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.

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§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.

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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 copy 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,

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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, dst: *mut u8)

🔬This is a nightly-only experimental API. (clone_to_uninit)
Performs copy-assignment from self to dst. 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
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impl<'a, T> Captures<'a> for T
where T: ?Sized,

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.