UniqueRc in alloc::rc - Rust
Description: A uniquely owned `Rc`.

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## UniqueRc

# Struct UniqueRc

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    pub struct UniqueRc<T: ?Sized, A: Allocator = Global> { /* private fields */ }

🔬This is a nightly-only experimental API. (`unique_rc_arc` #112566)
Expand description

A uniquely owned `Rc`.

This represents an `Rc` that is known to be uniquely owned – that is, have exactly one strong reference. Multiple weak pointers can be created, but attempts to upgrade those to strong references will fail unless the `UniqueRc` they point to has been converted into a regular `Rc`.

Because they are uniquely owned, the contents of a `UniqueRc` can be freely mutated. A common use case is to have an object be mutable during its initialization phase but then have it become immutable and converted to a normal `Rc`.

This can be used as a flexible way to create cyclic data structures, as in the example below.

    #![feature(unique_rc_arc)]
    use std::rc::{Rc, Weak, UniqueRc};

    struct Gadget {
        #[allow(dead_code)]
        me: Weak<Gadget>,
    }

    fn create_gadget() -> Option<Rc<Gadget>> {
        let mut rc = UniqueRc::new(Gadget {
            me: Weak::new(),
        });
        rc.me = UniqueRc::downgrade(&rc);
        Some(UniqueRc::into_rc(rc))
    }

    create_gadget().unwrap();

An advantage of using `UniqueRc` over `Rc::new_cyclic` to build cyclic data structures is that `Rc::new_cyclic`’s `data_fn` parameter cannot be async or return a `Result`. As shown in the previous example, `UniqueRc` allows for more flexibility in the construction of cyclic data, including fallible or async constructors.

## Implementations§

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### impl<T> UniqueRc<T>

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#### pub fn new(value: T) -> Self

🔬This is a nightly-only experimental API. (`unique_rc_arc` #112566)

Creates a new `UniqueRc`.

Weak references to this `UniqueRc` can be created with `UniqueRc::downgrade`. Upgrading these weak references will fail before the `UniqueRc` has been converted into an `Rc`. After converting the `UniqueRc` into an `Rc`, any weak references created beforehand will point to the new `Rc`.

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#### pub fn map<U>(this: Self, f: impl FnOnce(T) -> U) -> UniqueRc<U>

🔬This is a nightly-only experimental API. (`smart_pointer_try_map` #144419)

Maps the value in a `UniqueRc`, reusing the allocation if possible.

`f` is called on a reference to the value in the `UniqueRc`, and the result is returned, also in a `UniqueRc`.

Note: this is an associated function, which means that you have to call it as `UniqueRc::map(u, f)` instead of `u.map(f)`. This is so that there is no conflict with a method on the inner type.

##### §Examples

    #![feature(smart_pointer_try_map)]
    #![feature(unique_rc_arc)]

    use std::rc::UniqueRc;

    let r = UniqueRc::new(7);
    let new = UniqueRc::map(r, |i| i + 7);
    assert_eq!(*new, 14);

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#### pub fn try_map<R>( this: Self, f: impl FnOnce(T) -> R, ) -> <R::Residual as Residual<UniqueRc<R::Output>>>::TryType
where R: Try, R::Residual: Residual<UniqueRc<R::Output>>,

🔬This is a nightly-only experimental API. (`smart_pointer_try_map` #144419)

Attempts to map the value in a `UniqueRc`, reusing the allocation if possible.

`f` is called on a reference to the value in the `UniqueRc`, and if the operation succeeds, the result is returned, also in a `UniqueRc`.

Note: this is an associated function, which means that you have to call it as `UniqueRc::try_map(u, f)` instead of `u.try_map(f)`. This is so that there is no conflict with a method on the inner type.

##### §Examples

    #![feature(smart_pointer_try_map)]
    #![feature(unique_rc_arc)]

    use std::rc::UniqueRc;

    let b = UniqueRc::new(7);
    let new = UniqueRc::try_map(b, u32::try_from).unwrap();
    assert_eq!(*new, 7);

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### impl<T, A: Allocator> UniqueRc<T, A>

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#### pub fn new_in(value: T, alloc: A) -> Self

🔬This is a nightly-only experimental API. (`unique_rc_arc` #112566)

Creates a new `UniqueRc` in the provided allocator.

Weak references to this `UniqueRc` can be created with `UniqueRc::downgrade`. Upgrading these weak references will fail before the `UniqueRc` has been converted into an `Rc`. After converting the `UniqueRc` into an `Rc`, any weak references created beforehand will point to the new `Rc`.

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### impl<T: ?Sized, A: Allocator> UniqueRc<T, A>

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#### pub fn into_rc(this: Self) -> Rc<T, A>

🔬This is a nightly-only experimental API. (`unique_rc_arc` #112566)

Converts the `UniqueRc` into a regular `Rc`.

This consumes the `UniqueRc` and returns a regular `Rc` that contains the `value` that is passed to `into_rc`.

Any weak references created before this method is called can now be upgraded to strong references.

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### impl<T: ?Sized, A: Allocator + Clone> UniqueRc<T, A>

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#### pub fn downgrade(this: &Self) -> Weak<T, A>

🔬This is a nightly-only experimental API. (`unique_rc_arc` #112566)

Creates a new weak reference to the `UniqueRc`.

Attempting to upgrade this weak reference will fail before the `UniqueRc` has been converted to a `Rc` using `UniqueRc::into_rc`.

## Trait Implementations§

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### impl<T: ?Sized, A: Allocator> !Send for UniqueRc<T, A>

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### impl<T: ?Sized, A: Allocator> !Sync for UniqueRc<T, A>

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### impl<T: ?Sized, A: Allocator> AsMut<T> for UniqueRc<T, A>

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#### fn as_mut(&mut self) -> &mut T

Converts this type into a mutable reference of the (usually inferred) input type.
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### impl<T: ?Sized, A: Allocator> AsRef<T> for UniqueRc<T, A>

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#### fn as_ref(&self) -> &T

Converts this type into a shared reference of the (usually inferred) input type.
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### impl<T: ?Sized, A: Allocator> Borrow<T> for UniqueRc<T, A>

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T: ?Sized, A: Allocator> BorrowMut<T> for UniqueRc<T, A>

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<UniqueRc<U, A>> for UniqueRc<T, A>

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### impl<T: ?Sized + Debug, A: Allocator> Debug for UniqueRc<T, A>

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
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### impl<T: ?Sized, A: Allocator> Deref for UniqueRc<T, A>

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#### type Target = T

The resulting type after dereferencing.
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#### fn deref(&self) -> &T

Dereferences the value.
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### impl<T: ?Sized, A: Allocator> DerefMut for UniqueRc<T, A>

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#### fn deref_mut(&mut self) -> &mut T

Mutably dereferences the value.
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### impl<T: ?Sized, A: Allocator> DerefPure for UniqueRc<T, A>

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### impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<UniqueRc<U>> for UniqueRc<T>

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### impl<T: ?Sized + Display, A: Allocator> Display for UniqueRc<T, A>

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
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### impl<T: ?Sized, A: Allocator> Drop for UniqueRc<T, A>

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#### fn drop(&mut self)

Executes the destructor for this type. Read more
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#### fn pin_drop(self: Pin<&mut Self>)

🔬This is a nightly-only experimental API. (`pin_ergonomics` #130494)
Execute the destructor for this type, but different to `Drop::drop`, it requires `self` to be pinned. Read more
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### impl<T: ?Sized + Eq, A: Allocator> Eq for UniqueRc<T, A>

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### impl<T> From<T> for UniqueRc<T>

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#### fn from(value: T) -> Self

Converts to this type from the input type.
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### impl<T: ?Sized + Hash, A: Allocator> Hash for UniqueRc<T, A>

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#### fn hash<H: Hasher>(&self, state: &mut H)

Feeds this value into the given `Hasher`. Read more
1.3.0 · Source§

#### fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

Feeds a slice of this type into the given `Hasher`. Read more
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### impl<T: ?Sized + Ord, A: Allocator> Ord for UniqueRc<T, A>

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#### fn cmp(&self, other: &UniqueRc<T, A>) -> Ordering

Comparison for two `UniqueRc`s.

The two are compared by calling `cmp()` on their inner values.

##### §Examples

    #![feature(unique_rc_arc)]
    use std::rc::UniqueRc;
    use std::cmp::Ordering;

    let five = UniqueRc::new(5);

    assert_eq!(Ordering::Less, five.cmp(&UniqueRc::new(6)));

1.21.0 (const: unstable) · Source§

#### fn max(self, other: Self) -> Self
where Self: Sized,

Compares and returns the maximum of two values. Read more
1.21.0 (const: unstable) · Source§

#### fn min(self, other: Self) -> Self
where Self: Sized,

Compares and returns the minimum of two values. Read more
1.50.0 (const: unstable) · Source§

#### fn clamp(self, min: Self, max: Self) -> Self
where Self: Sized,

Restrict a value to a certain interval. Read more
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### impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for UniqueRc<T, A>

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#### fn eq(&self, other: &Self) -> bool

Equality for two `UniqueRc`s.

Two `UniqueRc`s are equal if their inner values are equal.

##### §Examples

    #![feature(unique_rc_arc)]
    use std::rc::UniqueRc;

    let five = UniqueRc::new(5);

    assert!(five == UniqueRc::new(5));

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#### fn ne(&self, other: &Self) -> bool

Inequality for two `UniqueRc`s.

Two `UniqueRc`s are not equal if their inner values are not equal.

##### §Examples

    #![feature(unique_rc_arc)]
    use std::rc::UniqueRc;

    let five = UniqueRc::new(5);

    assert!(five != UniqueRc::new(6));

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### impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for UniqueRc<T, A>

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#### fn partial_cmp(&self, other: &UniqueRc<T, A>) -> Option<Ordering>

Partial comparison for two `UniqueRc`s.

The two are compared by calling `partial_cmp()` on their inner values.

##### §Examples

    #![feature(unique_rc_arc)]
    use std::rc::UniqueRc;
    use std::cmp::Ordering;

    let five = UniqueRc::new(5);

    assert_eq!(Some(Ordering::Less), five.partial_cmp(&UniqueRc::new(6)));

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#### fn lt(&self, other: &UniqueRc<T, A>) -> bool

Less-than comparison for two `UniqueRc`s.

The two are compared by calling `<` on their inner values.

##### §Examples

    #![feature(unique_rc_arc)]
    use std::rc::UniqueRc;

    let five = UniqueRc::new(5);

    assert!(five < UniqueRc::new(6));

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#### fn le(&self, other: &UniqueRc<T, A>) -> bool

‘Less than or equal to’ comparison for two `UniqueRc`s.

The two are compared by calling `<=` on their inner values.

##### §Examples

    #![feature(unique_rc_arc)]
    use std::rc::UniqueRc;

    let five = UniqueRc::new(5);

    assert!(five <= UniqueRc::new(5));

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#### fn gt(&self, other: &UniqueRc<T, A>) -> bool

Greater-than comparison for two `UniqueRc`s.

The two are compared by calling `>` on their inner values.

##### §Examples

    #![feature(unique_rc_arc)]
    use std::rc::UniqueRc;

    let five = UniqueRc::new(5);

    assert!(five > UniqueRc::new(4));

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#### fn ge(&self, other: &UniqueRc<T, A>) -> bool

‘Greater than or equal to’ comparison for two `UniqueRc`s.

The two are compared by calling `>=` on their inner values.

##### §Examples

    #![feature(unique_rc_arc)]
    use std::rc::UniqueRc;

    let five = UniqueRc::new(5);

    assert!(five >= UniqueRc::new(5));

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### impl<T: ?Sized, A: Allocator> PinCoerceUnsized for UniqueRc<T, A>

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### impl<T: ?Sized, A: Allocator> Pointer for UniqueRc<T, A>

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
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### impl<T: ?Sized, A: Allocator> Unpin for UniqueRc<T, A>

## Auto Trait Implementations§

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### impl<T, A = Global> !RefUnwindSafe for UniqueRc<T, A>

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### impl<T, A = Global> !UnwindSafe for UniqueRc<T, A>

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### impl<T, A> Freeze for UniqueRc<T, A>
where A: Freeze, T: ?Sized,

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### impl<T, A> UnsafeUnpin for UniqueRc<T, A>
where A: UnsafeUnpin, T: ?Sized,

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<!> for T

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#### fn from(t: !) -> T

Converts to this type from the input type.
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<P, T> Receiver for P
where P: Deref<Target = T> + ?Sized, T: ?Sized,

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#### type Target = T

🔬This is a nightly-only experimental API. (`arbitrary_self_types` #44874)
The target type on which the method may be called.
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### impl<T> ToString for T
where T: Display + ?Sized,

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#### fn to_string(&self) -> String

Converts the given value to a `String`. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.