Struct Variance 

pub struct Variance(/* private fields */);

Variance of a type parameter in a generic type.

Variance determines how subtyping of generic types relates to subtyping of their type arguments. This is critical for O(1) generic assignability.

Variance Kinds

• Covariant (COVARIANT): T <: T iff U <: V

  • Example: Array, ReadonlyArray, Promise

• Most common for immutable containers

• Contravariant (CONTRAVARIANT): T <: T iff V <: U (reversed)

  • Example: Function parameters (in strict mode)

• Rare in practice, mostly for function types

• Invariant (COVARIANT | CONTRAVARIANT): T <: T iff U === V

  • Example: Mutable properties, Box<T> with read/write

• Requires both directions to hold

• Independent (empty): Type parameter not used in variance position

  • Example: Type parameter only used in non-variance positions

• Can be skipped in subtype checks (always compatible)

Examples

// Covariant: Array< Dog > <: Array< Animal >
type Covariant<T> = { readonly get(): T };

// Contravariant: Writer< Animal > <: Writer< Dog >
type Contravariant<T> = { write(x: T): void };

// Invariant: Box<Dog> NOT <: Box<Animal> (mutable!)
type Invariant<T> = { get(): T; set(x: T): void };

Implementations

impl Variance

pub const COVARIANT: Self

Covariant position (e.g., function return types)

pub const CONTRAVARIANT: Self

Contravariant position (e.g., function parameters)

impl Variance

pub const fn empty() -> Self

Get a flags value with all bits unset.

pub const fn all() -> Self

Get a flags value with all known bits set.

pub const fn bits(&self) -> u8

Get the underlying bits value.

The returned value is exactly the bits set in this flags value.

pub const fn from_bits(bits: u8) -> Option<Self>

Convert from a bits value.

This method will return None if any unknown bits are set.

pub const fn from_bits_truncate(bits: u8) -> Self

Convert from a bits value, unsetting any unknown bits.

pub const fn from_bits_retain(bits: u8) -> Self

Convert from a bits value exactly.

pub fn from_name(name: &str) -> Option<Self>

Get a flags value with the bits of a flag with the given name set.

This method will return None if name is empty or doesn’t correspond to any named flag.

pub const fn is_empty(&self) -> bool

Whether all bits in this flags value are unset.

pub const fn is_all(&self) -> bool

Whether all known bits in this flags value are set.

pub const fn intersects(&self, other: Self) -> bool

Whether any set bits in a source flags value are also set in a target flags value.

pub const fn contains(&self, other: Self) -> bool

Whether all set bits in a source flags value are also set in a target flags value.

pub fn insert(&mut self, other: Self)

The bitwise or (|) of the bits in two flags values.

pub fn remove(&mut self, other: Self)

The intersection of a source flags value with the complement of a target flags value (&!).

This method is not equivalent to self & !other when other has unknown bits set. remove won’t truncate other, but the ! operator will.

pub fn toggle(&mut self, other: Self)

The bitwise exclusive-or (^) of the bits in two flags values.

pub fn set(&mut self, other: Self, value: bool)

Call insert when value is true or remove when value is false.

pub const fn intersection(self, other: Self) -> Self

The bitwise and (&) of the bits in two flags values.

pub const fn union(self, other: Self) -> Self

The bitwise or (|) of the bits in two flags values.

pub const fn difference(self, other: Self) -> Self

The intersection of a source flags value with the complement of a target flags value (&!).

This method is not equivalent to self & !other when other has unknown bits set. difference won’t truncate other, but the ! operator will.

pub const fn symmetric_difference(self, other: Self) -> Self

The bitwise exclusive-or (^) of the bits in two flags values.

pub const fn complement(self) -> Self

The bitwise negation (!) of the bits in a flags value, truncating the result.

impl Variance

pub const fn iter(&self) -> Iter<Variance>

Yield a set of contained flags values.

Each yielded flags value will correspond to a defined named flag. Any unknown bits will be yielded together as a final flags value.

pub const fn iter_names(&self) -> IterNames<Variance>

Yield a set of contained named flags values.

This method is like iter, except only yields bits in contained named flags. Any unknown bits, or bits not corresponding to a contained flag will not be yielded.

impl Variance

pub const fn is_independent(&self) -> bool

Check if this is an independent type parameter (not used in variance position).

pub const fn is_covariant(&self) -> bool

Check if this is covariant only.

pub const fn is_contravariant(&self) -> bool

Check if this is contravariant only.

pub fn is_invariant(&self) -> bool

Check if this is invariant (both covariant and contravariant).

pub fn compose(&self, other: Self) -> Self

Compose two variances (for nested generics).

Rules:

• Independent × anything = Independent
• Covariant × Covariant = Covariant
• Covariant × Contravariant = Contravariant
• Contravariant × Covariant = Contravariant
• Contravariant × Contravariant = Covariant
• Invariant × anything = Invariant

Trait Implementations

impl Binary for Variance

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

Formats the value using the given formatter.

impl BitAnd for Variance

fn bitand(self, other: Self) -> Self

The bitwise and (&) of the bits in two flags values.

type Output = Variance

The resulting type after applying the & operator.

impl BitAndAssign for Variance

fn bitand_assign(&mut self, other: Self)

The bitwise and (&) of the bits in two flags values.

impl BitOr for Variance

fn bitor(self, other: Variance) -> Self

The bitwise or (|) of the bits in two flags values.

type Output = Variance

The resulting type after applying the | operator.

impl BitOrAssign for Variance

fn bitor_assign(&mut self, other: Self)

The bitwise or (|) of the bits in two flags values.

impl BitXor for Variance

fn bitxor(self, other: Self) -> Self

The bitwise exclusive-or (^) of the bits in two flags values.

type Output = Variance

The resulting type after applying the ^ operator.

impl BitXorAssign for Variance

fn bitxor_assign(&mut self, other: Self)

The bitwise exclusive-or (^) of the bits in two flags values.

impl Clone for Variance

fn clone(&self) -> Variance

Returns a duplicate 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 Default for Variance

fn default() -> Variance

Returns the “default value” for a type.

impl Extend<Variance> for Variance

fn extend<T: IntoIterator<Item = Self>>(&mut self, iterator: T)

The bitwise or (|) of the bits in each flags value.

fn extend_one(&mut self, item: A)

🔬This is a nightly-only experimental API. (extend_one)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one)

Reserves capacity in a collection for the given number of additional elements.

impl Flags for Variance

const FLAGS: &'static [Flag<Variance>]

The set of defined flags.

type Bits = u8

The underlying bits type.

fn bits(&self) -> u8

Get the underlying bits value.

fn from_bits_retain(bits: u8) -> Variance

Convert from a bits value exactly.

fn empty() -> Self

Get a flags value with all bits unset.

fn all() -> Self

Get a flags value with all known bits set.

fn known_bits(&self) -> Self::Bits

Get the known bits from a flags value.

fn unknown_bits(&self) -> Self::Bits

Get the unknown bits from a flags value.

fn contains_unknown_bits(&self) -> bool

This method will return true if any unknown bits are set.

fn from_bits(bits: Self::Bits) -> Option<Self>

Convert from a bits value.

fn from_bits_truncate(bits: Self::Bits) -> Self

Convert from a bits value, unsetting any unknown bits.

fn from_name(name: &str) -> Option<Self>

Get a flags value with the bits of a flag with the given name set.

fn iter(&self) -> Iter<Self>

Yield a set of contained flags values.

fn iter_names(&self) -> IterNames<Self>

Yield a set of contained named flags values.

fn iter_defined_names() -> IterDefinedNames<Self>

Yield a set of all named flags defined by Self::FLAGS.

fn is_empty(&self) -> bool

Whether all bits in this flags value are unset.

fn is_all(&self) -> bool

Whether all known bits in this flags value are set.

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

where Self: Sized,

Whether any set bits in a source flags value are also set in a target flags value.

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

where Self: Sized,

Whether all set bits in a source flags value are also set in a target flags value.

fn truncate(&mut self)

where Self: Sized,

Remove any unknown bits from the flags.

fn insert(&mut self, other: Self)

where Self: Sized,

The bitwise or (|) of the bits in two flags values.

fn remove(&mut self, other: Self)

where Self: Sized,

The intersection of a source flags value with the complement of a target flags value (&!).

fn toggle(&mut self, other: Self)

where Self: Sized,

The bitwise exclusive-or (^) of the bits in two flags values.

fn set(&mut self, other: Self, value: bool)

where Self: Sized,

Call Flags::insert when value is true or Flags::remove when value is false.

fn clear(&mut self)

where Self: Sized,

Unsets all bits in the flags.

fn intersection(self, other: Self) -> Self

The bitwise and (&) of the bits in two flags values.

fn union(self, other: Self) -> Self

The bitwise or (|) of the bits in two flags values.

fn difference(self, other: Self) -> Self

The intersection of a source flags value with the complement of a target flags value (&!).

fn symmetric_difference(self, other: Self) -> Self

The bitwise exclusive-or (^) of the bits in two flags values.

fn complement(self) -> Self

The bitwise negation (!) of the bits in a flags value, truncating the result.

impl FromIterator<Variance> for Variance

fn from_iter<T: IntoIterator<Item = Self>>(iterator: T) -> Self

The bitwise or (|) of the bits in each flags value.

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, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl IntoIterator for Variance

type Item = Variance

The type of the elements being iterated over.

type IntoIter = Iter<Variance>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.

impl LowerHex for Variance

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

Formats the value using the given formatter.

impl Not for Variance

fn not(self) -> Self

The bitwise negation (!) of the bits in a flags value, truncating the result.

type Output = Variance

The resulting type after applying the ! operator.

impl Octal for Variance

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

Formats the value using the given formatter.

impl PartialEq for Variance

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

Tests for self and other values to be equal, and is used by ==.

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

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl PublicFlags for Variance

type Primitive = u8

The type of the underlying storage.

type Internal = InternalBitFlags

The type of the internal field on the generated flags type.

impl Sub for Variance

fn sub(self, other: Self) -> Self

The intersection of a source flags value with the complement of a target flags value (&!).

This method is not equivalent to self & !other when other has unknown bits set. difference won’t truncate other, but the ! operator will.

type Output = Variance

The resulting type after applying the - operator.

impl SubAssign for Variance

fn sub_assign(&mut self, other: Self)

The intersection of a source flags value with the complement of a target flags value (&!).

This method is not equivalent to self & !other when other has unknown bits set. difference won’t truncate other, but the ! operator will.

impl UpperHex for Variance

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

Formats the value using the given formatter.

impl Copy for Variance

impl Eq for Variance

impl StructuralPartialEq for Variance