weak<usize> = Weak::new();
         assert!(foo.upgrade().is_none());
     }
+
+    #[test]
+    fn test_ptr_eq() {
+        let five = Rc::new(5);
+        let same_five = five.clone();
+        let other_five = Rc::new(5);
+
+        assert!(Rc::ptr_eq(&five, &same_five));
+        assert!(!Rc::ptr_eq(&five, &other_five));
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/ptr.rs

@@ -479,6 +479,40 @@ impl<T: ?Sized> PartialEq for *mut T {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Eq for *mut T {}
 
+/// Compare raw pointers for equality.
+///
+/// This is the same as using the `==` operator, but less generic:
+/// the arguments have to be `*const T` raw pointers,
+/// not anything that implements `PartialEq`.
+///
+/// This can be used to compare `&T` references (which coerce to `*const T` implicitly)
+/// by their address rather than comparing the values they point to
+/// (which is what the `PartialEq for &T` implementation does).
+///
+/// # Examples
+///
+/// ```
+/// #![feature(ptr_eq)]
+/// use std::ptr;
+///
+/// let five = 5;
+/// let other_five = 5;
+/// let five_ref = &five;
+/// let same_five_ref = &five;
+/// 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!(!ptr::eq(five_ref, other_five_ref));
+/// ```
+#[unstable(feature = "ptr_eq", reason = "newly added", issue = "36497")]
+#[inline]
+pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {
+    a == b
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Clone for *const T {
     #[inline]