## TaggedRef

# Struct TaggedRef

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    pub struct TaggedRef<'a, Pointee: Aligned + ?Sized, T: Tag> {
        packed: NonNull<Pointee>,
        tag_pointer_ghost: PhantomData<(&'a Pointee, T)>,
    }

Expand description

A covariant `Copy` tagged borrow. This is essentially `{ pointer: &'a P, tag: T }` packed in a single reference.

## Fields§

§`packed: NonNull<Pointee>`

This is semantically a pair of `pointer: &'a P` and `tag: T` fields, however we pack them in a single pointer, to save space.

We pack the tag into the most-significant bits of the pointer to ease retrieval of the value. A left shift is a multiplication and those are embeddable in instruction encoding, for example:

    // (<https://godbolt.org/z/jqcYPWEr3>)
    example::shift_read3:
        mov     eax, dword ptr [8*rdi]
        ret

    example::mask_read3:
        and     rdi, -8
        mov     eax, dword ptr [rdi]
        ret

This is ASM outputted by rustc for reads of values behind tagged pointers for different approaches of tagging:

- `shift_read3` uses `<< 3` (the tag is in the most-significant bits)
- `mask_read3` uses `& !0b111` (the tag is in the least-significant bits)

The shift approach thus produces less instructions and is likely faster (see https://godbolt.org/z/Y913sMdWb).

Encoding diagram:

    [ packed.addr                     ]
    [ tag ] [ pointer.addr >> T::BITS ] <-- usize::BITS - T::BITS bits
       ^
       |
    T::BITS bits

The tag can be retrieved by `packed.addr() >> T::BITS` and the pointer can be retrieved by `packed.map_addr(|addr| addr << T::BITS)`.

§`tag_pointer_ghost: PhantomData<(&'a Pointee, T)>`

## Implementations§

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### impl<'a, P, T> TaggedRef<'a, P, T>
where P: Aligned + ?Sized, T: Tag,

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#### const TAG_BIT_SHIFT: u32

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#### const ASSERTION: ()

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#### pub fn new(pointer: &'a P, tag: T) -> Self

Tags `pointer` with `tag`.

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#### pub fn pointer(self) -> &'a P

Retrieves the pointer.

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#### pub fn tag(&self) -> T

Retrieves the tag.

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#### pub fn set_tag(&mut self, tag: T)

Sets the tag to a new value.

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#### fn pack(ptr: NonNull<P>, tag: T) -> NonNull<P>

Pack pointer `ptr` with a `tag`, according to `self.packed` encoding scheme.

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#### pub(crate) fn pointer_raw(&self) -> NonNull<P>

Retrieves the original raw pointer from `self.packed`.

## Trait Implementations§

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### impl<P, T> Clone for TaggedRef<'_, P, T>
where P: Aligned + ?Sized, T: Tag,

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#### fn clone(&self) -> Self

Returns a duplicate of the value. Read more
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#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl<P, T> Debug for TaggedRef<'_, P, T>
where P: Aligned + Debug + ?Sized, T: Tag + Debug,

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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<P, T> Deref for TaggedRef<'_, P, T>
where P: Aligned + ?Sized, T: Tag,

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#### type Target = P

The resulting type after dereferencing.
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#### fn deref(&self) -> &Self::Target

Dereferences the value.
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### impl<P, T: Tag> Hash for TaggedRef<'_, P, T>

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

Feeds this value into the given `Hasher`. Read more
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#### 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<'a, P, T, HCX> HashStable<HCX> for TaggedRef<'a, P, T>
where P: HashStable<HCX> + Aligned + ?Sized, T: Tag + HashStable<HCX>,

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#### fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher)

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### impl<P, T> PartialEq for TaggedRef<'_, P, T>
where P: Aligned + ?Sized, T: Tag,

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

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &Rhs) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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### impl<P, T> Copy for TaggedRef<'_, P, T>
where P: Aligned + ?Sized, T: Tag,

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### impl<'a, P: Sync, T: Send + Tag> DynSend for TaggedRef<'a, P, T>

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### impl<'a, P: Sync, T: Sync + Tag> DynSync for TaggedRef<'a, P, T>

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### impl<P, T: Tag> Eq for TaggedRef<'_, P, T>

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### impl<P, T> Send for TaggedRef<'_, P, T>
where P: Sync + Aligned + ?Sized, T: Send + Tag,

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### impl<P, T> Sync for TaggedRef<'_, P, T>
where P: Sync + Aligned + ?Sized, T: Sync + Tag,

## Auto Trait Implementations§

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### impl<'a, Pointee, T> Freeze for TaggedRef<'a, Pointee, T>
where Pointee: ?Sized,

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### impl<'a, Pointee, T> RefUnwindSafe for TaggedRef<'a, Pointee, T>
where Pointee: RefUnwindSafe + ?Sized, T: RefUnwindSafe,

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### impl<'a, Pointee, T> Unpin for TaggedRef<'a, Pointee, T>
where T: Unpin, Pointee: ?Sized,

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### impl<'a, Pointee, T> UnwindSafe for TaggedRef<'a, Pointee, T>
where Pointee: RefUnwindSafe + ?Sized, T: UnwindSafe,

## Blanket Implementations§

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### impl<T> Aligned for T

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#### const ALIGN: Alignment = const ALIGN: Alignment = Alignment::of::<Self>();

Alignment of `Self`.
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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> CloneToUninit for T
where T: Clone,

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit`)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Compare self to `key` and return `true` if they are equal.
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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> Instrument for T

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#### fn instrument(self, span: Span) -> Instrumented<Self>

Instruments this type with the provided `Span`, returning an `Instrumented` wrapper. Read more
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#### fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current `Span`, returning an `Instrumented` wrapper. Read more
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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<T> IntoEither for T

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#### fn into_either(self, into_left: bool) -> Either<Self, Self>

Converts `self` into a `Left` variant of `Either<Self, Self>` if `into_left` is `true`. Converts `self` into a `Right` variant of `Either<Self, Self>` otherwise. Read more
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#### fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
where F: FnOnce(&Self) -> bool,

Converts `self` into a `Left` variant of `Either<Self, Self>` if `into_left(&self)` returns `true`. Converts `self` into a `Right` variant of `Either<Self, Self>` otherwise. Read more
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### impl<T> Pointable for T

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#### const ALIGN: usize

The alignment of pointer.
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#### type Init = T

The type for initializers.
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#### unsafe fn init(init: <T as Pointable>::Init) -> usize

Initializes a with the given initializer. Read more
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#### unsafe fn deref<'a>(ptr: usize) -> &'a T

Dereferences the given pointer. Read more
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#### unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut T

Mutably dereferences the given pointer. Read more
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#### unsafe fn drop(ptr: usize)

Drops the object pointed to by the given pointer. Read more
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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`)
The target type on which the method may be called.
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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. 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.
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### impl<T> WithSubscriber for T

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#### fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
where S: Into<Dispatch>,

Attaches the provided `Subscriber` to this type, returning a `WithDispatch` wrapper. Read more
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#### fn with_current_subscriber(self) -> WithDispatch<Self>

Attaches the current default `Subscriber` to this type, returning a `WithDispatch` wrapper. Read more

## Layout§

Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.