auto merge of #6241 : thestinger/rust/rc, r=pcwalton

To provide a reference counted pointer type with deterministic
destruction once managed boxes are switched over to a garbage
collector. Unlike managed boxes, these can be moved instead of just
copied/cloned which is helpful for avoiding reference counts.

Needs #5601 to be fixed in order for safety to be provided without the current ugly workaround of making the pointers contain `Option<@()>` and `Option<@mut ()>` (which are just set to `None`).

@brson: r?

src/libstd/rc.rs

@@ -0,0 +1,275 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+/** Task-local reference counted smart pointers
+
+Task-local reference counted smart pointers are an alternative to managed boxes with deterministic
+destruction. They are restricted to containing `Owned` types in order to prevent cycles.
+
+*/
+
+use core::libc::{c_void, size_t, malloc, free};
+use core::unstable::intrinsics;
+
+struct RcBox<T> {
+    value: T,
+    count: uint
+}
+
+/// Immutable reference counted pointer type
+pub struct Rc<T> {
+    priv ptr: *mut RcBox<T>,
+    priv non_owned: Option<@()> // FIXME: #5601: replace with `#[non_owned]`
+}
+
+pub impl<'self, T: Owned> Rc<T> {
+    fn new(value: T) -> Rc<T> {
+        unsafe {
+            let ptr = malloc(sys::size_of::<RcBox<T>>() as size_t) as *mut RcBox<T>;
+            assert!(!ptr::is_null(ptr));
+            intrinsics::move_val_init(&mut *ptr, RcBox{value: value, count: 1});
+            Rc{ptr: ptr, non_owned: None}
+        }
+    }
+
+    #[inline(always)]
+    fn borrow(&self) -> &'self T {
+        unsafe { cast::transmute_region(&(*self.ptr).value) }
+    }
+}
+
+#[unsafe_destructor]
+impl<T: Owned> Drop for Rc<T> {
+    fn finalize(&self) {
+        unsafe {
+            (*self.ptr).count -= 1;
+            if (*self.ptr).count == 0 {
+                let mut x = intrinsics::init();
+                x <-> *self.ptr;
+                free(self.ptr as *c_void)
+            }
+        }
+    }
+}
+
+impl<T: Owned> Clone for Rc<T> {
+    #[inline]
+    fn clone(&self) -> Rc<T> {
+        unsafe {
+            (*self.ptr).count += 1;
+            Rc{ptr: self.ptr, non_owned: None}
+        }
+    }
+}
+
+#[cfg(test)]
+mod test_rc {
+    use super::*;
+
+    #[test]
+    fn test_simple() {
+        let x = Rc::new(5);
+        assert_eq!(*x.borrow(), 5);
+    }
+
+    #[test]
+    fn test_clone() {
+        let x = Rc::new(5);
+        let y = x.clone();
+        assert_eq!(*x.borrow(), 5);
+        assert_eq!(*y.borrow(), 5);
+    }
+
+    #[test]
+    fn test_destructor() {
+        let x = Rc::new(~5);
+        assert_eq!(**x.borrow(), 5);
+    }
+}
+
+#[abi = "rust-intrinsic"]
+extern "rust-intrinsic" mod rusti {
+    fn init<T>() -> T;
+}
+
+#[deriving(Eq)]
+enum Borrow {
+    Mutable,
+    Immutable,
+    Nothing
+}
+
+struct RcMutBox<T> {
+    value: T,
+    count: uint,
+    borrow: Borrow
+}
+
+/// Mutable reference counted pointer type
+pub struct RcMut<T> {
+    priv ptr: *mut RcMutBox<T>,
+    priv non_owned: Option<@mut ()> // FIXME: #5601: replace with `#[non_owned]` and `#[non_const]`
+}
+
+pub impl<'self, T: Owned> RcMut<T> {
+    fn new(value: T) -> RcMut<T> {
+        unsafe {
+            let ptr = malloc(sys::size_of::<RcMutBox<T>>() as size_t) as *mut RcMutBox<T>;
+            assert!(!ptr::is_null(ptr));
+            intrinsics::move_val_init(&mut *ptr, RcMutBox{value: value, count: 1, borrow: Nothing});
+            RcMut{ptr: ptr, non_owned: None}
+        }
+    }
+
+    /// Fails if there is already a mutable borrow of the box
+    #[inline]
+    fn with_borrow(&self, f: &fn(&T)) {
+        unsafe {
+            assert!((*self.ptr).borrow != Mutable);
+            let previous = (*self.ptr).borrow;
+            (*self.ptr).borrow = Immutable;
+            f(cast::transmute_region(&(*self.ptr).value));
+            (*self.ptr).borrow = previous;
+        }
+    }
+
+    /// Fails if there is already a mutable or immutable borrow of the box
+    #[inline]
+    fn with_mut_borrow(&self, f: &fn(&mut T)) {
+        unsafe {
+            assert!((*self.ptr).borrow == Nothing);
+            (*self.ptr).borrow = Mutable;
+            f(cast::transmute_mut_region(&mut (*self.ptr).value));
+            (*self.ptr).borrow = Nothing;
+        }
+    }
+}
+
+#[unsafe_destructor]
+impl<T: Owned> Drop for RcMut<T> {
+    fn finalize(&self) {
+        unsafe {
+            (*self.ptr).count -= 1;
+            if (*self.ptr).count == 0 {
+                let mut x = rusti::init();
+                x <-> *self.ptr;
+                free(self.ptr as *c_void)
+            }
+        }
+    }
+}
+
+impl<T: Owned> Clone for RcMut<T> {
+    #[inline]
+    fn clone(&self) -> RcMut<T> {
+        unsafe {
+            (*self.ptr).count += 1;
+            RcMut{ptr: self.ptr, non_owned: None}
+        }
+    }
+}
+
+#[cfg(test)]
+mod test_rc_mut {
+    use super::*;
+
+    #[test]
+    fn borrow_many() {
+        let x = RcMut::new(5);
+        let y = x.clone();
+
+        do x.with_borrow |a| {
+            assert_eq!(*a, 5);
+            do y.with_borrow |b| {
+                assert_eq!(*b, 5);
+                do x.with_borrow |c| {
+                    assert_eq!(*c, 5);
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn modify() {
+        let x = RcMut::new(5);
+        let y = x.clone();
+
+        do y.with_mut_borrow |a| {
+            assert_eq!(*a, 5);
+            *a = 6;
+        }
+
+        do x.with_borrow |a| {
+            assert_eq!(*a, 6);
+        }
+    }
+
+    #[test]
+    fn release_immutable() {
+        let x = RcMut::new(5);
+        do x.with_borrow |_| {}
+        do x.with_mut_borrow |_| {}
+    }
+
+    #[test]
+    fn release_mutable() {
+        let x = RcMut::new(5);
+        do x.with_mut_borrow |_| {}
+        do x.with_borrow |_| {}
+    }
+
+    #[test]
+    #[should_fail]
+    fn frozen() {
+        let x = RcMut::new(5);
+        let y = x.clone();
+
+        do x.with_borrow |_| {
+            do y.with_mut_borrow |_| {
+            }
+        }
+    }
+
+    #[test]
+    #[should_fail]
+    fn mutable_dupe() {
+        let x = RcMut::new(5);
+        let y = x.clone();
+
+        do x.with_mut_borrow |_| {
+            do y.with_mut_borrow |_| {
+            }
+        }
+    }
+
+    #[test]
+    #[should_fail]
+    fn mutable_freeze() {
+        let x = RcMut::new(5);
+        let y = x.clone();
+
+        do x.with_mut_borrow |_| {
+            do y.with_borrow |_| {
+            }
+        }
+    }
+
+    #[test]
+    #[should_fail]
+    fn restore_freeze() {
+        let x = RcMut::new(5);
+        let y = x.clone();
+
+        do x.with_borrow |_| {
+            do x.with_borrow |_| {}
+            do y.with_mut_borrow |_| {}
+        }
+    }
+}

src/libstd/std.rc

@@ -50,6 +50,7 @@ pub mod uv_global_loop;
 pub mod c_vec;
 pub mod timer;
 pub mod io_util;
+pub mod rc;
 
 // Concurrency