relax Rc restrictions and add strong/weak reference counted pointer type

src/libextra/serialize.rs

@@ -406,14 +406,14 @@ impl<S:Encoder,T:Encodable<S>> Encodable<S> for @T {
     }
 }
 
-impl<S:Encoder,T:Encodable<S> + Freeze> Encodable<S> for Rc<T> {
+impl<S:Encoder,T:Encodable<S>> Encodable<S> for Rc<T> {
     #[inline]
     fn encode(&self, s: &mut S) {
         self.borrow().encode(s)
     }
 }
 
-impl<D:Decoder,T:Decodable<D> + Freeze> Decodable<D> for Rc<T> {
+impl<D:Decoder,T:Decodable<D>> Decodable<D> for Rc<T> {
     #[inline]
     fn decode(d: &mut D) -> Rc<T> {
         Rc::new(Decodable::decode(d))

src/librustuv/idle.rs

@@ -127,7 +127,7 @@ mod test {
     }
 
     fn mk(v: uint) -> (~IdleWatcher, Chan) {
-        let rc = Rc::from_send(RefCell::new((None, 0)));
+        let rc = Rc::new(RefCell::new((None, 0)));
         let cb = ~MyCallback(rc.clone(), v);
         let cb = cb as ~Callback:;
         let cb = unsafe { cast::transmute(cb) };

src/libstd/option.rs

@@ -509,7 +509,7 @@ mod tests {
             }
         }
 
-        let i = Rc::from_send(RefCell::new(0));
+        let i = Rc::new(RefCell::new(0));
         {
             let x = R(i.clone());
             let opt = Some(x);

src/libstd/rc.rs

@@ -14,19 +14,27 @@ The `Rc` type provides shared ownership of an immutable value. Destruction is de
 will occur as soon as the last owner is gone. It is marked as non-sendable because it avoids the
 overhead of atomic reference counting.
 
+The `downgrade` method can be used to create a non-owning `Weak` pointer to the box. A `Weak`
+pointer can be upgraded to an `Rc` pointer, but will return `None` if the value has already been
+freed.
+
+For example, a tree with parent pointers can be represented by putting the nodes behind `Strong`
+pointers, and then storing the parent pointers as `Weak` pointers.
+
 */
 
-use ptr::RawPtr;
-use unstable::intrinsics::transmute;
+use cast::transmute;
 use ops::Drop;
-use kinds::{Freeze, Send};
+use cmp::{Eq, Ord};
 use clone::{Clone, DeepClone};
-use cell::RefCell;
-use cmp::{Eq, TotalEq, Ord, TotalOrd, Ordering};
+use rt::global_heap::exchange_free;
+use ptr::read_ptr;
+use option::{Option, Some, None};
 
 struct RcBox<T> {
     value: T,
-    count: uint
+    strong: uint,
+    weak: uint
 }
 
 /// Immutable reference counted pointer type
@@ -36,147 +44,141 @@ pub struct Rc<T> {
     priv ptr: *mut RcBox<T>
 }
 
-impl<T: Freeze> Rc<T> {
-    /// Construct a new reference-counted box from a `Freeze` value
-    #[inline]
+impl<T> Rc<T> {
+    /// Construct a new reference-counted box
     pub fn new(value: T) -> Rc<T> {
         unsafe {
-            Rc::new_unchecked(value)
+            Rc { ptr: transmute(~RcBox { value: value, strong: 1, weak: 0 }) }
         }
     }
 }
 
-impl<T: Send> Rc<T> {
-    /// Construct a new reference-counted box from a `Send` value
-    #[inline]
-    pub fn from_send(value: T) -> Rc<T> {
+impl<T> Rc<T> {
+    /// Borrow the value contained in the reference-counted box
+    #[inline(always)]
+    pub fn borrow<'a>(&'a self) -> &'a T {
+        unsafe { &(*self.ptr).value }
+    }
+
+    /// Downgrade the reference-counted pointer to a weak reference
+    pub fn downgrade(&self) -> Weak<T> {
         unsafe {
-            Rc::new_unchecked(value)
+            (*self.ptr).weak += 1;
+            Weak { ptr: self.ptr }
         }
     }
 }
 
-impl<T: Freeze> Rc<RefCell<T>> {
-    /// Construct a new reference-counted box from a `RefCell`-wrapped `Freeze` value
-    #[inline]
-    pub fn from_mut(value: RefCell<T>) -> Rc<RefCell<T>> {
+#[unsafe_destructor]
+impl<T> Drop for Rc<T> {
+    fn drop(&mut self) {
         unsafe {
-            Rc::new_unchecked(value)
+            if self.ptr != 0 as *mut RcBox<T> {
+                (*self.ptr).strong -= 1;
+                if (*self.ptr).strong == 0 {
+                    read_ptr(self.borrow()); // destroy the contained object
+                    if (*self.ptr).weak == 0 {
+                        exchange_free(self.ptr as *mut u8 as *i8)
+                    }
+                }
+            }
         }
     }
 }
 
-impl<T> Rc<T> {
-    /// Unsafety construct a new reference-counted box from any value.
-    ///
-    /// It is possible to create cycles, which will leak, and may interact
-    /// poorly with managed pointers.
-    #[inline]
-    pub unsafe fn new_unchecked(value: T) -> Rc<T> {
-        Rc{ptr: transmute(~RcBox{value: value, count: 1})}
-    }
-
-    /// Borrow the value contained in the reference-counted box
+impl<T> Clone for Rc<T> {
     #[inline]
-    pub fn borrow<'r>(&'r self) -> &'r T {
-        unsafe { &(*self.ptr).value }
+    fn clone(&self) -> Rc<T> {
+        unsafe {
+            (*self.ptr).strong += 1;
+            Rc { ptr: self.ptr }
+        }
     }
+}
 
-    /// Determine if two reference-counted pointers point to the same object
+impl<T: DeepClone> DeepClone for Rc<T> {
     #[inline]
-    pub fn ptr_eq(&self, other: &Rc<T>) -> bool {
-        self.ptr == other.ptr
+    fn deep_clone(&self) -> Rc<T> {
+        Rc::new(self.borrow().deep_clone())
     }
 }
 
 impl<T: Eq> Eq for Rc<T> {
-    #[inline]
-    fn eq(&self, other: &Rc<T>) -> bool {
-        unsafe { (*self.ptr).value == (*other.ptr).value }
-    }
+    #[inline(always)]
+    fn eq(&self, other: &Rc<T>) -> bool { *self.borrow() == *other.borrow() }
 
-    #[inline]
-    fn ne(&self, other: &Rc<T>) -> bool {
-        unsafe { (*self.ptr).value != (*other.ptr).value }
-    }
-}
-
-impl<T: TotalEq> TotalEq for Rc<T> {
-    #[inline]
-    fn equals(&self, other: &Rc<T>) -> bool {
-        unsafe { (*self.ptr).value.equals(&(*other.ptr).value) }
-    }
+    #[inline(always)]
+    fn ne(&self, other: &Rc<T>) -> bool { *self.borrow() != *other.borrow() }
 }
 
 impl<T: Ord> Ord for Rc<T> {
-    #[inline]
-    fn lt(&self, other: &Rc<T>) -> bool {
-        unsafe { (*self.ptr).value < (*other.ptr).value }
-    }
+    #[inline(always)]
+    fn lt(&self, other: &Rc<T>) -> bool { *self.borrow() < *other.borrow() }
 
-    #[inline]
-    fn le(&self, other: &Rc<T>) -> bool {
-        unsafe { (*self.ptr).value <= (*other.ptr).value }
-    }
+    #[inline(always)]
+    fn le(&self, other: &Rc<T>) -> bool { *self.borrow() <= *other.borrow() }
 
-    #[inline]
-    fn ge(&self, other: &Rc<T>) -> bool {
-        unsafe { (*self.ptr).value >= (*other.ptr).value }
-    }
+    #[inline(always)]
+    fn gt(&self, other: &Rc<T>) -> bool { *self.borrow() > *other.borrow() }
 
-    #[inline]
-    fn gt(&self, other: &Rc<T>) -> bool {
-        unsafe { (*self.ptr).value > (*other.ptr).value }
-    }
+    #[inline(always)]
+    fn ge(&self, other: &Rc<T>) -> bool { *self.borrow() >= *other.borrow() }
 }
 
-impl<T: TotalOrd> TotalOrd for Rc<T> {
-    #[inline]
-    fn cmp(&self, other: &Rc<T>) -> Ordering {
-        unsafe { (*self.ptr).value.cmp(&(*other.ptr).value) }
-    }
+/// Weak reference to a reference-counted box
+#[unsafe_no_drop_flag]
+#[no_send]
+pub struct Weak<T> {
+    priv ptr: *mut RcBox<T>
 }
 
-impl<T> Clone for Rc<T> {
-    #[inline]
-    fn clone(&self) -> Rc<T> {
+impl<T> Weak<T> {
+    /// Upgrade a weak reference to a strong reference
+    pub fn upgrade(&self) -> Option<Rc<T>> {
         unsafe {
-            (*self.ptr).count += 1;
-            Rc{ptr: self.ptr}
+            if (*self.ptr).strong == 0 {
+                None
+            } else {
+                (*self.ptr).strong += 1;
+                Some(Rc { ptr: self.ptr })
+            }
         }
     }
 }
 
-impl<T: DeepClone> DeepClone for Rc<T> {
-    #[inline]
-    fn deep_clone(&self) -> Rc<T> {
-        unsafe { Rc::new_unchecked(self.borrow().deep_clone()) }
-    }
-}
-
 #[unsafe_destructor]
-impl<T> Drop for Rc<T> {
+impl<T> Drop for Weak<T> {
     fn drop(&mut self) {
         unsafe {
-            if self.ptr.is_not_null() {
-                (*self.ptr).count -= 1;
-                if (*self.ptr).count == 0 {
-                    let _: ~RcBox<T> = transmute(self.ptr);
+            if self.ptr != 0 as *mut RcBox<T> {
+                (*self.ptr).weak -= 1;
+                if (*self.ptr).weak == 0 && (*self.ptr).strong == 0 {
+                    exchange_free(self.ptr as *mut u8 as *i8)
                 }
             }
         }
     }
 }
 
+impl<T> Clone for Weak<T> {
+    #[inline]
+    fn clone(&self) -> Weak<T> {
+        unsafe {
+            (*self.ptr).weak += 1;
+            Weak { ptr: self.ptr }
+        }
+    }
+}
+
 #[cfg(test)]
-mod test_rc {
+mod tests {
     use prelude::*;
     use super::*;
     use cell::RefCell;
 
     #[test]
     fn test_clone() {
-        let x = Rc::from_send(RefCell::new(5));
+        let x = Rc::new(RefCell::new(5));
         let y = x.clone();
         x.borrow().with_mut(|inner| {
             *inner = 20;
@@ -186,7 +188,7 @@ mod test_rc {
 
     #[test]
     fn test_deep_clone() {
-        let x = Rc::from_send(RefCell::new(5));
+        let x = Rc::new(RefCell::new(5));
         let y = x.deep_clone();
         x.borrow().with_mut(|inner| {
             *inner = 20;
@@ -210,13 +212,22 @@ mod test_rc {
 
     #[test]
     fn test_destructor() {
-        let x = Rc::from_send(~5);
+        let x = Rc::new(~5);
         assert_eq!(**x.borrow(), 5);
     }
 
     #[test]
-    fn test_from_mut() {
-        let a = 10;
-        let _x = Rc::from_mut(RefCell::new(&a));
+    fn test_live() {
+        let x = Rc::new(5);
+        let y = x.downgrade();
+        assert!(y.upgrade().is_some());
+    }
+
+    #[test]
+    fn test_dead() {
+        let x = Rc::new(5);
+        let y = x.downgrade();
+        drop(x);
+        assert!(y.upgrade().is_none());
     }
 }

src/test/compile-fail/issue-7013.rs

@@ -37,7 +37,7 @@ struct A
 fn main()
 {
     let a = A {v: ~B{v: None} as ~Foo}; //~ ERROR cannot pack type `~B`, which does not fulfill `Send`
-    let v = Rc::from_send(RefCell::new(a));
+    let v = Rc::new(RefCell::new(a));
     let w = v.clone();
     let b = v.borrow();
     let mut b = b.borrow_mut();

src/test/compile-fail/no_freeze-rc.rs

@@ -14,6 +14,6 @@ use std::cell::RefCell;
 fn bar<T: Freeze>(_: T) {}
 
 fn main() {
-    let x = Rc::from_send(RefCell::new(5));
+    let x = Rc::new(RefCell::new(5));
     bar(x); //~ ERROR instantiating a type parameter with an incompatible type `std::rc::Rc<std::cell::RefCell<int>>`, which does not fulfill `Freeze`
 }