Enum rustc_type_ir::Variance

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

Variants

Covariant

Invariant

Contravariant

Bivariant

Implementations

impl Variance

pub fn xform(self, v: 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.

Trait Implementations

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 Debug for Variance

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

Formats the value using the given formatter.

impl<__D: Decoder> Decodable<__D> for Variance

fn decode(__decoder: &mut __D) -> Result<Self, <__D as Decoder>::Error>

impl<__E: Encoder> Encodable<__E> for Variance

fn encode(&self, __encoder: &mut __E) -> Result<(), <__E as Encoder>::Error>

impl Hash for Variance

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

Feeds this value into the given Hasher.

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

Feeds a slice of this type into the given Hasher.

impl<CTX> HashStable<CTX> for Variance

fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher)

impl PartialEq<Variance> for Variance

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

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

#[must_use]

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

This method tests for !=.

impl Copy for Variance

impl StructuralPartialEq for Variance