1 // Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
13 //! Single-threaded reference-counting pointers. 'Rc' stands for 'Reference
16 //! The type [`Rc<T>`][`Rc`] provides shared ownership of a value of type `T`,
17 //! allocated in the heap. Invoking [`clone`][clone] on [`Rc`] produces a new
18 //! pointer to the same value in the heap. When the last [`Rc`] pointer to a
19 //! given value is destroyed, the pointed-to value is also destroyed.
21 //! Shared references in Rust disallow mutation by default, and [`Rc`]
22 //! is no exception: you cannot generally obtain a mutable reference to
23 //! something inside an [`Rc`]. If you need mutability, put a [`Cell`]
24 //! or [`RefCell`] inside the [`Rc`]; see [an example of mutability
25 //! inside an Rc][mutability].
27 //! [`Rc`] uses non-atomic reference counting. This means that overhead is very
28 //! low, but an [`Rc`] cannot be sent between threads, and consequently [`Rc`]
29 //! does not implement [`Send`][send]. As a result, the Rust compiler
30 //! will check *at compile time* that you are not sending [`Rc`]s between
31 //! threads. If you need multi-threaded, atomic reference counting, use
32 //! [`sync::Arc`][arc].
34 //! The [`downgrade`][downgrade] method can be used to create a non-owning
35 //! [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
36 //! to an [`Rc`], but this will return [`None`] if the value has
37 //! already been dropped.
39 //! A cycle between [`Rc`] pointers will never be deallocated. For this reason,
40 //! [`Weak`] is used to break cycles. For example, a tree could have strong
41 //! [`Rc`] pointers from parent nodes to children, and [`Weak`] pointers from
42 //! children back to their parents.
44 //! `Rc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
45 //! so you can call `T`'s methods on a value of type [`Rc<T>`][`Rc`]. To avoid name
46 //! clashes with `T`'s methods, the methods of [`Rc<T>`][`Rc`] itself are [associated
47 //! functions][assoc], called using function-like syntax:
51 //! let my_rc = Rc::new(());
53 //! Rc::downgrade(&my_rc);
56 //! [`Weak<T>`][`Weak`] does not auto-dereference to `T`, because the value may have
57 //! already been destroyed.
59 //! # Cloning references
61 //! Creating a new reference from an existing reference counted pointer is done using the
62 //! `Clone` trait implemented for [`Rc<T>`][`Rc`] and [`Weak<T>`][`Weak`].
66 //! let foo = Rc::new(vec![1.0, 2.0, 3.0]);
67 //! // The two syntaxes below are equivalent.
68 //! let a = foo.clone();
69 //! let b = Rc::clone(&foo);
70 //! // a and b both point to the same memory location as foo.
73 //! The `Rc::clone(&from)` syntax is the most idiomatic because it conveys more explicitly
74 //! the meaning of the code. In the example above, this syntax makes it easier to see that
75 //! this code is creating a new reference rather than copying the whole content of foo.
79 //! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`.
80 //! We want to have our `Gadget`s point to their `Owner`. We can't do this with
81 //! unique ownership, because more than one gadget may belong to the same
82 //! `Owner`. [`Rc`] allows us to share an `Owner` between multiple `Gadget`s,
83 //! and have the `Owner` remain allocated as long as any `Gadget` points at it.
90 //! // ...other fields
96 //! // ...other fields
100 //! // Create a reference-counted `Owner`.
101 //! let gadget_owner: Rc<Owner> = Rc::new(
103 //! name: "Gadget Man".to_string(),
107 //! // Create `Gadget`s belonging to `gadget_owner`. Cloning the `Rc<Owner>`
108 //! // value gives us a new pointer to the same `Owner` value, incrementing
109 //! // the reference count in the process.
110 //! let gadget1 = Gadget {
112 //! owner: Rc::clone(&gadget_owner),
114 //! let gadget2 = Gadget {
116 //! owner: Rc::clone(&gadget_owner),
119 //! // Dispose of our local variable `gadget_owner`.
120 //! drop(gadget_owner);
122 //! // Despite dropping `gadget_owner`, we're still able to print out the name
123 //! // of the `Owner` of the `Gadget`s. This is because we've only dropped a
124 //! // single `Rc<Owner>`, not the `Owner` it points to. As long as there are
125 //! // other `Rc<Owner>` values pointing at the same `Owner`, it will remain
126 //! // allocated. The field projection `gadget1.owner.name` works because
127 //! // `Rc<Owner>` automatically dereferences to `Owner`.
128 //! println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name);
129 //! println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name);
131 //! // At the end of the function, `gadget1` and `gadget2` are destroyed, and
132 //! // with them the last counted references to our `Owner`. Gadget Man now
133 //! // gets destroyed as well.
137 //! If our requirements change, and we also need to be able to traverse from
138 //! `Owner` to `Gadget`, we will run into problems. An [`Rc`] pointer from `Owner`
139 //! to `Gadget` introduces a cycle between the values. This means that their
140 //! reference counts can never reach 0, and the values will remain allocated
141 //! forever: a memory leak. In order to get around this, we can use [`Weak`]
144 //! Rust actually makes it somewhat difficult to produce this loop in the first
145 //! place. In order to end up with two values that point at each other, one of
146 //! them needs to be mutable. This is difficult because [`Rc`] enforces
147 //! memory safety by only giving out shared references to the value it wraps,
148 //! and these don't allow direct mutation. We need to wrap the part of the
149 //! value we wish to mutate in a [`RefCell`], which provides *interior
150 //! mutability*: a method to achieve mutability through a shared reference.
151 //! [`RefCell`] enforces Rust's borrowing rules at runtime.
155 //! use std::rc::Weak;
156 //! use std::cell::RefCell;
160 //! gadgets: RefCell<Vec<Weak<Gadget>>>,
161 //! // ...other fields
166 //! owner: Rc<Owner>,
167 //! // ...other fields
171 //! // Create a reference-counted `Owner`. Note that we've put the `Owner`'s
172 //! // vector of `Gadget`s inside a `RefCell` so that we can mutate it through
173 //! // a shared reference.
174 //! let gadget_owner: Rc<Owner> = Rc::new(
176 //! name: "Gadget Man".to_string(),
177 //! gadgets: RefCell::new(vec![]),
181 //! // Create `Gadget`s belonging to `gadget_owner`, as before.
182 //! let gadget1 = Rc::new(
185 //! owner: Rc::clone(&gadget_owner),
188 //! let gadget2 = Rc::new(
191 //! owner: Rc::clone(&gadget_owner),
195 //! // Add the `Gadget`s to their `Owner`.
197 //! let mut gadgets = gadget_owner.gadgets.borrow_mut();
198 //! gadgets.push(Rc::downgrade(&gadget1));
199 //! gadgets.push(Rc::downgrade(&gadget2));
201 //! // `RefCell` dynamic borrow ends here.
204 //! // Iterate over our `Gadget`s, printing their details out.
205 //! for gadget_weak in gadget_owner.gadgets.borrow().iter() {
207 //! // `gadget_weak` is a `Weak<Gadget>`. Since `Weak` pointers can't
208 //! // guarantee the value is still allocated, we need to call
209 //! // `upgrade`, which returns an `Option<Rc<Gadget>>`.
211 //! // In this case we know the value still exists, so we simply
212 //! // `unwrap` the `Option`. In a more complicated program, you might
213 //! // need graceful error handling for a `None` result.
215 //! let gadget = gadget_weak.upgrade().unwrap();
216 //! println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
219 //! // At the end of the function, `gadget_owner`, `gadget1`, and `gadget2`
220 //! // are destroyed. There are now no strong (`Rc`) pointers to the
221 //! // gadgets, so they are destroyed. This zeroes the reference count on
222 //! // Gadget Man, so he gets destroyed as well.
226 //! [`Rc`]: struct.Rc.html
227 //! [`Weak`]: struct.Weak.html
228 //! [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
229 //! [`Cell`]: ../../std/cell/struct.Cell.html
230 //! [`RefCell`]: ../../std/cell/struct.RefCell.html
231 //! [send]: ../../std/marker/trait.Send.html
232 //! [arc]: ../../std/sync/struct.Arc.html
233 //! [`Deref`]: ../../std/ops/trait.Deref.html
234 //! [downgrade]: struct.Rc.html#method.downgrade
235 //! [upgrade]: struct.Weak.html#method.upgrade
236 //! [`None`]: ../../std/option/enum.Option.html#variant.None
237 //! [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
238 //! [mutability]: ../../std/cell/index.html#introducing-mutability-inside-of-something-immutable
240 #![stable(feature = "rust1", since = "1.0.0")]
249 use core::cell::Cell;
250 use core::cmp::Ordering;
252 use core::hash::{Hash, Hasher};
253 use core::intrinsics::abort;
255 use core::marker::{Unsize, PhantomData};
256 use core::mem::{self, align_of_val, forget, size_of_val, uninitialized};
257 use core::ops::Deref;
258 use core::ops::CoerceUnsized;
259 use core::ptr::{self, NonNull};
260 use core::convert::From;
262 use heap::{Heap, Alloc, Layout, box_free};
266 struct RcBox<T: ?Sized> {
272 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
275 /// See the [module-level documentation](./index.html) for more details.
277 /// The inherent methods of `Rc` are all associated functions, which means
278 /// that you have to call them as e.g. [`Rc::get_mut(&mut value)`][get_mut] instead of
279 /// `value.get_mut()`. This avoids conflicts with methods of the inner
282 /// [get_mut]: #method.get_mut
283 #[stable(feature = "rust1", since = "1.0.0")]
284 pub struct Rc<T: ?Sized> {
285 ptr: NonNull<RcBox<T>>,
286 phantom: PhantomData<T>,
289 #[stable(feature = "rust1", since = "1.0.0")]
290 impl<T: ?Sized> !marker::Send for Rc<T> {}
291 #[stable(feature = "rust1", since = "1.0.0")]
292 impl<T: ?Sized> !marker::Sync for Rc<T> {}
294 #[unstable(feature = "coerce_unsized", issue = "27732")]
295 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
298 /// Constructs a new `Rc<T>`.
305 /// let five = Rc::new(5);
307 #[stable(feature = "rust1", since = "1.0.0")]
308 pub fn new(value: T) -> Rc<T> {
310 // there is an implicit weak pointer owned by all the strong
311 // pointers, which ensures that the weak destructor never frees
312 // the allocation while the strong destructor is running, even
313 // if the weak pointer is stored inside the strong one.
314 ptr: Box::into_nonnull_raw(box RcBox {
315 strong: Cell::new(1),
319 phantom: PhantomData,
323 /// Returns the contained value, if the `Rc` has exactly one strong reference.
325 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
328 /// This will succeed even if there are outstanding weak references.
330 /// [result]: ../../std/result/enum.Result.html
337 /// let x = Rc::new(3);
338 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
340 /// let x = Rc::new(4);
341 /// let _y = Rc::clone(&x);
342 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
345 #[stable(feature = "rc_unique", since = "1.4.0")]
346 pub fn try_unwrap(this: Self) -> Result<T, Self> {
347 if Rc::strong_count(&this) == 1 {
349 let val = ptr::read(&*this); // copy the contained object
351 // Indicate to Weaks that they can't be promoted by decrementing
352 // the strong count, and then remove the implicit "strong weak"
353 // pointer while also handling drop logic by just crafting a
356 let _weak = Weak { ptr: this.ptr };
366 impl<T: ?Sized> Rc<T> {
367 /// Consumes the `Rc`, returning the wrapped pointer.
369 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
370 /// [`Rc::from_raw`][from_raw].
372 /// [from_raw]: struct.Rc.html#method.from_raw
379 /// let x = Rc::new(10);
380 /// let x_ptr = Rc::into_raw(x);
381 /// assert_eq!(unsafe { *x_ptr }, 10);
383 #[stable(feature = "rc_raw", since = "1.17.0")]
384 pub fn into_raw(this: Self) -> *const T {
385 let ptr: *const T = &*this;
390 /// Constructs an `Rc` from a raw pointer.
392 /// The raw pointer must have been previously returned by a call to a
393 /// [`Rc::into_raw`][into_raw].
395 /// This function is unsafe because improper use may lead to memory problems. For example, a
396 /// double-free may occur if the function is called twice on the same raw pointer.
398 /// [into_raw]: struct.Rc.html#method.into_raw
405 /// let x = Rc::new(10);
406 /// let x_ptr = Rc::into_raw(x);
409 /// // Convert back to an `Rc` to prevent leak.
410 /// let x = Rc::from_raw(x_ptr);
411 /// assert_eq!(*x, 10);
413 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
416 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
418 #[stable(feature = "rc_raw", since = "1.17.0")]
419 pub unsafe fn from_raw(ptr: *const T) -> Self {
420 // Align the unsized value to the end of the RcBox.
421 // Because it is ?Sized, it will always be the last field in memory.
422 let align = align_of_val(&*ptr);
423 let layout = Layout::new::<RcBox<()>>();
424 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
426 // Reverse the offset to find the original RcBox.
427 let fake_ptr = ptr as *mut RcBox<T>;
428 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
431 ptr: NonNull::new_unchecked(rc_ptr),
432 phantom: PhantomData,
436 /// Creates a new [`Weak`][weak] pointer to this value.
438 /// [weak]: struct.Weak.html
445 /// let five = Rc::new(5);
447 /// let weak_five = Rc::downgrade(&five);
449 #[stable(feature = "rc_weak", since = "1.4.0")]
450 pub fn downgrade(this: &Self) -> Weak<T> {
452 Weak { ptr: this.ptr }
455 /// Gets the number of [`Weak`][weak] pointers to this value.
457 /// [weak]: struct.Weak.html
464 /// let five = Rc::new(5);
465 /// let _weak_five = Rc::downgrade(&five);
467 /// assert_eq!(1, Rc::weak_count(&five));
470 #[stable(feature = "rc_counts", since = "1.15.0")]
471 pub fn weak_count(this: &Self) -> usize {
475 /// Gets the number of strong (`Rc`) pointers to this value.
482 /// let five = Rc::new(5);
483 /// let _also_five = Rc::clone(&five);
485 /// assert_eq!(2, Rc::strong_count(&five));
488 #[stable(feature = "rc_counts", since = "1.15.0")]
489 pub fn strong_count(this: &Self) -> usize {
493 /// Returns true if there are no other `Rc` or [`Weak`][weak] pointers to
494 /// this inner value.
496 /// [weak]: struct.Weak.html
498 fn is_unique(this: &Self) -> bool {
499 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
502 /// Returns a mutable reference to the inner value, if there are
503 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
505 /// Returns [`None`] otherwise, because it is not safe to
506 /// mutate a shared value.
508 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
509 /// the inner value when it's shared.
511 /// [weak]: struct.Weak.html
512 /// [`None`]: ../../std/option/enum.Option.html#variant.None
513 /// [make_mut]: struct.Rc.html#method.make_mut
514 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
521 /// let mut x = Rc::new(3);
522 /// *Rc::get_mut(&mut x).unwrap() = 4;
523 /// assert_eq!(*x, 4);
525 /// let _y = Rc::clone(&x);
526 /// assert!(Rc::get_mut(&mut x).is_none());
529 #[stable(feature = "rc_unique", since = "1.4.0")]
530 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
531 if Rc::is_unique(this) {
533 Some(&mut this.ptr.as_mut().value)
541 #[stable(feature = "ptr_eq", since = "1.17.0")]
542 /// Returns true if the two `Rc`s point to the same value (not
543 /// just values that compare as equal).
550 /// let five = Rc::new(5);
551 /// let same_five = Rc::clone(&five);
552 /// let other_five = Rc::new(5);
554 /// assert!(Rc::ptr_eq(&five, &same_five));
555 /// assert!(!Rc::ptr_eq(&five, &other_five));
557 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
558 this.ptr.as_ptr() == other.ptr.as_ptr()
562 impl<T: Clone> Rc<T> {
563 /// Makes a mutable reference into the given `Rc`.
565 /// If there are other `Rc` or [`Weak`][weak] pointers to the same value,
566 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
567 /// ensure unique ownership. This is also referred to as clone-on-write.
569 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
571 /// [weak]: struct.Weak.html
572 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
573 /// [get_mut]: struct.Rc.html#method.get_mut
580 /// let mut data = Rc::new(5);
582 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
583 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
584 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
585 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
586 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
588 /// // Now `data` and `other_data` point to different values.
589 /// assert_eq!(*data, 8);
590 /// assert_eq!(*other_data, 12);
593 #[stable(feature = "rc_unique", since = "1.4.0")]
594 pub fn make_mut(this: &mut Self) -> &mut T {
595 if Rc::strong_count(this) != 1 {
596 // Gotta clone the data, there are other Rcs
597 *this = Rc::new((**this).clone())
598 } else if Rc::weak_count(this) != 0 {
599 // Can just steal the data, all that's left is Weaks
601 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
602 mem::swap(this, &mut swap);
604 // Remove implicit strong-weak ref (no need to craft a fake
605 // Weak here -- we know other Weaks can clean up for us)
610 // This unsafety is ok because we're guaranteed that the pointer
611 // returned is the *only* pointer that will ever be returned to T. Our
612 // reference count is guaranteed to be 1 at this point, and we required
613 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
614 // reference to the inner value.
616 &mut this.ptr.as_mut().value
623 #[unstable(feature = "rc_downcast", issue = "44608")]
624 /// Attempt to downcast the `Rc<Any>` to a concrete type.
629 /// #![feature(rc_downcast)]
630 /// use std::any::Any;
633 /// fn print_if_string(value: Rc<Any>) {
634 /// if let Ok(string) = value.downcast::<String>() {
635 /// println!("String ({}): {}", string.len(), string);
640 /// let my_string = "Hello World".to_string();
641 /// print_if_string(Rc::new(my_string));
642 /// print_if_string(Rc::new(0i8));
645 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<Any>> {
646 if (*self).is::<T>() {
647 // avoid the pointer arithmetic in from_raw
649 let raw: *const RcBox<Any> = self.ptr.as_ptr();
652 ptr: NonNull::new_unchecked(raw as *const RcBox<T> as *mut _),
653 phantom: PhantomData,
662 impl<T: ?Sized> Rc<T> {
663 // Allocates an `RcBox<T>` with sufficient space for an unsized value
664 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
665 // Create a fake RcBox to find allocation size and alignment
666 let fake_ptr = ptr as *mut RcBox<T>;
668 let layout = Layout::for_value(&*fake_ptr);
670 let mem = Heap.alloc(layout)
671 .unwrap_or_else(|e| Heap.oom(e));
673 // Initialize the real RcBox
674 let inner = set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>;
676 ptr::write(&mut (*inner).strong, Cell::new(1));
677 ptr::write(&mut (*inner).weak, Cell::new(1));
682 fn from_box(v: Box<T>) -> Rc<T> {
684 let bptr = Box::into_raw(v);
686 let value_size = size_of_val(&*bptr);
687 let ptr = Self::allocate_for_ptr(bptr);
689 // Copy value as bytes
690 ptr::copy_nonoverlapping(
691 bptr as *const T as *const u8,
692 &mut (*ptr).value as *mut _ as *mut u8,
695 // Free the allocation without dropping its contents
698 Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
703 // Sets the data pointer of a `?Sized` raw pointer.
705 // For a slice/trait object, this sets the `data` field and leaves the rest
706 // unchanged. For a sized raw pointer, this simply sets the pointer.
707 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
708 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
713 // Copy elements from slice into newly allocated Rc<[T]>
715 // Unsafe because the caller must either take ownership or bind `T: Copy`
716 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
717 let v_ptr = v as *const [T];
718 let ptr = Self::allocate_for_ptr(v_ptr);
720 ptr::copy_nonoverlapping(
722 &mut (*ptr).value as *mut [T] as *mut T,
725 Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
729 trait RcFromSlice<T> {
730 fn from_slice(slice: &[T]) -> Self;
733 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
735 default fn from_slice(v: &[T]) -> Self {
736 // Panic guard while cloning T elements.
737 // In the event of a panic, elements that have been written
738 // into the new RcBox will be dropped, then the memory freed.
746 impl<T> Drop for Guard<T> {
748 use core::slice::from_raw_parts_mut;
751 let slice = from_raw_parts_mut(self.elems, self.n_elems);
752 ptr::drop_in_place(slice);
754 Heap.dealloc(self.mem, self.layout.clone());
760 let v_ptr = v as *const [T];
761 let ptr = Self::allocate_for_ptr(v_ptr);
763 let mem = ptr as *mut _ as *mut u8;
764 let layout = Layout::for_value(&*ptr);
766 // Pointer to first element
767 let elems = &mut (*ptr).value as *mut [T] as *mut T;
769 let mut guard = Guard{
776 for (i, item) in v.iter().enumerate() {
777 ptr::write(elems.offset(i as isize), item.clone());
781 // All clear. Forget the guard so it doesn't free the new RcBox.
784 Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
789 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
791 fn from_slice(v: &[T]) -> Self {
792 unsafe { Rc::copy_from_slice(v) }
796 #[stable(feature = "rust1", since = "1.0.0")]
797 impl<T: ?Sized> Deref for Rc<T> {
801 fn deref(&self) -> &T {
806 #[stable(feature = "rust1", since = "1.0.0")]
807 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
810 /// This will decrement the strong reference count. If the strong reference
811 /// count reaches zero then the only other references (if any) are
812 /// [`Weak`][weak], so we `drop` the inner value.
814 /// [weak]: struct.Weak.html
823 /// impl Drop for Foo {
824 /// fn drop(&mut self) {
825 /// println!("dropped!");
829 /// let foo = Rc::new(Foo);
830 /// let foo2 = Rc::clone(&foo);
832 /// drop(foo); // Doesn't print anything
833 /// drop(foo2); // Prints "dropped!"
837 let ptr = self.ptr.as_ptr();
840 if self.strong() == 0 {
841 // destroy the contained object
842 ptr::drop_in_place(self.ptr.as_mut());
844 // remove the implicit "strong weak" pointer now that we've
845 // destroyed the contents.
848 if self.weak() == 0 {
849 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr));
856 #[stable(feature = "rust1", since = "1.0.0")]
857 impl<T: ?Sized> Clone for Rc<T> {
858 /// Makes a clone of the `Rc` pointer.
860 /// This creates another pointer to the same inner value, increasing the
861 /// strong reference count.
868 /// let five = Rc::new(5);
870 /// Rc::clone(&five);
873 fn clone(&self) -> Rc<T> {
875 Rc { ptr: self.ptr, phantom: PhantomData }
879 #[stable(feature = "rust1", since = "1.0.0")]
880 impl<T: Default> Default for Rc<T> {
881 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
888 /// let x: Rc<i32> = Default::default();
889 /// assert_eq!(*x, 0);
892 fn default() -> Rc<T> {
893 Rc::new(Default::default())
897 #[stable(feature = "rust1", since = "1.0.0")]
898 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
899 /// Equality for two `Rc`s.
901 /// Two `Rc`s are equal if their inner values are equal.
908 /// let five = Rc::new(5);
910 /// assert!(five == Rc::new(5));
913 fn eq(&self, other: &Rc<T>) -> bool {
917 /// Inequality for two `Rc`s.
919 /// Two `Rc`s are unequal if their inner values are unequal.
926 /// let five = Rc::new(5);
928 /// assert!(five != Rc::new(6));
931 fn ne(&self, other: &Rc<T>) -> bool {
936 #[stable(feature = "rust1", since = "1.0.0")]
937 impl<T: ?Sized + Eq> Eq for Rc<T> {}
939 #[stable(feature = "rust1", since = "1.0.0")]
940 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
941 /// Partial comparison for two `Rc`s.
943 /// The two are compared by calling `partial_cmp()` on their inner values.
949 /// use std::cmp::Ordering;
951 /// let five = Rc::new(5);
953 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
956 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
957 (**self).partial_cmp(&**other)
960 /// Less-than comparison for two `Rc`s.
962 /// The two are compared by calling `<` on their inner values.
969 /// let five = Rc::new(5);
971 /// assert!(five < Rc::new(6));
974 fn lt(&self, other: &Rc<T>) -> bool {
978 /// 'Less than or equal to' comparison for two `Rc`s.
980 /// The two are compared by calling `<=` on their inner values.
987 /// let five = Rc::new(5);
989 /// assert!(five <= Rc::new(5));
992 fn le(&self, other: &Rc<T>) -> bool {
996 /// Greater-than comparison for two `Rc`s.
998 /// The two are compared by calling `>` on their inner values.
1003 /// use std::rc::Rc;
1005 /// let five = Rc::new(5);
1007 /// assert!(five > Rc::new(4));
1010 fn gt(&self, other: &Rc<T>) -> bool {
1014 /// 'Greater than or equal to' comparison for two `Rc`s.
1016 /// The two are compared by calling `>=` on their inner values.
1021 /// use std::rc::Rc;
1023 /// let five = Rc::new(5);
1025 /// assert!(five >= Rc::new(5));
1028 fn ge(&self, other: &Rc<T>) -> bool {
1033 #[stable(feature = "rust1", since = "1.0.0")]
1034 impl<T: ?Sized + Ord> Ord for Rc<T> {
1035 /// Comparison for two `Rc`s.
1037 /// The two are compared by calling `cmp()` on their inner values.
1042 /// use std::rc::Rc;
1043 /// use std::cmp::Ordering;
1045 /// let five = Rc::new(5);
1047 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1050 fn cmp(&self, other: &Rc<T>) -> Ordering {
1051 (**self).cmp(&**other)
1055 #[stable(feature = "rust1", since = "1.0.0")]
1056 impl<T: ?Sized + Hash> Hash for Rc<T> {
1057 fn hash<H: Hasher>(&self, state: &mut H) {
1058 (**self).hash(state);
1062 #[stable(feature = "rust1", since = "1.0.0")]
1063 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1064 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1065 fmt::Display::fmt(&**self, f)
1069 #[stable(feature = "rust1", since = "1.0.0")]
1070 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1071 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1072 fmt::Debug::fmt(&**self, f)
1076 #[stable(feature = "rust1", since = "1.0.0")]
1077 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1078 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1079 fmt::Pointer::fmt(&(&**self as *const T), f)
1083 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1084 impl<T> From<T> for Rc<T> {
1085 fn from(t: T) -> Self {
1090 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1091 impl<'a, T: Clone> From<&'a [T]> for Rc<[T]> {
1093 fn from(v: &[T]) -> Rc<[T]> {
1094 <Self as RcFromSlice<T>>::from_slice(v)
1098 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1099 impl<'a> From<&'a str> for Rc<str> {
1101 fn from(v: &str) -> Rc<str> {
1102 let rc = Rc::<[u8]>::from(v.as_bytes());
1103 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1107 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1108 impl From<String> for Rc<str> {
1110 fn from(v: String) -> Rc<str> {
1115 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1116 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1118 fn from(v: Box<T>) -> Rc<T> {
1123 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1124 impl<T> From<Vec<T>> for Rc<[T]> {
1126 fn from(mut v: Vec<T>) -> Rc<[T]> {
1128 let rc = Rc::copy_from_slice(&v);
1130 // Allow the Vec to free its memory, but not destroy its contents
1138 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1139 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
1140 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1142 /// Since a `Weak` reference does not count towards ownership, it will not
1143 /// prevent the inner value from being dropped, and `Weak` itself makes no
1144 /// guarantees about the value still being present and may return [`None`]
1145 /// when [`upgrade`]d.
1147 /// A `Weak` pointer is useful for keeping a temporary reference to the value
1148 /// within [`Rc`] without extending its lifetime. It is also used to prevent
1149 /// circular references between [`Rc`] pointers, since mutual owning references
1150 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1151 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1152 /// pointers from children back to their parents.
1154 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1156 /// [`Rc`]: struct.Rc.html
1157 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1158 /// [`upgrade`]: struct.Weak.html#method.upgrade
1159 /// [`Option`]: ../../std/option/enum.Option.html
1160 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1161 #[stable(feature = "rc_weak", since = "1.4.0")]
1162 pub struct Weak<T: ?Sized> {
1163 ptr: NonNull<RcBox<T>>,
1166 #[stable(feature = "rc_weak", since = "1.4.0")]
1167 impl<T: ?Sized> !marker::Send for Weak<T> {}
1168 #[stable(feature = "rc_weak", since = "1.4.0")]
1169 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1171 #[unstable(feature = "coerce_unsized", issue = "27732")]
1172 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1175 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1176 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1178 /// [`upgrade`]: struct.Weak.html#method.upgrade
1179 /// [`None`]: ../../std/option/enum.Option.html
1184 /// use std::rc::Weak;
1186 /// let empty: Weak<i64> = Weak::new();
1187 /// assert!(empty.upgrade().is_none());
1189 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1190 pub fn new() -> Weak<T> {
1193 ptr: Box::into_nonnull_raw(box RcBox {
1194 strong: Cell::new(0),
1196 value: uninitialized(),
1203 impl<T: ?Sized> Weak<T> {
1204 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1205 /// the lifetime of the value if successful.
1207 /// Returns [`None`] if the value has since been dropped.
1209 /// [`Rc`]: struct.Rc.html
1210 /// [`None`]: ../../std/option/enum.Option.html
1215 /// use std::rc::Rc;
1217 /// let five = Rc::new(5);
1219 /// let weak_five = Rc::downgrade(&five);
1221 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1222 /// assert!(strong_five.is_some());
1224 /// // Destroy all strong pointers.
1225 /// drop(strong_five);
1228 /// assert!(weak_five.upgrade().is_none());
1230 #[stable(feature = "rc_weak", since = "1.4.0")]
1231 pub fn upgrade(&self) -> Option<Rc<T>> {
1232 if self.strong() == 0 {
1236 Some(Rc { ptr: self.ptr, phantom: PhantomData })
1241 #[stable(feature = "rc_weak", since = "1.4.0")]
1242 impl<T: ?Sized> Drop for Weak<T> {
1243 /// Drops the `Weak` pointer.
1248 /// use std::rc::{Rc, Weak};
1252 /// impl Drop for Foo {
1253 /// fn drop(&mut self) {
1254 /// println!("dropped!");
1258 /// let foo = Rc::new(Foo);
1259 /// let weak_foo = Rc::downgrade(&foo);
1260 /// let other_weak_foo = Weak::clone(&weak_foo);
1262 /// drop(weak_foo); // Doesn't print anything
1263 /// drop(foo); // Prints "dropped!"
1265 /// assert!(other_weak_foo.upgrade().is_none());
1267 fn drop(&mut self) {
1269 let ptr = self.ptr.as_ptr();
1272 // the weak count starts at 1, and will only go to zero if all
1273 // the strong pointers have disappeared.
1274 if self.weak() == 0 {
1275 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr));
1281 #[stable(feature = "rc_weak", since = "1.4.0")]
1282 impl<T: ?Sized> Clone for Weak<T> {
1283 /// Makes a clone of the `Weak` pointer that points to the same value.
1288 /// use std::rc::{Rc, Weak};
1290 /// let weak_five = Rc::downgrade(&Rc::new(5));
1292 /// Weak::clone(&weak_five);
1295 fn clone(&self) -> Weak<T> {
1297 Weak { ptr: self.ptr }
1301 #[stable(feature = "rc_weak", since = "1.4.0")]
1302 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1303 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1308 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1309 impl<T> Default for Weak<T> {
1310 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1311 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1313 /// [`upgrade`]: struct.Weak.html#method.upgrade
1314 /// [`None`]: ../../std/option/enum.Option.html
1319 /// use std::rc::Weak;
1321 /// let empty: Weak<i64> = Default::default();
1322 /// assert!(empty.upgrade().is_none());
1324 fn default() -> Weak<T> {
1329 // NOTE: We checked_add here to deal with mem::forget safety. In particular
1330 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1331 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1332 // We abort because this is such a degenerate scenario that we don't care about
1333 // what happens -- no real program should ever experience this.
1335 // This should have negligible overhead since you don't actually need to
1336 // clone these much in Rust thanks to ownership and move-semantics.
1339 trait RcBoxPtr<T: ?Sized> {
1340 fn inner(&self) -> &RcBox<T>;
1343 fn strong(&self) -> usize {
1344 self.inner().strong.get()
1348 fn inc_strong(&self) {
1349 self.inner().strong.set(self.strong().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1353 fn dec_strong(&self) {
1354 self.inner().strong.set(self.strong() - 1);
1358 fn weak(&self) -> usize {
1359 self.inner().weak.get()
1363 fn inc_weak(&self) {
1364 self.inner().weak.set(self.weak().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1368 fn dec_weak(&self) {
1369 self.inner().weak.set(self.weak() - 1);
1373 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1375 fn inner(&self) -> &RcBox<T> {
1382 impl<T: ?Sized> RcBoxPtr<T> for Weak<T> {
1384 fn inner(&self) -> &RcBox<T> {
1393 use super::{Rc, Weak};
1394 use std::boxed::Box;
1395 use std::cell::RefCell;
1396 use std::option::Option;
1397 use std::option::Option::{None, Some};
1398 use std::result::Result::{Err, Ok};
1400 use std::clone::Clone;
1401 use std::convert::From;
1405 let x = Rc::new(RefCell::new(5));
1407 *x.borrow_mut() = 20;
1408 assert_eq!(*y.borrow(), 20);
1418 fn test_simple_clone() {
1426 fn test_destructor() {
1427 let x: Rc<Box<_>> = Rc::new(box 5);
1434 let y = Rc::downgrade(&x);
1435 assert!(y.upgrade().is_some());
1441 let y = Rc::downgrade(&x);
1443 assert!(y.upgrade().is_none());
1447 fn weak_self_cyclic() {
1449 x: RefCell<Option<Weak<Cycle>>>,
1452 let a = Rc::new(Cycle { x: RefCell::new(None) });
1453 let b = Rc::downgrade(&a.clone());
1454 *a.x.borrow_mut() = Some(b);
1456 // hopefully we don't double-free (or leak)...
1462 assert!(Rc::is_unique(&x));
1464 assert!(!Rc::is_unique(&x));
1466 assert!(Rc::is_unique(&x));
1467 let w = Rc::downgrade(&x);
1468 assert!(!Rc::is_unique(&x));
1470 assert!(Rc::is_unique(&x));
1474 fn test_strong_count() {
1476 assert!(Rc::strong_count(&a) == 1);
1477 let w = Rc::downgrade(&a);
1478 assert!(Rc::strong_count(&a) == 1);
1479 let b = w.upgrade().expect("upgrade of live rc failed");
1480 assert!(Rc::strong_count(&b) == 2);
1481 assert!(Rc::strong_count(&a) == 2);
1484 assert!(Rc::strong_count(&b) == 1);
1486 assert!(Rc::strong_count(&b) == 2);
1487 assert!(Rc::strong_count(&c) == 2);
1491 fn test_weak_count() {
1493 assert!(Rc::strong_count(&a) == 1);
1494 assert!(Rc::weak_count(&a) == 0);
1495 let w = Rc::downgrade(&a);
1496 assert!(Rc::strong_count(&a) == 1);
1497 assert!(Rc::weak_count(&a) == 1);
1499 assert!(Rc::strong_count(&a) == 1);
1500 assert!(Rc::weak_count(&a) == 0);
1502 assert!(Rc::strong_count(&a) == 2);
1503 assert!(Rc::weak_count(&a) == 0);
1510 assert_eq!(Rc::try_unwrap(x), Ok(3));
1513 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
1515 let _w = Rc::downgrade(&x);
1516 assert_eq!(Rc::try_unwrap(x), Ok(5));
1520 fn into_from_raw() {
1521 let x = Rc::new(box "hello");
1524 let x_ptr = Rc::into_raw(x);
1527 assert_eq!(**x_ptr, "hello");
1529 let x = Rc::from_raw(x_ptr);
1530 assert_eq!(**x, "hello");
1532 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
1537 fn test_into_from_raw_unsized() {
1538 use std::fmt::Display;
1539 use std::string::ToString;
1541 let rc: Rc<str> = Rc::from("foo");
1543 let ptr = Rc::into_raw(rc.clone());
1544 let rc2 = unsafe { Rc::from_raw(ptr) };
1546 assert_eq!(unsafe { &*ptr }, "foo");
1547 assert_eq!(rc, rc2);
1549 let rc: Rc<Display> = Rc::new(123);
1551 let ptr = Rc::into_raw(rc.clone());
1552 let rc2 = unsafe { Rc::from_raw(ptr) };
1554 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1555 assert_eq!(rc2.to_string(), "123");
1560 let mut x = Rc::new(3);
1561 *Rc::get_mut(&mut x).unwrap() = 4;
1564 assert!(Rc::get_mut(&mut x).is_none());
1566 assert!(Rc::get_mut(&mut x).is_some());
1567 let _w = Rc::downgrade(&x);
1568 assert!(Rc::get_mut(&mut x).is_none());
1572 fn test_cowrc_clone_make_unique() {
1573 let mut cow0 = Rc::new(75);
1574 let mut cow1 = cow0.clone();
1575 let mut cow2 = cow1.clone();
1577 assert!(75 == *Rc::make_mut(&mut cow0));
1578 assert!(75 == *Rc::make_mut(&mut cow1));
1579 assert!(75 == *Rc::make_mut(&mut cow2));
1581 *Rc::make_mut(&mut cow0) += 1;
1582 *Rc::make_mut(&mut cow1) += 2;
1583 *Rc::make_mut(&mut cow2) += 3;
1585 assert!(76 == *cow0);
1586 assert!(77 == *cow1);
1587 assert!(78 == *cow2);
1589 // none should point to the same backing memory
1590 assert!(*cow0 != *cow1);
1591 assert!(*cow0 != *cow2);
1592 assert!(*cow1 != *cow2);
1596 fn test_cowrc_clone_unique2() {
1597 let mut cow0 = Rc::new(75);
1598 let cow1 = cow0.clone();
1599 let cow2 = cow1.clone();
1601 assert!(75 == *cow0);
1602 assert!(75 == *cow1);
1603 assert!(75 == *cow2);
1605 *Rc::make_mut(&mut cow0) += 1;
1607 assert!(76 == *cow0);
1608 assert!(75 == *cow1);
1609 assert!(75 == *cow2);
1611 // cow1 and cow2 should share the same contents
1612 // cow0 should have a unique reference
1613 assert!(*cow0 != *cow1);
1614 assert!(*cow0 != *cow2);
1615 assert!(*cow1 == *cow2);
1619 fn test_cowrc_clone_weak() {
1620 let mut cow0 = Rc::new(75);
1621 let cow1_weak = Rc::downgrade(&cow0);
1623 assert!(75 == *cow0);
1624 assert!(75 == *cow1_weak.upgrade().unwrap());
1626 *Rc::make_mut(&mut cow0) += 1;
1628 assert!(76 == *cow0);
1629 assert!(cow1_weak.upgrade().is_none());
1634 let foo = Rc::new(75);
1635 assert_eq!(format!("{:?}", foo), "75");
1640 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
1641 assert_eq!(foo, foo.clone());
1645 fn test_from_owned() {
1647 let foo_rc = Rc::from(foo);
1648 assert!(123 == *foo_rc);
1652 fn test_new_weak() {
1653 let foo: Weak<usize> = Weak::new();
1654 assert!(foo.upgrade().is_none());
1659 let five = Rc::new(5);
1660 let same_five = five.clone();
1661 let other_five = Rc::new(5);
1663 assert!(Rc::ptr_eq(&five, &same_five));
1664 assert!(!Rc::ptr_eq(&five, &other_five));
1668 fn test_from_str() {
1669 let r: Rc<str> = Rc::from("foo");
1671 assert_eq!(&r[..], "foo");
1675 fn test_copy_from_slice() {
1676 let s: &[u32] = &[1, 2, 3];
1677 let r: Rc<[u32]> = Rc::from(s);
1679 assert_eq!(&r[..], [1, 2, 3]);
1683 fn test_clone_from_slice() {
1684 #[derive(Clone, Debug, Eq, PartialEq)]
1687 let s: &[X] = &[X(1), X(2), X(3)];
1688 let r: Rc<[X]> = Rc::from(s);
1690 assert_eq!(&r[..], s);
1695 fn test_clone_from_slice_panic() {
1696 use std::string::{String, ToString};
1698 struct Fail(u32, String);
1700 impl Clone for Fail {
1701 fn clone(&self) -> Fail {
1705 Fail(self.0, self.1.clone())
1710 Fail(0, "foo".to_string()),
1711 Fail(1, "bar".to_string()),
1712 Fail(2, "baz".to_string()),
1715 // Should panic, but not cause memory corruption
1716 let _r: Rc<[Fail]> = Rc::from(s);
1720 fn test_from_box() {
1721 let b: Box<u32> = box 123;
1722 let r: Rc<u32> = Rc::from(b);
1724 assert_eq!(*r, 123);
1728 fn test_from_box_str() {
1729 use std::string::String;
1731 let s = String::from("foo").into_boxed_str();
1732 let r: Rc<str> = Rc::from(s);
1734 assert_eq!(&r[..], "foo");
1738 fn test_from_box_slice() {
1739 let s = vec![1, 2, 3].into_boxed_slice();
1740 let r: Rc<[u32]> = Rc::from(s);
1742 assert_eq!(&r[..], [1, 2, 3]);
1746 fn test_from_box_trait() {
1747 use std::fmt::Display;
1748 use std::string::ToString;
1750 let b: Box<Display> = box 123;
1751 let r: Rc<Display> = Rc::from(b);
1753 assert_eq!(r.to_string(), "123");
1757 fn test_from_box_trait_zero_sized() {
1758 use std::fmt::Debug;
1760 let b: Box<Debug> = box ();
1761 let r: Rc<Debug> = Rc::from(b);
1763 assert_eq!(format!("{:?}", r), "()");
1767 fn test_from_vec() {
1768 let v = vec![1, 2, 3];
1769 let r: Rc<[u32]> = Rc::from(v);
1771 assert_eq!(&r[..], [1, 2, 3]);
1775 fn test_downcast() {
1778 let r1: Rc<Any> = Rc::new(i32::max_value());
1779 let r2: Rc<Any> = Rc::new("abc");
1781 assert!(r1.clone().downcast::<u32>().is_err());
1783 let r1i32 = r1.downcast::<i32>();
1784 assert!(r1i32.is_ok());
1785 assert_eq!(r1i32.unwrap(), Rc::new(i32::max_value()));
1787 assert!(r2.clone().downcast::<i32>().is_err());
1789 let r2str = r2.downcast::<&'static str>();
1790 assert!(r2str.is_ok());
1791 assert_eq!(r2str.unwrap(), Rc::new("abc"));
1795 #[stable(feature = "rust1", since = "1.0.0")]
1796 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
1797 fn borrow(&self) -> &T {
1802 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1803 impl<T: ?Sized> AsRef<T> for Rc<T> {
1804 fn as_ref(&self) -> &T {