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};
257 use core::ops::Deref;
258 use core::ops::CoerceUnsized;
259 use core::ptr::{self, NonNull};
260 use core::convert::From;
263 use alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
267 struct RcBox<T: ?Sized> {
273 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
276 /// See the [module-level documentation](./index.html) for more details.
278 /// The inherent methods of `Rc` are all associated functions, which means
279 /// that you have to call them as e.g. [`Rc::get_mut(&mut value)`][get_mut] instead of
280 /// `value.get_mut()`. This avoids conflicts with methods of the inner
283 /// [get_mut]: #method.get_mut
284 #[stable(feature = "rust1", since = "1.0.0")]
285 pub struct Rc<T: ?Sized> {
286 ptr: NonNull<RcBox<T>>,
287 phantom: PhantomData<T>,
290 #[stable(feature = "rust1", since = "1.0.0")]
291 impl<T: ?Sized> !marker::Send for Rc<T> {}
292 #[stable(feature = "rust1", since = "1.0.0")]
293 impl<T: ?Sized> !marker::Sync for Rc<T> {}
295 #[unstable(feature = "coerce_unsized", issue = "27732")]
296 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
299 /// Constructs a new `Rc<T>`.
306 /// let five = Rc::new(5);
308 #[stable(feature = "rust1", since = "1.0.0")]
309 pub fn new(value: T) -> Rc<T> {
311 // there is an implicit weak pointer owned by all the strong
312 // pointers, which ensures that the weak destructor never frees
313 // the allocation while the strong destructor is running, even
314 // if the weak pointer is stored inside the strong one.
315 ptr: Box::into_raw_non_null(box RcBox {
316 strong: Cell::new(1),
320 phantom: PhantomData,
324 /// Returns the contained value, if the `Rc` has exactly one strong reference.
326 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
329 /// This will succeed even if there are outstanding weak references.
331 /// [result]: ../../std/result/enum.Result.html
338 /// let x = Rc::new(3);
339 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
341 /// let x = Rc::new(4);
342 /// let _y = Rc::clone(&x);
343 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
346 #[stable(feature = "rc_unique", since = "1.4.0")]
347 pub fn try_unwrap(this: Self) -> Result<T, Self> {
348 if Rc::strong_count(&this) == 1 {
350 let val = ptr::read(&*this); // copy the contained object
352 // Indicate to Weaks that they can't be promoted by decrementing
353 // the strong count, and then remove the implicit "strong weak"
354 // pointer while also handling drop logic by just crafting a
357 let _weak = Weak { ptr: this.ptr };
367 impl<T: ?Sized> Rc<T> {
368 /// Consumes the `Rc`, returning the wrapped pointer.
370 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
371 /// [`Rc::from_raw`][from_raw].
373 /// [from_raw]: struct.Rc.html#method.from_raw
380 /// let x = Rc::new(10);
381 /// let x_ptr = Rc::into_raw(x);
382 /// assert_eq!(unsafe { *x_ptr }, 10);
384 #[stable(feature = "rc_raw", since = "1.17.0")]
385 pub fn into_raw(this: Self) -> *const T {
386 let ptr: *const T = &*this;
391 /// Constructs an `Rc` from a raw pointer.
393 /// The raw pointer must have been previously returned by a call to a
394 /// [`Rc::into_raw`][into_raw].
396 /// This function is unsafe because improper use may lead to memory problems. For example, a
397 /// double-free may occur if the function is called twice on the same raw pointer.
399 /// [into_raw]: struct.Rc.html#method.into_raw
406 /// let x = Rc::new(10);
407 /// let x_ptr = Rc::into_raw(x);
410 /// // Convert back to an `Rc` to prevent leak.
411 /// let x = Rc::from_raw(x_ptr);
412 /// assert_eq!(*x, 10);
414 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
417 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
419 #[stable(feature = "rc_raw", since = "1.17.0")]
420 pub unsafe fn from_raw(ptr: *const T) -> Self {
421 // Align the unsized value to the end of the RcBox.
422 // Because it is ?Sized, it will always be the last field in memory.
423 let align = align_of_val(&*ptr);
424 let layout = Layout::new::<RcBox<()>>();
425 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
427 // Reverse the offset to find the original RcBox.
428 let fake_ptr = ptr as *mut RcBox<T>;
429 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
432 ptr: NonNull::new_unchecked(rc_ptr),
433 phantom: PhantomData,
437 /// Creates a new [`Weak`][weak] pointer to this value.
439 /// [weak]: struct.Weak.html
446 /// let five = Rc::new(5);
448 /// let weak_five = Rc::downgrade(&five);
450 #[stable(feature = "rc_weak", since = "1.4.0")]
451 pub fn downgrade(this: &Self) -> Weak<T> {
453 // Make sure we do not create a dangling Weak
454 debug_assert!(!is_dangling(this.ptr));
455 Weak { ptr: this.ptr }
458 /// Gets the number of [`Weak`][weak] pointers to this value.
460 /// [weak]: struct.Weak.html
467 /// let five = Rc::new(5);
468 /// let _weak_five = Rc::downgrade(&five);
470 /// assert_eq!(1, Rc::weak_count(&five));
473 #[stable(feature = "rc_counts", since = "1.15.0")]
474 pub fn weak_count(this: &Self) -> usize {
478 /// Gets the number of strong (`Rc`) pointers to this value.
485 /// let five = Rc::new(5);
486 /// let _also_five = Rc::clone(&five);
488 /// assert_eq!(2, Rc::strong_count(&five));
491 #[stable(feature = "rc_counts", since = "1.15.0")]
492 pub fn strong_count(this: &Self) -> usize {
496 /// Returns true if there are no other `Rc` or [`Weak`][weak] pointers to
497 /// this inner value.
499 /// [weak]: struct.Weak.html
501 fn is_unique(this: &Self) -> bool {
502 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
505 /// Returns a mutable reference to the inner value, if there are
506 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
508 /// Returns [`None`] otherwise, because it is not safe to
509 /// mutate a shared value.
511 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
512 /// the inner value when it's shared.
514 /// [weak]: struct.Weak.html
515 /// [`None`]: ../../std/option/enum.Option.html#variant.None
516 /// [make_mut]: struct.Rc.html#method.make_mut
517 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
524 /// let mut x = Rc::new(3);
525 /// *Rc::get_mut(&mut x).unwrap() = 4;
526 /// assert_eq!(*x, 4);
528 /// let _y = Rc::clone(&x);
529 /// assert!(Rc::get_mut(&mut x).is_none());
532 #[stable(feature = "rc_unique", since = "1.4.0")]
533 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
534 if Rc::is_unique(this) {
536 Some(&mut this.ptr.as_mut().value)
544 #[stable(feature = "ptr_eq", since = "1.17.0")]
545 /// Returns true if the two `Rc`s point to the same value (not
546 /// just values that compare as equal).
553 /// let five = Rc::new(5);
554 /// let same_five = Rc::clone(&five);
555 /// let other_five = Rc::new(5);
557 /// assert!(Rc::ptr_eq(&five, &same_five));
558 /// assert!(!Rc::ptr_eq(&five, &other_five));
560 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
561 this.ptr.as_ptr() == other.ptr.as_ptr()
565 impl<T: Clone> Rc<T> {
566 /// Makes a mutable reference into the given `Rc`.
568 /// If there are other `Rc` or [`Weak`][weak] pointers to the same value,
569 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
570 /// ensure unique ownership. This is also referred to as clone-on-write.
572 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
574 /// [weak]: struct.Weak.html
575 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
576 /// [get_mut]: struct.Rc.html#method.get_mut
583 /// let mut data = Rc::new(5);
585 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
586 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
587 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
588 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
589 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
591 /// // Now `data` and `other_data` point to different values.
592 /// assert_eq!(*data, 8);
593 /// assert_eq!(*other_data, 12);
596 #[stable(feature = "rc_unique", since = "1.4.0")]
597 pub fn make_mut(this: &mut Self) -> &mut T {
598 if Rc::strong_count(this) != 1 {
599 // Gotta clone the data, there are other Rcs
600 *this = Rc::new((**this).clone())
601 } else if Rc::weak_count(this) != 0 {
602 // Can just steal the data, all that's left is Weaks
604 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
605 mem::swap(this, &mut swap);
607 // Remove implicit strong-weak ref (no need to craft a fake
608 // Weak here -- we know other Weaks can clean up for us)
613 // This unsafety is ok because we're guaranteed that the pointer
614 // returned is the *only* pointer that will ever be returned to T. Our
615 // reference count is guaranteed to be 1 at this point, and we required
616 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
617 // reference to the inner value.
619 &mut this.ptr.as_mut().value
626 #[stable(feature = "rc_downcast", since = "1.29.0")]
627 /// Attempt to downcast the `Rc<Any>` to a concrete type.
632 /// use std::any::Any;
635 /// fn print_if_string(value: Rc<Any>) {
636 /// if let Ok(string) = value.downcast::<String>() {
637 /// println!("String ({}): {}", string.len(), string);
642 /// let my_string = "Hello World".to_string();
643 /// print_if_string(Rc::new(my_string));
644 /// print_if_string(Rc::new(0i8));
647 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
648 if (*self).is::<T>() {
649 let ptr = self.ptr.cast::<RcBox<T>>();
651 Ok(Rc { ptr, phantom: PhantomData })
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 = Global.alloc(layout)
667 .unwrap_or_else(|_| handle_alloc_error(layout));
669 // Initialize the real RcBox
670 let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) 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 box_unique = Box::into_unique(v);
681 let bptr = box_unique.as_ptr();
683 let value_size = size_of_val(&*bptr);
684 let ptr = Self::allocate_for_ptr(bptr);
686 // Copy value as bytes
687 ptr::copy_nonoverlapping(
688 bptr as *const T as *const u8,
689 &mut (*ptr).value as *mut _ as *mut u8,
692 // Free the allocation without dropping its contents
693 box_free(box_unique);
695 Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
700 // Sets the data pointer of a `?Sized` raw pointer.
702 // For a slice/trait object, this sets the `data` field and leaves the rest
703 // unchanged. For a sized raw pointer, this simply sets the pointer.
704 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
705 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
710 // Copy elements from slice into newly allocated Rc<[T]>
712 // Unsafe because the caller must either take ownership or bind `T: Copy`
713 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
714 let v_ptr = v as *const [T];
715 let ptr = Self::allocate_for_ptr(v_ptr);
717 ptr::copy_nonoverlapping(
719 &mut (*ptr).value as *mut [T] as *mut T,
722 Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
726 trait RcFromSlice<T> {
727 fn from_slice(slice: &[T]) -> Self;
730 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
732 default fn from_slice(v: &[T]) -> Self {
733 // Panic guard while cloning T elements.
734 // In the event of a panic, elements that have been written
735 // into the new RcBox will be dropped, then the memory freed.
743 impl<T> Drop for Guard<T> {
745 use core::slice::from_raw_parts_mut;
748 let slice = from_raw_parts_mut(self.elems, self.n_elems);
749 ptr::drop_in_place(slice);
751 Global.dealloc(self.mem, self.layout.clone());
757 let v_ptr = v as *const [T];
758 let ptr = Self::allocate_for_ptr(v_ptr);
760 let mem = ptr as *mut _ as *mut u8;
761 let layout = Layout::for_value(&*ptr);
763 // Pointer to first element
764 let elems = &mut (*ptr).value as *mut [T] as *mut T;
766 let mut guard = Guard{
767 mem: NonNull::new_unchecked(mem),
773 for (i, item) in v.iter().enumerate() {
774 ptr::write(elems.add(i), item.clone());
778 // All clear. Forget the guard so it doesn't free the new RcBox.
781 Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
786 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
788 fn from_slice(v: &[T]) -> Self {
789 unsafe { Rc::copy_from_slice(v) }
793 #[stable(feature = "rust1", since = "1.0.0")]
794 impl<T: ?Sized> Deref for Rc<T> {
798 fn deref(&self) -> &T {
803 #[stable(feature = "rust1", since = "1.0.0")]
804 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
807 /// This will decrement the strong reference count. If the strong reference
808 /// count reaches zero then the only other references (if any) are
809 /// [`Weak`], so we `drop` the inner value.
818 /// impl Drop for Foo {
819 /// fn drop(&mut self) {
820 /// println!("dropped!");
824 /// let foo = Rc::new(Foo);
825 /// let foo2 = Rc::clone(&foo);
827 /// drop(foo); // Doesn't print anything
828 /// drop(foo2); // Prints "dropped!"
833 if self.strong() == 0 {
834 // destroy the contained object
835 ptr::drop_in_place(self.ptr.as_mut());
837 // remove the implicit "strong weak" pointer now that we've
838 // destroyed the contents.
841 if self.weak() == 0 {
842 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
849 #[stable(feature = "rust1", since = "1.0.0")]
850 impl<T: ?Sized> Clone for Rc<T> {
851 /// Makes a clone of the `Rc` pointer.
853 /// This creates another pointer to the same inner value, increasing the
854 /// strong reference count.
861 /// let five = Rc::new(5);
863 /// Rc::clone(&five);
866 fn clone(&self) -> Rc<T> {
868 Rc { ptr: self.ptr, phantom: PhantomData }
872 #[stable(feature = "rust1", since = "1.0.0")]
873 impl<T: Default> Default for Rc<T> {
874 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
881 /// let x: Rc<i32> = Default::default();
882 /// assert_eq!(*x, 0);
885 fn default() -> Rc<T> {
886 Rc::new(Default::default())
890 #[stable(feature = "rust1", since = "1.0.0")]
891 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
892 /// Equality for two `Rc`s.
894 /// Two `Rc`s are equal if their inner values are equal.
901 /// let five = Rc::new(5);
903 /// assert!(five == Rc::new(5));
906 fn eq(&self, other: &Rc<T>) -> bool {
910 /// Inequality for two `Rc`s.
912 /// Two `Rc`s are unequal if their inner values are unequal.
919 /// let five = Rc::new(5);
921 /// assert!(five != Rc::new(6));
924 fn ne(&self, other: &Rc<T>) -> bool {
929 #[stable(feature = "rust1", since = "1.0.0")]
930 impl<T: ?Sized + Eq> Eq for Rc<T> {}
932 #[stable(feature = "rust1", since = "1.0.0")]
933 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
934 /// Partial comparison for two `Rc`s.
936 /// The two are compared by calling `partial_cmp()` on their inner values.
942 /// use std::cmp::Ordering;
944 /// let five = Rc::new(5);
946 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
949 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
950 (**self).partial_cmp(&**other)
953 /// Less-than comparison for two `Rc`s.
955 /// The two are compared by calling `<` on their inner values.
962 /// let five = Rc::new(5);
964 /// assert!(five < Rc::new(6));
967 fn lt(&self, other: &Rc<T>) -> bool {
971 /// 'Less than or equal to' comparison for two `Rc`s.
973 /// The two are compared by calling `<=` on their inner values.
980 /// let five = Rc::new(5);
982 /// assert!(five <= Rc::new(5));
985 fn le(&self, other: &Rc<T>) -> bool {
989 /// Greater-than comparison for two `Rc`s.
991 /// The two are compared by calling `>` on their inner values.
998 /// let five = Rc::new(5);
1000 /// assert!(five > Rc::new(4));
1003 fn gt(&self, other: &Rc<T>) -> bool {
1007 /// 'Greater than or equal to' comparison for two `Rc`s.
1009 /// The two are compared by calling `>=` on their inner values.
1014 /// use std::rc::Rc;
1016 /// let five = Rc::new(5);
1018 /// assert!(five >= Rc::new(5));
1021 fn ge(&self, other: &Rc<T>) -> bool {
1026 #[stable(feature = "rust1", since = "1.0.0")]
1027 impl<T: ?Sized + Ord> Ord for Rc<T> {
1028 /// Comparison for two `Rc`s.
1030 /// The two are compared by calling `cmp()` on their inner values.
1035 /// use std::rc::Rc;
1036 /// use std::cmp::Ordering;
1038 /// let five = Rc::new(5);
1040 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1043 fn cmp(&self, other: &Rc<T>) -> Ordering {
1044 (**self).cmp(&**other)
1048 #[stable(feature = "rust1", since = "1.0.0")]
1049 impl<T: ?Sized + Hash> Hash for Rc<T> {
1050 fn hash<H: Hasher>(&self, state: &mut H) {
1051 (**self).hash(state);
1055 #[stable(feature = "rust1", since = "1.0.0")]
1056 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1057 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1058 fmt::Display::fmt(&**self, f)
1062 #[stable(feature = "rust1", since = "1.0.0")]
1063 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1064 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1065 fmt::Debug::fmt(&**self, f)
1069 #[stable(feature = "rust1", since = "1.0.0")]
1070 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1071 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1072 fmt::Pointer::fmt(&(&**self as *const T), f)
1076 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1077 impl<T> From<T> for Rc<T> {
1078 fn from(t: T) -> Self {
1083 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1084 impl<'a, T: Clone> From<&'a [T]> for Rc<[T]> {
1086 fn from(v: &[T]) -> Rc<[T]> {
1087 <Self as RcFromSlice<T>>::from_slice(v)
1091 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1092 impl<'a> From<&'a str> for Rc<str> {
1094 fn from(v: &str) -> Rc<str> {
1095 let rc = Rc::<[u8]>::from(v.as_bytes());
1096 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1100 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1101 impl From<String> for Rc<str> {
1103 fn from(v: String) -> Rc<str> {
1108 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1109 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1111 fn from(v: Box<T>) -> Rc<T> {
1116 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1117 impl<T> From<Vec<T>> for Rc<[T]> {
1119 fn from(mut v: Vec<T>) -> Rc<[T]> {
1121 let rc = Rc::copy_from_slice(&v);
1123 // Allow the Vec to free its memory, but not destroy its contents
1131 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1132 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
1133 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1135 /// Since a `Weak` reference does not count towards ownership, it will not
1136 /// prevent the inner value from being dropped, and `Weak` itself makes no
1137 /// guarantees about the value still being present and may return [`None`]
1138 /// when [`upgrade`]d.
1140 /// A `Weak` pointer is useful for keeping a temporary reference to the value
1141 /// within [`Rc`] without extending its lifetime. It is also used to prevent
1142 /// circular references between [`Rc`] pointers, since mutual owning references
1143 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1144 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1145 /// pointers from children back to their parents.
1147 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1149 /// [`Rc`]: struct.Rc.html
1150 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1151 /// [`upgrade`]: struct.Weak.html#method.upgrade
1152 /// [`Option`]: ../../std/option/enum.Option.html
1153 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1154 #[stable(feature = "rc_weak", since = "1.4.0")]
1155 pub struct Weak<T: ?Sized> {
1156 // This is a `NonNull` to allow optimizing the size of this type in enums,
1157 // but it is not necessarily a valid pointer.
1158 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1159 // to allocate space on the heap. That's not a value a real pointer
1160 // will ever have because RcBox has alignment at least 2.
1161 ptr: NonNull<RcBox<T>>,
1164 #[stable(feature = "rc_weak", since = "1.4.0")]
1165 impl<T: ?Sized> !marker::Send for Weak<T> {}
1166 #[stable(feature = "rc_weak", since = "1.4.0")]
1167 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1169 #[unstable(feature = "coerce_unsized", issue = "27732")]
1170 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1173 /// Constructs a new `Weak<T>`, without allocating any memory.
1174 /// Calling [`upgrade`][Weak::upgrade] on the return value always gives [`None`].
1176 /// [`None`]: ../../std/option/enum.Option.html
1181 /// use std::rc::Weak;
1183 /// let empty: Weak<i64> = Weak::new();
1184 /// assert!(empty.upgrade().is_none());
1186 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1187 pub fn new() -> Weak<T> {
1189 ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"),
1194 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1195 let address = ptr.as_ptr() as *mut () as usize;
1196 address == usize::MAX
1199 impl<T: ?Sized> Weak<T> {
1200 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1201 /// the lifetime of the value if successful.
1203 /// Returns [`None`] if the value has since been dropped.
1205 /// [`Rc`]: struct.Rc.html
1206 /// [`None`]: ../../std/option/enum.Option.html
1211 /// use std::rc::Rc;
1213 /// let five = Rc::new(5);
1215 /// let weak_five = Rc::downgrade(&five);
1217 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1218 /// assert!(strong_five.is_some());
1220 /// // Destroy all strong pointers.
1221 /// drop(strong_five);
1224 /// assert!(weak_five.upgrade().is_none());
1226 #[stable(feature = "rc_weak", since = "1.4.0")]
1227 pub fn upgrade(&self) -> Option<Rc<T>> {
1228 let inner = self.inner()?;
1229 if inner.strong() == 0 {
1233 Some(Rc { ptr: self.ptr, phantom: PhantomData })
1237 /// Return `None` when the pointer is dangling and there is no allocated `RcBox`,
1238 /// i.e. this `Weak` was created by `Weak::new`
1240 fn inner(&self) -> Option<&RcBox<T>> {
1241 if is_dangling(self.ptr) {
1244 Some(unsafe { self.ptr.as_ref() })
1249 #[stable(feature = "rc_weak", since = "1.4.0")]
1250 impl<T: ?Sized> Drop for Weak<T> {
1251 /// Drops the `Weak` pointer.
1256 /// use std::rc::{Rc, Weak};
1260 /// impl Drop for Foo {
1261 /// fn drop(&mut self) {
1262 /// println!("dropped!");
1266 /// let foo = Rc::new(Foo);
1267 /// let weak_foo = Rc::downgrade(&foo);
1268 /// let other_weak_foo = Weak::clone(&weak_foo);
1270 /// drop(weak_foo); // Doesn't print anything
1271 /// drop(foo); // Prints "dropped!"
1273 /// assert!(other_weak_foo.upgrade().is_none());
1275 fn drop(&mut self) {
1276 if let Some(inner) = self.inner() {
1278 // the weak count starts at 1, and will only go to zero if all
1279 // the strong pointers have disappeared.
1280 if inner.weak() == 0 {
1282 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1289 #[stable(feature = "rc_weak", since = "1.4.0")]
1290 impl<T: ?Sized> Clone for Weak<T> {
1291 /// Makes a clone of the `Weak` pointer that points to the same value.
1296 /// use std::rc::{Rc, Weak};
1298 /// let weak_five = Rc::downgrade(&Rc::new(5));
1300 /// Weak::clone(&weak_five);
1303 fn clone(&self) -> Weak<T> {
1304 if let Some(inner) = self.inner() {
1307 Weak { ptr: self.ptr }
1311 #[stable(feature = "rc_weak", since = "1.4.0")]
1312 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1313 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1318 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1319 impl<T> Default for Weak<T> {
1320 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1321 /// it. Calling [`upgrade`][Weak::upgrade] on the return value always gives [`None`].
1323 /// [`None`]: ../../std/option/enum.Option.html
1328 /// use std::rc::Weak;
1330 /// let empty: Weak<i64> = Default::default();
1331 /// assert!(empty.upgrade().is_none());
1333 fn default() -> Weak<T> {
1338 // NOTE: We checked_add here to deal with mem::forget safely. In particular
1339 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1340 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1341 // We abort because this is such a degenerate scenario that we don't care about
1342 // what happens -- no real program should ever experience this.
1344 // This should have negligible overhead since you don't actually need to
1345 // clone these much in Rust thanks to ownership and move-semantics.
1348 trait RcBoxPtr<T: ?Sized> {
1349 fn inner(&self) -> &RcBox<T>;
1352 fn strong(&self) -> usize {
1353 self.inner().strong.get()
1357 fn inc_strong(&self) {
1358 // We want to abort on overflow instead of dropping the value.
1359 // The reference count will never be zero when this is called;
1360 // nevertheless, we insert an abort here to hint LLVM at
1361 // an otherwise missed optimization.
1362 if self.strong() == 0 || self.strong() == usize::max_value() {
1365 self.inner().strong.set(self.strong() + 1);
1369 fn dec_strong(&self) {
1370 self.inner().strong.set(self.strong() - 1);
1374 fn weak(&self) -> usize {
1375 self.inner().weak.get()
1379 fn inc_weak(&self) {
1380 // We want to abort on overflow instead of dropping the value.
1381 // The reference count will never be zero when this is called;
1382 // nevertheless, we insert an abort here to hint LLVM at
1383 // an otherwise missed optimization.
1384 if self.weak() == 0 || self.weak() == usize::max_value() {
1387 self.inner().weak.set(self.weak() + 1);
1391 fn dec_weak(&self) {
1392 self.inner().weak.set(self.weak() - 1);
1396 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1398 fn inner(&self) -> &RcBox<T> {
1405 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
1407 fn inner(&self) -> &RcBox<T> {
1414 use super::{Rc, Weak};
1415 use std::boxed::Box;
1416 use std::cell::RefCell;
1417 use std::option::Option;
1418 use std::option::Option::{None, Some};
1419 use std::result::Result::{Err, Ok};
1421 use std::clone::Clone;
1422 use std::convert::From;
1426 let x = Rc::new(RefCell::new(5));
1428 *x.borrow_mut() = 20;
1429 assert_eq!(*y.borrow(), 20);
1439 fn test_simple_clone() {
1447 fn test_destructor() {
1448 let x: Rc<Box<_>> = Rc::new(box 5);
1455 let y = Rc::downgrade(&x);
1456 assert!(y.upgrade().is_some());
1462 let y = Rc::downgrade(&x);
1464 assert!(y.upgrade().is_none());
1468 fn weak_self_cyclic() {
1470 x: RefCell<Option<Weak<Cycle>>>,
1473 let a = Rc::new(Cycle { x: RefCell::new(None) });
1474 let b = Rc::downgrade(&a.clone());
1475 *a.x.borrow_mut() = Some(b);
1477 // hopefully we don't double-free (or leak)...
1483 assert!(Rc::is_unique(&x));
1485 assert!(!Rc::is_unique(&x));
1487 assert!(Rc::is_unique(&x));
1488 let w = Rc::downgrade(&x);
1489 assert!(!Rc::is_unique(&x));
1491 assert!(Rc::is_unique(&x));
1495 fn test_strong_count() {
1497 assert!(Rc::strong_count(&a) == 1);
1498 let w = Rc::downgrade(&a);
1499 assert!(Rc::strong_count(&a) == 1);
1500 let b = w.upgrade().expect("upgrade of live rc failed");
1501 assert!(Rc::strong_count(&b) == 2);
1502 assert!(Rc::strong_count(&a) == 2);
1505 assert!(Rc::strong_count(&b) == 1);
1507 assert!(Rc::strong_count(&b) == 2);
1508 assert!(Rc::strong_count(&c) == 2);
1512 fn test_weak_count() {
1514 assert!(Rc::strong_count(&a) == 1);
1515 assert!(Rc::weak_count(&a) == 0);
1516 let w = Rc::downgrade(&a);
1517 assert!(Rc::strong_count(&a) == 1);
1518 assert!(Rc::weak_count(&a) == 1);
1520 assert!(Rc::strong_count(&a) == 1);
1521 assert!(Rc::weak_count(&a) == 0);
1523 assert!(Rc::strong_count(&a) == 2);
1524 assert!(Rc::weak_count(&a) == 0);
1531 assert_eq!(Rc::try_unwrap(x), Ok(3));
1534 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
1536 let _w = Rc::downgrade(&x);
1537 assert_eq!(Rc::try_unwrap(x), Ok(5));
1541 fn into_from_raw() {
1542 let x = Rc::new(box "hello");
1545 let x_ptr = Rc::into_raw(x);
1548 assert_eq!(**x_ptr, "hello");
1550 let x = Rc::from_raw(x_ptr);
1551 assert_eq!(**x, "hello");
1553 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
1558 fn test_into_from_raw_unsized() {
1559 use std::fmt::Display;
1560 use std::string::ToString;
1562 let rc: Rc<str> = Rc::from("foo");
1564 let ptr = Rc::into_raw(rc.clone());
1565 let rc2 = unsafe { Rc::from_raw(ptr) };
1567 assert_eq!(unsafe { &*ptr }, "foo");
1568 assert_eq!(rc, rc2);
1570 let rc: Rc<dyn Display> = Rc::new(123);
1572 let ptr = Rc::into_raw(rc.clone());
1573 let rc2 = unsafe { Rc::from_raw(ptr) };
1575 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1576 assert_eq!(rc2.to_string(), "123");
1581 let mut x = Rc::new(3);
1582 *Rc::get_mut(&mut x).unwrap() = 4;
1585 assert!(Rc::get_mut(&mut x).is_none());
1587 assert!(Rc::get_mut(&mut x).is_some());
1588 let _w = Rc::downgrade(&x);
1589 assert!(Rc::get_mut(&mut x).is_none());
1593 fn test_cowrc_clone_make_unique() {
1594 let mut cow0 = Rc::new(75);
1595 let mut cow1 = cow0.clone();
1596 let mut cow2 = cow1.clone();
1598 assert!(75 == *Rc::make_mut(&mut cow0));
1599 assert!(75 == *Rc::make_mut(&mut cow1));
1600 assert!(75 == *Rc::make_mut(&mut cow2));
1602 *Rc::make_mut(&mut cow0) += 1;
1603 *Rc::make_mut(&mut cow1) += 2;
1604 *Rc::make_mut(&mut cow2) += 3;
1606 assert!(76 == *cow0);
1607 assert!(77 == *cow1);
1608 assert!(78 == *cow2);
1610 // none should point to the same backing memory
1611 assert!(*cow0 != *cow1);
1612 assert!(*cow0 != *cow2);
1613 assert!(*cow1 != *cow2);
1617 fn test_cowrc_clone_unique2() {
1618 let mut cow0 = Rc::new(75);
1619 let cow1 = cow0.clone();
1620 let cow2 = cow1.clone();
1622 assert!(75 == *cow0);
1623 assert!(75 == *cow1);
1624 assert!(75 == *cow2);
1626 *Rc::make_mut(&mut cow0) += 1;
1628 assert!(76 == *cow0);
1629 assert!(75 == *cow1);
1630 assert!(75 == *cow2);
1632 // cow1 and cow2 should share the same contents
1633 // cow0 should have a unique reference
1634 assert!(*cow0 != *cow1);
1635 assert!(*cow0 != *cow2);
1636 assert!(*cow1 == *cow2);
1640 fn test_cowrc_clone_weak() {
1641 let mut cow0 = Rc::new(75);
1642 let cow1_weak = Rc::downgrade(&cow0);
1644 assert!(75 == *cow0);
1645 assert!(75 == *cow1_weak.upgrade().unwrap());
1647 *Rc::make_mut(&mut cow0) += 1;
1649 assert!(76 == *cow0);
1650 assert!(cow1_weak.upgrade().is_none());
1655 let foo = Rc::new(75);
1656 assert_eq!(format!("{:?}", foo), "75");
1661 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
1662 assert_eq!(foo, foo.clone());
1666 fn test_from_owned() {
1668 let foo_rc = Rc::from(foo);
1669 assert!(123 == *foo_rc);
1673 fn test_new_weak() {
1674 let foo: Weak<usize> = Weak::new();
1675 assert!(foo.upgrade().is_none());
1680 let five = Rc::new(5);
1681 let same_five = five.clone();
1682 let other_five = Rc::new(5);
1684 assert!(Rc::ptr_eq(&five, &same_five));
1685 assert!(!Rc::ptr_eq(&five, &other_five));
1689 fn test_from_str() {
1690 let r: Rc<str> = Rc::from("foo");
1692 assert_eq!(&r[..], "foo");
1696 fn test_copy_from_slice() {
1697 let s: &[u32] = &[1, 2, 3];
1698 let r: Rc<[u32]> = Rc::from(s);
1700 assert_eq!(&r[..], [1, 2, 3]);
1704 fn test_clone_from_slice() {
1705 #[derive(Clone, Debug, Eq, PartialEq)]
1708 let s: &[X] = &[X(1), X(2), X(3)];
1709 let r: Rc<[X]> = Rc::from(s);
1711 assert_eq!(&r[..], s);
1716 fn test_clone_from_slice_panic() {
1717 use std::string::{String, ToString};
1719 struct Fail(u32, String);
1721 impl Clone for Fail {
1722 fn clone(&self) -> Fail {
1726 Fail(self.0, self.1.clone())
1731 Fail(0, "foo".to_string()),
1732 Fail(1, "bar".to_string()),
1733 Fail(2, "baz".to_string()),
1736 // Should panic, but not cause memory corruption
1737 let _r: Rc<[Fail]> = Rc::from(s);
1741 fn test_from_box() {
1742 let b: Box<u32> = box 123;
1743 let r: Rc<u32> = Rc::from(b);
1745 assert_eq!(*r, 123);
1749 fn test_from_box_str() {
1750 use std::string::String;
1752 let s = String::from("foo").into_boxed_str();
1753 let r: Rc<str> = Rc::from(s);
1755 assert_eq!(&r[..], "foo");
1759 fn test_from_box_slice() {
1760 let s = vec![1, 2, 3].into_boxed_slice();
1761 let r: Rc<[u32]> = Rc::from(s);
1763 assert_eq!(&r[..], [1, 2, 3]);
1767 fn test_from_box_trait() {
1768 use std::fmt::Display;
1769 use std::string::ToString;
1771 let b: Box<dyn Display> = box 123;
1772 let r: Rc<dyn Display> = Rc::from(b);
1774 assert_eq!(r.to_string(), "123");
1778 fn test_from_box_trait_zero_sized() {
1779 use std::fmt::Debug;
1781 let b: Box<dyn Debug> = box ();
1782 let r: Rc<dyn Debug> = Rc::from(b);
1784 assert_eq!(format!("{:?}", r), "()");
1788 fn test_from_vec() {
1789 let v = vec![1, 2, 3];
1790 let r: Rc<[u32]> = Rc::from(v);
1792 assert_eq!(&r[..], [1, 2, 3]);
1796 fn test_downcast() {
1799 let r1: Rc<dyn Any> = Rc::new(i32::max_value());
1800 let r2: Rc<dyn Any> = Rc::new("abc");
1802 assert!(r1.clone().downcast::<u32>().is_err());
1804 let r1i32 = r1.downcast::<i32>();
1805 assert!(r1i32.is_ok());
1806 assert_eq!(r1i32.unwrap(), Rc::new(i32::max_value()));
1808 assert!(r2.clone().downcast::<i32>().is_err());
1810 let r2str = r2.downcast::<&'static str>();
1811 assert!(r2str.is_ok());
1812 assert_eq!(r2str.unwrap(), Rc::new("abc"));
1816 #[stable(feature = "rust1", since = "1.0.0")]
1817 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
1818 fn borrow(&self) -> &T {
1823 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1824 impl<T: ?Sized> AsRef<T> for Rc<T> {
1825 fn as_ref(&self) -> &T {