Enum rustc::ty::Variance

pub enum Variance {
    Covariant,
    Invariant,
    Contravariant,
    Bivariant,
}

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Variants

Covariant

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Invariant

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Contravariant

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Bivariant

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Methods

impl Variance

pub fn xform(self, v: Variance) -> Variance

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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.

Trait Implementations

impl<'tcx> HashStable<StableHashingContext<'tcx>> for Variance

fn hash_stable<W: StableHasherResult>(
    &self,
    __ctx: &mut StableHashingContext<'tcx>,
    __hasher: &mut StableHasher<W>
)

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impl Clone for Variance

fn clone(&self) -> Variance

Returns a copy of the value.

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

Performs copy-assignment from source.

impl PartialEq for Variance

fn eq(&self, __arg_0: &Variance) -> bool

This method tests for self and other values to be equal, and is used by ==.

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

This method tests for !=.

impl Decodable for Variance

fn decode<__D: Decoder>(__arg_0: &mut __D) -> Result<Variance, __D::Error>

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impl Encodable for Variance

fn encode<__S: Encoder>(&self, __arg_0: &mut __S) -> Result<(), __S::Error>

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impl Copy for Variance

impl Print for Variance

fn print<F: Write>(&self, f: &mut F, cx: &mut PrintContext) -> Result

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fn print_to_string(&self, cx: &mut PrintContext) -> String

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fn print_display<F: Write>(&self, f: &mut F, cx: &mut PrintContext) -> Result

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fn print_display_to_string(&self, cx: &mut PrintContext) -> String

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fn print_debug<F: Write>(&self, f: &mut F, cx: &mut PrintContext) -> Result

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fn print_debug_to_string(&self, cx: &mut PrintContext) -> String

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impl Debug for Variance

fn fmt(&self, f: &mut Formatter) -> Result

Formats the value using the given formatter.