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;
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, Shared};
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: Shared<RcBox<T>>,
288 #[stable(feature = "rust1", since = "1.0.0")]
289 impl<T: ?Sized> !marker::Send for Rc<T> {}
290 #[stable(feature = "rust1", since = "1.0.0")]
291 impl<T: ?Sized> !marker::Sync for Rc<T> {}
293 #[unstable(feature = "coerce_unsized", issue = "27732")]
294 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
297 /// Constructs a new `Rc<T>`.
304 /// let five = Rc::new(5);
306 #[stable(feature = "rust1", since = "1.0.0")]
307 pub fn new(value: T) -> Rc<T> {
309 // there is an implicit weak pointer owned by all the strong
310 // pointers, which ensures that the weak destructor never frees
311 // the allocation while the strong destructor is running, even
312 // if the weak pointer is stored inside the strong one.
313 ptr: Shared::from(Box::into_unique(box RcBox {
314 strong: Cell::new(1),
321 /// Returns the contained value, if the `Rc` has exactly one strong reference.
323 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
326 /// This will succeed even if there are outstanding weak references.
328 /// [result]: ../../std/result/enum.Result.html
335 /// let x = Rc::new(3);
336 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
338 /// let x = Rc::new(4);
339 /// let _y = Rc::clone(&x);
340 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
343 #[stable(feature = "rc_unique", since = "1.4.0")]
344 pub fn try_unwrap(this: Self) -> Result<T, Self> {
345 if Rc::strong_count(&this) == 1 {
347 let val = ptr::read(&*this); // copy the contained object
349 // Indicate to Weaks that they can't be promoted by decrememting
350 // the strong count, and then remove the implicit "strong weak"
351 // pointer while also handling drop logic by just crafting a
354 let _weak = Weak { ptr: this.ptr };
364 impl<T: ?Sized> Rc<T> {
365 /// Consumes the `Rc`, returning the wrapped pointer.
367 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
368 /// [`Rc::from_raw`][from_raw].
370 /// [from_raw]: struct.Rc.html#method.from_raw
377 /// let x = Rc::new(10);
378 /// let x_ptr = Rc::into_raw(x);
379 /// assert_eq!(unsafe { *x_ptr }, 10);
381 #[stable(feature = "rc_raw", since = "1.17.0")]
382 pub fn into_raw(this: Self) -> *const T {
383 let ptr: *const T = &*this;
388 /// Constructs an `Rc` from a raw pointer.
390 /// The raw pointer must have been previously returned by a call to a
391 /// [`Rc::into_raw`][into_raw].
393 /// This function is unsafe because improper use may lead to memory problems. For example, a
394 /// double-free may occur if the function is called twice on the same raw pointer.
396 /// [into_raw]: struct.Rc.html#method.into_raw
403 /// let x = Rc::new(10);
404 /// let x_ptr = Rc::into_raw(x);
407 /// // Convert back to an `Rc` to prevent leak.
408 /// let x = Rc::from_raw(x_ptr);
409 /// assert_eq!(*x, 10);
411 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
414 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
416 #[stable(feature = "rc_raw", since = "1.17.0")]
417 pub unsafe fn from_raw(ptr: *const T) -> Self {
418 // Align the unsized value to the end of the RcBox.
419 // Because it is ?Sized, it will always be the last field in memory.
420 let align = align_of_val(&*ptr);
421 let layout = Layout::new::<RcBox<()>>();
422 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
424 // Reverse the offset to find the original RcBox.
425 let fake_ptr = ptr as *mut RcBox<T>;
426 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
429 ptr: Shared::new_unchecked(rc_ptr),
433 /// Creates a new [`Weak`][weak] pointer to this value.
435 /// [weak]: struct.Weak.html
442 /// let five = Rc::new(5);
444 /// let weak_five = Rc::downgrade(&five);
446 #[stable(feature = "rc_weak", since = "1.4.0")]
447 pub fn downgrade(this: &Self) -> Weak<T> {
449 Weak { ptr: this.ptr }
452 /// Gets the number of [`Weak`][weak] pointers to this value.
454 /// [weak]: struct.Weak.html
461 /// let five = Rc::new(5);
462 /// let _weak_five = Rc::downgrade(&five);
464 /// assert_eq!(1, Rc::weak_count(&five));
467 #[stable(feature = "rc_counts", since = "1.15.0")]
468 pub fn weak_count(this: &Self) -> usize {
472 /// Gets the number of strong (`Rc`) pointers to this value.
479 /// let five = Rc::new(5);
480 /// let _also_five = Rc::clone(&five);
482 /// assert_eq!(2, Rc::strong_count(&five));
485 #[stable(feature = "rc_counts", since = "1.15.0")]
486 pub fn strong_count(this: &Self) -> usize {
490 /// Returns true if there are no other `Rc` or [`Weak`][weak] pointers to
491 /// this inner value.
493 /// [weak]: struct.Weak.html
495 fn is_unique(this: &Self) -> bool {
496 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
499 /// Returns a mutable reference to the inner value, if there are
500 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
502 /// Returns [`None`] otherwise, because it is not safe to
503 /// mutate a shared value.
505 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
506 /// the inner value when it's shared.
508 /// [weak]: struct.Weak.html
509 /// [`None`]: ../../std/option/enum.Option.html#variant.None
510 /// [make_mut]: struct.Rc.html#method.make_mut
511 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
518 /// let mut x = Rc::new(3);
519 /// *Rc::get_mut(&mut x).unwrap() = 4;
520 /// assert_eq!(*x, 4);
522 /// let _y = Rc::clone(&x);
523 /// assert!(Rc::get_mut(&mut x).is_none());
526 #[stable(feature = "rc_unique", since = "1.4.0")]
527 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
528 if Rc::is_unique(this) {
530 Some(&mut this.ptr.as_mut().value)
538 #[stable(feature = "ptr_eq", since = "1.17.0")]
539 /// Returns true if the two `Rc`s point to the same value (not
540 /// just values that compare as equal).
547 /// let five = Rc::new(5);
548 /// let same_five = Rc::clone(&five);
549 /// let other_five = Rc::new(5);
551 /// assert!(Rc::ptr_eq(&five, &same_five));
552 /// assert!(!Rc::ptr_eq(&five, &other_five));
554 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
555 this.ptr.as_ptr() == other.ptr.as_ptr()
559 impl<T: Clone> Rc<T> {
560 /// Makes a mutable reference into the given `Rc`.
562 /// If there are other `Rc` or [`Weak`][weak] pointers to the same value,
563 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
564 /// ensure unique ownership. This is also referred to as clone-on-write.
566 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
568 /// [weak]: struct.Weak.html
569 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
570 /// [get_mut]: struct.Rc.html#method.get_mut
577 /// let mut data = Rc::new(5);
579 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
580 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
581 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
582 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
583 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
585 /// // Now `data` and `other_data` point to different values.
586 /// assert_eq!(*data, 8);
587 /// assert_eq!(*other_data, 12);
590 #[stable(feature = "rc_unique", since = "1.4.0")]
591 pub fn make_mut(this: &mut Self) -> &mut T {
592 if Rc::strong_count(this) != 1 {
593 // Gotta clone the data, there are other Rcs
594 *this = Rc::new((**this).clone())
595 } else if Rc::weak_count(this) != 0 {
596 // Can just steal the data, all that's left is Weaks
598 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
599 mem::swap(this, &mut swap);
601 // Remove implicit strong-weak ref (no need to craft a fake
602 // Weak here -- we know other Weaks can clean up for us)
607 // This unsafety is ok because we're guaranteed that the pointer
608 // returned is the *only* pointer that will ever be returned to T. Our
609 // reference count is guaranteed to be 1 at this point, and we required
610 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
611 // reference to the inner value.
613 &mut this.ptr.as_mut().value
620 #[unstable(feature = "rc_downcast", issue = "44608")]
621 /// Attempt to downcast the `Rc<Any>` to a concrete type.
626 /// #![feature(rc_downcast)]
627 /// use std::any::Any;
630 /// fn print_if_string(value: Rc<Any>) {
631 /// if let Ok(string) = value.downcast::<String>() {
632 /// println!("String ({}): {}", string.len(), string);
637 /// let my_string = "Hello World".to_string();
638 /// print_if_string(Rc::new(my_string));
639 /// print_if_string(Rc::new(0i8));
642 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<Any>> {
643 if (*self).is::<T>() {
644 // avoid the pointer arithmetic in from_raw
646 let raw: *const RcBox<Any> = self.ptr.as_ptr();
649 ptr: Shared::new_unchecked(raw as *const RcBox<T> as *mut _),
658 impl<T: ?Sized> Rc<T> {
659 // Allocates an `RcBox<T>` with sufficient space for an unsized value
660 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
661 // Create a fake RcBox to find allocation size and alignment
662 let fake_ptr = ptr as *mut RcBox<T>;
664 let layout = Layout::for_value(&*fake_ptr);
666 let mem = Heap.alloc(layout)
667 .unwrap_or_else(|e| Heap.oom(e));
669 // Initialize the real RcBox
670 let inner = set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>;
672 ptr::write(&mut (*inner).strong, Cell::new(1));
673 ptr::write(&mut (*inner).weak, Cell::new(1));
678 fn from_box(v: Box<T>) -> Rc<T> {
680 let bptr = Box::into_raw(v);
682 let value_size = size_of_val(&*bptr);
683 let ptr = Self::allocate_for_ptr(bptr);
685 // Copy value as bytes
686 ptr::copy_nonoverlapping(
687 bptr as *const T as *const u8,
688 &mut (*ptr).value as *mut _ as *mut u8,
691 // Free the allocation without dropping its contents
694 Rc { ptr: Shared::new_unchecked(ptr) }
699 // Sets the data pointer of a `?Sized` raw pointer.
701 // For a slice/trait object, this sets the `data` field and leaves the rest
702 // unchanged. For a sized raw pointer, this simply sets the pointer.
703 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
704 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
709 // Copy elements from slice into newly allocated Rc<[T]>
711 // Unsafe because the caller must either take ownership or bind `T: Copy`
712 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
713 let v_ptr = v as *const [T];
714 let ptr = Self::allocate_for_ptr(v_ptr);
716 ptr::copy_nonoverlapping(
718 &mut (*ptr).value as *mut [T] as *mut T,
721 Rc { ptr: Shared::new_unchecked(ptr) }
725 trait RcFromSlice<T> {
726 fn from_slice(slice: &[T]) -> Self;
729 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
731 default fn from_slice(v: &[T]) -> Self {
732 // Panic guard while cloning T elements.
733 // In the event of a panic, elements that have been written
734 // into the new RcBox will be dropped, then the memory freed.
742 impl<T> Drop for Guard<T> {
744 use core::slice::from_raw_parts_mut;
747 let slice = from_raw_parts_mut(self.elems, self.n_elems);
748 ptr::drop_in_place(slice);
750 Heap.dealloc(self.mem, self.layout.clone());
756 let v_ptr = v as *const [T];
757 let ptr = Self::allocate_for_ptr(v_ptr);
759 let mem = ptr as *mut _ as *mut u8;
760 let layout = Layout::for_value(&*ptr);
762 // Pointer to first element
763 let elems = &mut (*ptr).value as *mut [T] as *mut T;
765 let mut guard = Guard{
772 for (i, item) in v.iter().enumerate() {
773 ptr::write(elems.offset(i as isize), item.clone());
777 // All clear. Forget the guard so it doesn't free the new RcBox.
780 Rc { ptr: Shared::new_unchecked(ptr) }
785 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
787 fn from_slice(v: &[T]) -> Self {
788 unsafe { Rc::copy_from_slice(v) }
792 #[stable(feature = "rust1", since = "1.0.0")]
793 impl<T: ?Sized> Deref for Rc<T> {
797 fn deref(&self) -> &T {
802 #[stable(feature = "rust1", since = "1.0.0")]
803 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
806 /// This will decrement the strong reference count. If the strong reference
807 /// count reaches zero then the only other references (if any) are
808 /// [`Weak`][weak], so we `drop` the inner value.
810 /// [weak]: struct.Weak.html
819 /// impl Drop for Foo {
820 /// fn drop(&mut self) {
821 /// println!("dropped!");
825 /// let foo = Rc::new(Foo);
826 /// let foo2 = Rc::clone(&foo);
828 /// drop(foo); // Doesn't print anything
829 /// drop(foo2); // Prints "dropped!"
833 let ptr = self.ptr.as_ptr();
836 if self.strong() == 0 {
837 // destroy the contained object
838 ptr::drop_in_place(self.ptr.as_mut());
840 // remove the implicit "strong weak" pointer now that we've
841 // destroyed the contents.
844 if self.weak() == 0 {
845 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr));
852 #[stable(feature = "rust1", since = "1.0.0")]
853 impl<T: ?Sized> Clone for Rc<T> {
854 /// Makes a clone of the `Rc` pointer.
856 /// This creates another pointer to the same inner value, increasing the
857 /// strong reference count.
864 /// let five = Rc::new(5);
866 /// Rc::clone(&five);
869 fn clone(&self) -> Rc<T> {
875 #[stable(feature = "rust1", since = "1.0.0")]
876 impl<T: Default> Default for Rc<T> {
877 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
884 /// let x: Rc<i32> = Default::default();
885 /// assert_eq!(*x, 0);
888 fn default() -> Rc<T> {
889 Rc::new(Default::default())
893 #[stable(feature = "rust1", since = "1.0.0")]
894 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
895 /// Equality for two `Rc`s.
897 /// Two `Rc`s are equal if their inner values are equal.
904 /// let five = Rc::new(5);
906 /// assert!(five == Rc::new(5));
909 fn eq(&self, other: &Rc<T>) -> bool {
913 /// Inequality for two `Rc`s.
915 /// Two `Rc`s are unequal if their inner values are unequal.
922 /// let five = Rc::new(5);
924 /// assert!(five != Rc::new(6));
927 fn ne(&self, other: &Rc<T>) -> bool {
932 #[stable(feature = "rust1", since = "1.0.0")]
933 impl<T: ?Sized + Eq> Eq for Rc<T> {}
935 #[stable(feature = "rust1", since = "1.0.0")]
936 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
937 /// Partial comparison for two `Rc`s.
939 /// The two are compared by calling `partial_cmp()` on their inner values.
945 /// use std::cmp::Ordering;
947 /// let five = Rc::new(5);
949 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
952 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
953 (**self).partial_cmp(&**other)
956 /// Less-than comparison for two `Rc`s.
958 /// The two are compared by calling `<` on their inner values.
965 /// let five = Rc::new(5);
967 /// assert!(five < Rc::new(6));
970 fn lt(&self, other: &Rc<T>) -> bool {
974 /// 'Less than or equal to' comparison for two `Rc`s.
976 /// The two are compared by calling `<=` on their inner values.
983 /// let five = Rc::new(5);
985 /// assert!(five <= Rc::new(5));
988 fn le(&self, other: &Rc<T>) -> bool {
992 /// Greater-than comparison for two `Rc`s.
994 /// The two are compared by calling `>` on their inner values.
1001 /// let five = Rc::new(5);
1003 /// assert!(five > Rc::new(4));
1006 fn gt(&self, other: &Rc<T>) -> bool {
1010 /// 'Greater than or equal to' comparison for two `Rc`s.
1012 /// The two are compared by calling `>=` on their inner values.
1017 /// use std::rc::Rc;
1019 /// let five = Rc::new(5);
1021 /// assert!(five >= Rc::new(5));
1024 fn ge(&self, other: &Rc<T>) -> bool {
1029 #[stable(feature = "rust1", since = "1.0.0")]
1030 impl<T: ?Sized + Ord> Ord for Rc<T> {
1031 /// Comparison for two `Rc`s.
1033 /// The two are compared by calling `cmp()` on their inner values.
1038 /// use std::rc::Rc;
1039 /// use std::cmp::Ordering;
1041 /// let five = Rc::new(5);
1043 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1046 fn cmp(&self, other: &Rc<T>) -> Ordering {
1047 (**self).cmp(&**other)
1051 #[stable(feature = "rust1", since = "1.0.0")]
1052 impl<T: ?Sized + Hash> Hash for Rc<T> {
1053 fn hash<H: Hasher>(&self, state: &mut H) {
1054 (**self).hash(state);
1058 #[stable(feature = "rust1", since = "1.0.0")]
1059 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1060 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1061 fmt::Display::fmt(&**self, f)
1065 #[stable(feature = "rust1", since = "1.0.0")]
1066 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1067 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1068 fmt::Debug::fmt(&**self, f)
1072 #[stable(feature = "rust1", since = "1.0.0")]
1073 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1074 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1075 fmt::Pointer::fmt(&self.ptr, f)
1079 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1080 impl<T> From<T> for Rc<T> {
1081 fn from(t: T) -> Self {
1086 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1087 impl<'a, T: Clone> From<&'a [T]> for Rc<[T]> {
1089 fn from(v: &[T]) -> Rc<[T]> {
1090 <Self as RcFromSlice<T>>::from_slice(v)
1094 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1095 impl<'a> From<&'a str> for Rc<str> {
1097 fn from(v: &str) -> Rc<str> {
1098 unsafe { mem::transmute(<Rc<[u8]>>::from(v.as_bytes())) }
1102 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1103 impl From<String> for Rc<str> {
1105 fn from(v: String) -> Rc<str> {
1110 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1111 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1113 fn from(v: Box<T>) -> Rc<T> {
1118 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1119 impl<T> From<Vec<T>> for Rc<[T]> {
1121 fn from(mut v: Vec<T>) -> Rc<[T]> {
1123 let rc = Rc::copy_from_slice(&v);
1125 // Allow the Vec to free its memory, but not destroy its contents
1133 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1134 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
1135 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1137 /// Since a `Weak` reference does not count towards ownership, it will not
1138 /// prevent the inner value from being dropped, and `Weak` itself makes no
1139 /// guarantees about the value still being present and may return [`None`]
1140 /// when [`upgrade`]d.
1142 /// A `Weak` pointer is useful for keeping a temporary reference to the value
1143 /// within [`Rc`] without extending its lifetime. It is also used to prevent
1144 /// circular references between [`Rc`] pointers, since mutual owning references
1145 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1146 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1147 /// pointers from children back to their parents.
1149 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1151 /// [`Rc`]: struct.Rc.html
1152 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1153 /// [`upgrade`]: struct.Weak.html#method.upgrade
1154 /// [`Option`]: ../../std/option/enum.Option.html
1155 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1156 #[stable(feature = "rc_weak", since = "1.4.0")]
1157 pub struct Weak<T: ?Sized> {
1158 ptr: Shared<RcBox<T>>,
1161 #[stable(feature = "rc_weak", since = "1.4.0")]
1162 impl<T: ?Sized> !marker::Send for Weak<T> {}
1163 #[stable(feature = "rc_weak", since = "1.4.0")]
1164 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1166 #[unstable(feature = "coerce_unsized", issue = "27732")]
1167 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1170 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1171 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1173 /// [`upgrade`]: struct.Weak.html#method.upgrade
1174 /// [`None`]: ../../std/option/enum.Option.html
1179 /// use std::rc::Weak;
1181 /// let empty: Weak<i64> = Weak::new();
1182 /// assert!(empty.upgrade().is_none());
1184 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1185 pub fn new() -> Weak<T> {
1188 ptr: Shared::from(Box::into_unique(box RcBox {
1189 strong: Cell::new(0),
1191 value: uninitialized(),
1198 impl<T: ?Sized> Weak<T> {
1199 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1200 /// the lifetime of the value if successful.
1202 /// Returns [`None`] if the value has since been dropped.
1204 /// [`Rc`]: struct.Rc.html
1205 /// [`None`]: ../../std/option/enum.Option.html
1210 /// use std::rc::Rc;
1212 /// let five = Rc::new(5);
1214 /// let weak_five = Rc::downgrade(&five);
1216 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1217 /// assert!(strong_five.is_some());
1219 /// // Destroy all strong pointers.
1220 /// drop(strong_five);
1223 /// assert!(weak_five.upgrade().is_none());
1225 #[stable(feature = "rc_weak", since = "1.4.0")]
1226 pub fn upgrade(&self) -> Option<Rc<T>> {
1227 if self.strong() == 0 {
1231 Some(Rc { ptr: self.ptr })
1236 #[stable(feature = "rc_weak", since = "1.4.0")]
1237 impl<T: ?Sized> Drop for Weak<T> {
1238 /// Drops the `Weak` pointer.
1243 /// use std::rc::{Rc, Weak};
1247 /// impl Drop for Foo {
1248 /// fn drop(&mut self) {
1249 /// println!("dropped!");
1253 /// let foo = Rc::new(Foo);
1254 /// let weak_foo = Rc::downgrade(&foo);
1255 /// let other_weak_foo = Weak::clone(&weak_foo);
1257 /// drop(weak_foo); // Doesn't print anything
1258 /// drop(foo); // Prints "dropped!"
1260 /// assert!(other_weak_foo.upgrade().is_none());
1262 fn drop(&mut self) {
1264 let ptr = self.ptr.as_ptr();
1267 // the weak count starts at 1, and will only go to zero if all
1268 // the strong pointers have disappeared.
1269 if self.weak() == 0 {
1270 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr));
1276 #[stable(feature = "rc_weak", since = "1.4.0")]
1277 impl<T: ?Sized> Clone for Weak<T> {
1278 /// Makes a clone of the `Weak` pointer that points to the same value.
1283 /// use std::rc::{Rc, Weak};
1285 /// let weak_five = Rc::downgrade(&Rc::new(5));
1287 /// Weak::clone(&weak_five);
1290 fn clone(&self) -> Weak<T> {
1292 Weak { ptr: self.ptr }
1296 #[stable(feature = "rc_weak", since = "1.4.0")]
1297 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1298 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1303 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1304 impl<T> Default for Weak<T> {
1305 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1306 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1308 /// [`upgrade`]: struct.Weak.html#method.upgrade
1309 /// [`None`]: ../../std/option/enum.Option.html
1314 /// use std::rc::Weak;
1316 /// let empty: Weak<i64> = Default::default();
1317 /// assert!(empty.upgrade().is_none());
1319 fn default() -> Weak<T> {
1324 // NOTE: We checked_add here to deal with mem::forget safety. In particular
1325 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1326 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1327 // We abort because this is such a degenerate scenario that we don't care about
1328 // what happens -- no real program should ever experience this.
1330 // This should have negligible overhead since you don't actually need to
1331 // clone these much in Rust thanks to ownership and move-semantics.
1334 trait RcBoxPtr<T: ?Sized> {
1335 fn inner(&self) -> &RcBox<T>;
1338 fn strong(&self) -> usize {
1339 self.inner().strong.get()
1343 fn inc_strong(&self) {
1344 self.inner().strong.set(self.strong().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1348 fn dec_strong(&self) {
1349 self.inner().strong.set(self.strong() - 1);
1353 fn weak(&self) -> usize {
1354 self.inner().weak.get()
1358 fn inc_weak(&self) {
1359 self.inner().weak.set(self.weak().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1363 fn dec_weak(&self) {
1364 self.inner().weak.set(self.weak() - 1);
1368 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1370 fn inner(&self) -> &RcBox<T> {
1377 impl<T: ?Sized> RcBoxPtr<T> for Weak<T> {
1379 fn inner(&self) -> &RcBox<T> {
1388 use super::{Rc, Weak};
1389 use std::boxed::Box;
1390 use std::cell::RefCell;
1391 use std::option::Option;
1392 use std::option::Option::{None, Some};
1393 use std::result::Result::{Err, Ok};
1395 use std::clone::Clone;
1396 use std::convert::From;
1400 let x = Rc::new(RefCell::new(5));
1402 *x.borrow_mut() = 20;
1403 assert_eq!(*y.borrow(), 20);
1413 fn test_simple_clone() {
1421 fn test_destructor() {
1422 let x: Rc<Box<_>> = Rc::new(box 5);
1429 let y = Rc::downgrade(&x);
1430 assert!(y.upgrade().is_some());
1436 let y = Rc::downgrade(&x);
1438 assert!(y.upgrade().is_none());
1442 fn weak_self_cyclic() {
1444 x: RefCell<Option<Weak<Cycle>>>,
1447 let a = Rc::new(Cycle { x: RefCell::new(None) });
1448 let b = Rc::downgrade(&a.clone());
1449 *a.x.borrow_mut() = Some(b);
1451 // hopefully we don't double-free (or leak)...
1457 assert!(Rc::is_unique(&x));
1459 assert!(!Rc::is_unique(&x));
1461 assert!(Rc::is_unique(&x));
1462 let w = Rc::downgrade(&x);
1463 assert!(!Rc::is_unique(&x));
1465 assert!(Rc::is_unique(&x));
1469 fn test_strong_count() {
1471 assert!(Rc::strong_count(&a) == 1);
1472 let w = Rc::downgrade(&a);
1473 assert!(Rc::strong_count(&a) == 1);
1474 let b = w.upgrade().expect("upgrade of live rc failed");
1475 assert!(Rc::strong_count(&b) == 2);
1476 assert!(Rc::strong_count(&a) == 2);
1479 assert!(Rc::strong_count(&b) == 1);
1481 assert!(Rc::strong_count(&b) == 2);
1482 assert!(Rc::strong_count(&c) == 2);
1486 fn test_weak_count() {
1488 assert!(Rc::strong_count(&a) == 1);
1489 assert!(Rc::weak_count(&a) == 0);
1490 let w = Rc::downgrade(&a);
1491 assert!(Rc::strong_count(&a) == 1);
1492 assert!(Rc::weak_count(&a) == 1);
1494 assert!(Rc::strong_count(&a) == 1);
1495 assert!(Rc::weak_count(&a) == 0);
1497 assert!(Rc::strong_count(&a) == 2);
1498 assert!(Rc::weak_count(&a) == 0);
1505 assert_eq!(Rc::try_unwrap(x), Ok(3));
1508 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
1510 let _w = Rc::downgrade(&x);
1511 assert_eq!(Rc::try_unwrap(x), Ok(5));
1515 fn into_from_raw() {
1516 let x = Rc::new(box "hello");
1519 let x_ptr = Rc::into_raw(x);
1522 assert_eq!(**x_ptr, "hello");
1524 let x = Rc::from_raw(x_ptr);
1525 assert_eq!(**x, "hello");
1527 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
1532 fn test_into_from_raw_unsized() {
1533 use std::fmt::Display;
1534 use std::string::ToString;
1536 let rc: Rc<str> = Rc::from("foo");
1538 let ptr = Rc::into_raw(rc.clone());
1539 let rc2 = unsafe { Rc::from_raw(ptr) };
1541 assert_eq!(unsafe { &*ptr }, "foo");
1542 assert_eq!(rc, rc2);
1544 let rc: Rc<Display> = Rc::new(123);
1546 let ptr = Rc::into_raw(rc.clone());
1547 let rc2 = unsafe { Rc::from_raw(ptr) };
1549 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1550 assert_eq!(rc2.to_string(), "123");
1555 let mut x = Rc::new(3);
1556 *Rc::get_mut(&mut x).unwrap() = 4;
1559 assert!(Rc::get_mut(&mut x).is_none());
1561 assert!(Rc::get_mut(&mut x).is_some());
1562 let _w = Rc::downgrade(&x);
1563 assert!(Rc::get_mut(&mut x).is_none());
1567 fn test_cowrc_clone_make_unique() {
1568 let mut cow0 = Rc::new(75);
1569 let mut cow1 = cow0.clone();
1570 let mut cow2 = cow1.clone();
1572 assert!(75 == *Rc::make_mut(&mut cow0));
1573 assert!(75 == *Rc::make_mut(&mut cow1));
1574 assert!(75 == *Rc::make_mut(&mut cow2));
1576 *Rc::make_mut(&mut cow0) += 1;
1577 *Rc::make_mut(&mut cow1) += 2;
1578 *Rc::make_mut(&mut cow2) += 3;
1580 assert!(76 == *cow0);
1581 assert!(77 == *cow1);
1582 assert!(78 == *cow2);
1584 // none should point to the same backing memory
1585 assert!(*cow0 != *cow1);
1586 assert!(*cow0 != *cow2);
1587 assert!(*cow1 != *cow2);
1591 fn test_cowrc_clone_unique2() {
1592 let mut cow0 = Rc::new(75);
1593 let cow1 = cow0.clone();
1594 let cow2 = cow1.clone();
1596 assert!(75 == *cow0);
1597 assert!(75 == *cow1);
1598 assert!(75 == *cow2);
1600 *Rc::make_mut(&mut cow0) += 1;
1602 assert!(76 == *cow0);
1603 assert!(75 == *cow1);
1604 assert!(75 == *cow2);
1606 // cow1 and cow2 should share the same contents
1607 // cow0 should have a unique reference
1608 assert!(*cow0 != *cow1);
1609 assert!(*cow0 != *cow2);
1610 assert!(*cow1 == *cow2);
1614 fn test_cowrc_clone_weak() {
1615 let mut cow0 = Rc::new(75);
1616 let cow1_weak = Rc::downgrade(&cow0);
1618 assert!(75 == *cow0);
1619 assert!(75 == *cow1_weak.upgrade().unwrap());
1621 *Rc::make_mut(&mut cow0) += 1;
1623 assert!(76 == *cow0);
1624 assert!(cow1_weak.upgrade().is_none());
1629 let foo = Rc::new(75);
1630 assert_eq!(format!("{:?}", foo), "75");
1635 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
1636 assert_eq!(foo, foo.clone());
1640 fn test_from_owned() {
1642 let foo_rc = Rc::from(foo);
1643 assert!(123 == *foo_rc);
1647 fn test_new_weak() {
1648 let foo: Weak<usize> = Weak::new();
1649 assert!(foo.upgrade().is_none());
1654 let five = Rc::new(5);
1655 let same_five = five.clone();
1656 let other_five = Rc::new(5);
1658 assert!(Rc::ptr_eq(&five, &same_five));
1659 assert!(!Rc::ptr_eq(&five, &other_five));
1663 fn test_from_str() {
1664 let r: Rc<str> = Rc::from("foo");
1666 assert_eq!(&r[..], "foo");
1670 fn test_copy_from_slice() {
1671 let s: &[u32] = &[1, 2, 3];
1672 let r: Rc<[u32]> = Rc::from(s);
1674 assert_eq!(&r[..], [1, 2, 3]);
1678 fn test_clone_from_slice() {
1679 #[derive(Clone, Debug, Eq, PartialEq)]
1682 let s: &[X] = &[X(1), X(2), X(3)];
1683 let r: Rc<[X]> = Rc::from(s);
1685 assert_eq!(&r[..], s);
1690 fn test_clone_from_slice_panic() {
1691 use std::string::{String, ToString};
1693 struct Fail(u32, String);
1695 impl Clone for Fail {
1696 fn clone(&self) -> Fail {
1700 Fail(self.0, self.1.clone())
1705 Fail(0, "foo".to_string()),
1706 Fail(1, "bar".to_string()),
1707 Fail(2, "baz".to_string()),
1710 // Should panic, but not cause memory corruption
1711 let _r: Rc<[Fail]> = Rc::from(s);
1715 fn test_from_box() {
1716 let b: Box<u32> = box 123;
1717 let r: Rc<u32> = Rc::from(b);
1719 assert_eq!(*r, 123);
1723 fn test_from_box_str() {
1724 use std::string::String;
1726 let s = String::from("foo").into_boxed_str();
1727 let r: Rc<str> = Rc::from(s);
1729 assert_eq!(&r[..], "foo");
1733 fn test_from_box_slice() {
1734 let s = vec![1, 2, 3].into_boxed_slice();
1735 let r: Rc<[u32]> = Rc::from(s);
1737 assert_eq!(&r[..], [1, 2, 3]);
1741 fn test_from_box_trait() {
1742 use std::fmt::Display;
1743 use std::string::ToString;
1745 let b: Box<Display> = box 123;
1746 let r: Rc<Display> = Rc::from(b);
1748 assert_eq!(r.to_string(), "123");
1752 fn test_from_box_trait_zero_sized() {
1753 use std::fmt::Debug;
1755 let b: Box<Debug> = box ();
1756 let r: Rc<Debug> = Rc::from(b);
1758 assert_eq!(format!("{:?}", r), "()");
1762 fn test_from_vec() {
1763 let v = vec![1, 2, 3];
1764 let r: Rc<[u32]> = Rc::from(v);
1766 assert_eq!(&r[..], [1, 2, 3]);
1770 fn test_downcast() {
1773 let r1: Rc<Any> = Rc::new(i32::max_value());
1774 let r2: Rc<Any> = Rc::new("abc");
1776 assert!(r1.clone().downcast::<u32>().is_err());
1778 let r1i32 = r1.downcast::<i32>();
1779 assert!(r1i32.is_ok());
1780 assert_eq!(r1i32.unwrap(), Rc::new(i32::max_value()));
1782 assert!(r2.clone().downcast::<i32>().is_err());
1784 let r2str = r2.downcast::<&'static str>();
1785 assert!(r2str.is_ok());
1786 assert_eq!(r2str.unwrap(), Rc::new("abc"));
1790 #[stable(feature = "rust1", since = "1.0.0")]
1791 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
1792 fn borrow(&self) -> &T {
1797 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1798 impl<T: ?Sized> AsRef<T> for Rc<T> {
1799 fn as_ref(&self) -> &T {