# Enum chalk_ir::Variance

``````pub enum Variance {
Covariant,
Invariant,
Contravariant,
}``````
Expand description

Variance

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a <: b

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a == b

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b <: a

## Implementations§

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### impl Variance

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#### pub fn xform(self, other: Variance) -> Variance

`a.xform(b)` combines the variance of a context with the variance of a type with the following meaning. If we are in a context with variance `a`, and we encounter a type argument in a position with variance `b`, then `a.xform(b)` is the new variance with which the argument appears.

Example 1:

``*mut Vec<i32>``

Here, the “ambient” variance starts as covariant. `*mut T` is invariant with respect to `T`, so the variance in which the `Vec<i32>` appears is `Covariant.xform(Invariant)`, which yields `Invariant`. Now, the type `Vec<T>` is covariant with respect to its type argument `T`, and hence the variance of the `i32` here is `Invariant.xform(Covariant)`, which results (again) in `Invariant`.

Example 2:

``fn(*const Vec<i32>, *mut Vec<i32)``

The ambient variance is covariant. A `fn` type is contravariant with respect to its parameters, so the variance within which both pointer types appear is `Covariant.xform(Contravariant)`, or `Contravariant`. `*const T` is covariant with respect to `T`, so the variance within which the first `Vec<i32>` appears is `Contravariant.xform(Covariant)` or `Contravariant`. The same is true for its `i32` argument. In the `*mut T` case, the variance of `Vec<i32>` is `Contravariant.xform(Invariant)`, and hence the outermost type is `Invariant` with respect to `Vec<i32>` (and its `i32` argument).

Source: Figure 1 of “Taming the Wildcards: Combining Definition- and Use-Site Variance” published in PLDI’11. (Doc from rustc)

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#### pub fn invert(self) -> Variance

Converts `Covariant` into `Contravariant` and vice-versa. `Invariant` stays the same.

## Trait Implementations§

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### impl Clone for Variance

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

Returns a copy of the value. Read more
1.0.0 · source§

#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl Debug for Variance

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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 Hash for Variance

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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 PartialEq<Variance> for Variance

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

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

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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## Blanket Implementations§

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### impl<T> Any for Twhere 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 Twhere T: ?Sized,

const: unstable · source§

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

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

const: unstable · source§

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

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

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#### fn cast<U>(self, interner: U::Interner) -> Uwhere Self: CastTo<U>, U: HasInterner,

Cast a value to type `U` using `CastTo`.
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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 Twhere 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> ToOwned for Twhere 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 Twhere 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 Twhere U: TryFrom<T>,

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

The type returned in the event of a conversion error.
const: unstable · source§

#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.