[][src]Enum rustc::ty::Variance

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

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Methods

impl Variance
[src]

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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<'a, 'tcx> HashStable<StableHashingContext<'a>> for Variance
[src]

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impl<'tcx> TypeFoldable<'tcx> for Variance
[src]

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True if self has any late-bound regions that are either bound by binder or bound by some binder outside of binder. If binder is ty::INNERMOST, this indicates whether there are any late-bound regions that appear free. Read more

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True if this self has any regions that escape binder (and hence are not bound by it). Read more

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"Free" regions in this context means that it has any region that is not (a) erased or (b) late-bound. Read more

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True if there any any un-erased free regions.

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Indicates whether this value references only 'global' types/lifetimes that are the same regardless of what fn we are in. This is used for caching. Read more

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True if there are any late-bound regions

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A visitor that does not recurse into types, works like fn walk_shallow in Ty.

impl<'tcx> Lift<'tcx> for Variance
[src]

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impl Clone for Variance
[src]

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

impl PartialEq for Variance
[src]

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

This method tests for !=.

impl Decodable for Variance
[src]

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impl Encodable for Variance
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impl Copy for Variance
[src]

impl Print for Variance
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impl Debug for Variance
[src]

Formats the value using the given formatter. Read more

Auto Trait Implementations

impl Send for Variance

impl Sync for Variance

Blanket Implementations

impl<T> MaybeResult for T
[src]

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impl<'tcx, T> Subst for T where
    T: TypeFoldable<'tcx>, 
[src]

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impl<'a, T> Captures for T where
    T: ?Sized
[src]

impl<T> ToOwned for T where
    T: Clone
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Creates owned data from borrowed data, usually by cloning. Read more

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recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

impl<T> From for T
[src]

Performs the conversion.

impl<T, U> Into for T where
    U: From<T>, 
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Performs the conversion.

impl<T, U> TryFrom for T where
    T: From<U>, 
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The type returned in the event of a conversion error.

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Performs the conversion.

impl<T> Borrow for T where
    T: ?Sized
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Important traits for &'a mut R

Immutably borrows from an owned value. Read more

impl<T, U> TryInto for T where
    U: TryFrom<T>, 
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The type returned in the event of a conversion error.

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Performs the conversion.

impl<T> BorrowMut for T where
    T: ?Sized
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Important traits for &'a mut R

Mutably borrows from an owned value. Read more

impl<T> Any for T where
    T: 'static + ?Sized
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Gets the TypeId of self. Read more

impl<T> Encodable for T where
    T: UseSpecializedEncodable + ?Sized
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impl<T> Decodable for T where
    T: UseSpecializedDecodable
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impl<E> SpecializationError for E
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Create an error for a missing method specialization. Defaults to panicking with type, trait & method names. S is the encoder/decoder state type, T is the type being encoded/decoded, and the arguments are the names of the trait and method that should've been overridden. Read more

impl<T> Erased for T
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impl<T> Send for T where
    T: ?Sized
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impl<T> Sync for T where
    T: ?Sized
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impl<T> Erased for T