Auto merge of #56550 - chpio:rc-eq, r=alexcrichton

Short-circuit Rc/Arc equality checking on equal pointers where T: Eq

based on #42965

Is the use of the private trait ok this way? Is there anything else needed for this to get pulled?

src/liballoc/rc.rs

@@ -900,12 +900,47 @@ impl<T: Default> Default for Rc<T> {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+trait RcEqIdent<T: ?Sized + PartialEq> {
+    fn eq(&self, other: &Rc<T>) -> bool;
+    fn ne(&self, other: &Rc<T>) -> bool;
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
+    #[inline]
+    default fn eq(&self, other: &Rc<T>) -> bool {
+        **self == **other
+    }
+
+    #[inline]
+    default fn ne(&self, other: &Rc<T>) -> bool {
+        **self != **other
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
+    #[inline]
+    fn eq(&self, other: &Rc<T>) -> bool {
+        Rc::ptr_eq(self, other) || **self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &Rc<T>) -> bool {
+        !Rc::ptr_eq(self, other) && **self != **other
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
     /// Equality for two `Rc`s.
     ///
     /// Two `Rc`s are equal if their inner values are equal.
     ///
+    /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
+    /// always equal.
+    ///
     /// # Examples
     ///
     /// ```
@@ -915,15 +950,18 @@ impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
     ///
     /// assert!(five == Rc::new(5));
     /// ```
-    #[inline(always)]
+    #[inline]
     fn eq(&self, other: &Rc<T>) -> bool {
-        **self == **other
+        RcEqIdent::eq(self, other)
     }
 
     /// Inequality for two `Rc`s.
     ///
     /// Two `Rc`s are unequal if their inner values are unequal.
     ///
+    /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
+    /// never unequal.
+    ///
     /// # Examples
     ///
     /// ```
@@ -933,9 +971,9 @@ impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
     ///
     /// assert!(five != Rc::new(6));
     /// ```
-    #[inline(always)]
+    #[inline]
     fn ne(&self, other: &Rc<T>) -> bool {
-        **self != **other
+        RcEqIdent::ne(self, other)
     }
 }
 

src/liballoc/sync.rs

@@ -1287,12 +1287,46 @@ impl<T: ?Sized> Drop for Weak<T> {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+trait ArcEqIdent<T: ?Sized + PartialEq> {
+    fn eq(&self, other: &Arc<T>) -> bool;
+    fn ne(&self, other: &Arc<T>) -> bool;
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
+    #[inline]
+    default fn eq(&self, other: &Arc<T>) -> bool {
+        **self == **other
+    }
+    #[inline]
+    default fn ne(&self, other: &Arc<T>) -> bool {
+        **self != **other
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + Eq> ArcEqIdent<T> for Arc<T> {
+    #[inline]
+    fn eq(&self, other: &Arc<T>) -> bool {
+        Arc::ptr_eq(self, other) || **self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &Arc<T>) -> bool {
+        !Arc::ptr_eq(self, other) && **self != **other
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
     /// Equality for two `Arc`s.
     ///
     /// Two `Arc`s are equal if their inner values are equal.
     ///
+    /// If `T` also implements `Eq`, two `Arc`s that point to the same value are
+    /// always equal.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1302,14 +1336,18 @@ impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
     ///
     /// assert!(five == Arc::new(5));
     /// ```
+    #[inline]
     fn eq(&self, other: &Arc<T>) -> bool {
-        *(*self) == *(*other)
+        ArcEqIdent::eq(self, other)
     }
 
     /// Inequality for two `Arc`s.
     ///
     /// Two `Arc`s are unequal if their inner values are unequal.
     ///
+    /// If `T` also implements `Eq`, two `Arc`s that point to the same value are
+    /// never unequal.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1319,10 +1357,12 @@ impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
     ///
     /// assert!(five != Arc::new(6));
     /// ```
+    #[inline]
     fn ne(&self, other: &Arc<T>) -> bool {
-        *(*self) != *(*other)
+        ArcEqIdent::ne(self, other)
     }
 }
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
     /// Partial comparison for two `Arc`s.

src/liballoc/tests/arc.rs

@@ -10,6 +10,8 @@
 
 use std::any::Any;
 use std::sync::{Arc, Weak};
+use std::cell::RefCell;
+use std::cmp::PartialEq;
 
 #[test]
 fn uninhabited() {
@@ -53,3 +55,43 @@ fn trait_object() {
     b = b.clone();
     assert!(b.upgrade().is_none());
 }
+
+#[test]
+fn float_nan_ne() {
+    let x = Arc::new(std::f32::NAN);
+    assert!(x != x);
+    assert!(!(x == x));
+}
+
+#[test]
+fn partial_eq() {
+    struct TestPEq (RefCell<usize>);
+    impl PartialEq for TestPEq {
+        fn eq(&self, other: &TestPEq) -> bool {
+            *self.0.borrow_mut() += 1;
+            *other.0.borrow_mut() += 1;
+            true
+        }
+    }
+    let x = Arc::new(TestPEq(RefCell::new(0)));
+    assert!(x == x);
+    assert!(!(x != x));
+    assert_eq!(*x.0.borrow(), 4);
+}
+
+#[test]
+fn eq() {
+    #[derive(Eq)]
+    struct TestEq (RefCell<usize>);
+    impl PartialEq for TestEq {
+        fn eq(&self, other: &TestEq) -> bool {
+            *self.0.borrow_mut() += 1;
+            *other.0.borrow_mut() += 1;
+            true
+        }
+    }
+    let x = Arc::new(TestEq(RefCell::new(0)));
+    assert!(x == x);
+    assert!(!(x != x));
+    assert_eq!(*x.0.borrow(), 0);
+}

src/liballoc/tests/rc.rs

@@ -10,6 +10,8 @@
 
 use std::any::Any;
 use std::rc::{Rc, Weak};
+use std::cell::RefCell;
+use std::cmp::PartialEq;
 
 #[test]
 fn uninhabited() {
@@ -53,3 +55,43 @@ fn trait_object() {
     b = b.clone();
     assert!(b.upgrade().is_none());
 }
+
+#[test]
+fn float_nan_ne() {
+    let x = Rc::new(std::f32::NAN);
+    assert!(x != x);
+    assert!(!(x == x));
+}
+
+#[test]
+fn partial_eq() {
+    struct TestPEq (RefCell<usize>);
+    impl PartialEq for TestPEq {
+        fn eq(&self, other: &TestPEq) -> bool {
+            *self.0.borrow_mut() += 1;
+            *other.0.borrow_mut() += 1;
+            true
+        }
+    }
+    let x = Rc::new(TestPEq(RefCell::new(0)));
+    assert!(x == x);
+    assert!(!(x != x));
+    assert_eq!(*x.0.borrow(), 4);
+}
+
+#[test]
+fn eq() {
+    #[derive(Eq)]
+    struct TestEq (RefCell<usize>);
+    impl PartialEq for TestEq {
+        fn eq(&self, other: &TestEq) -> bool {
+            *self.0.borrow_mut() += 1;
+            *other.0.borrow_mut() += 1;
+            true
+        }
+    }
+    let x = Rc::new(TestEq(RefCell::new(0)));
+    assert!(x == x);
+    assert!(!(x != x));
+    assert_eq!(*x.0.borrow(), 0);
+}