pub enum Variance {
Covariant,
Invariant,
Contravariant,
Bivariant,
}
Variants§
Implementations§
source§impl Variance
impl Variance
sourcepub fn xform(self, v: Variance) -> 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§
source§impl<__D> Decodable<__D> for Variancewhere
__D: SpanDecoder,
impl<__D> Decodable<__D> for Variancewhere
__D: SpanDecoder,
source§impl<__E> Encodable<__E> for Variancewhere
__E: SpanEncoder,
impl<__E> Encodable<__E> for Variancewhere
__E: SpanEncoder,
source§impl<__CTX> HashStable<__CTX> for Variance
impl<__CTX> HashStable<__CTX> for Variance
fn hash_stable( &self, __hcx: &mut __CTX, __hasher: &mut StableHasher<SipHasher128>, )
source§impl<'tcx, D> RefDecodable<'tcx, D> for Variance
impl<'tcx, D> RefDecodable<'tcx, D> for Variance
source§impl<I> TypeFoldable<I> for Variancewhere
I: Interner,
impl<I> TypeFoldable<I> for Variancewhere
I: Interner,
source§fn try_fold_with<F>(
self,
_: &mut F,
) -> Result<Variance, <F as FallibleTypeFolder<I>>::Error>where
F: FallibleTypeFolder<I>,
fn try_fold_with<F>(
self,
_: &mut F,
) -> Result<Variance, <F as FallibleTypeFolder<I>>::Error>where
F: FallibleTypeFolder<I>,
source§impl<I> TypeVisitable<I> for Variancewhere
I: Interner,
impl<I> TypeVisitable<I> for Variancewhere
I: Interner,
source§fn visit_with<F>(&self, _: &mut F) -> <F as TypeVisitor<I>>::Resultwhere
F: TypeVisitor<I>,
fn visit_with<F>(&self, _: &mut F) -> <F as TypeVisitor<I>>::Resultwhere
F: TypeVisitor<I>,
impl Copy for Variance
impl Eq for Variance
impl StructuralPartialEq for Variance
Auto Trait Implementations§
impl Freeze for Variance
impl RefUnwindSafe for Variance
impl Send for Variance
impl Sync for Variance
impl Unpin for Variance
impl UnwindSafe for Variance
Blanket Implementations§
source§impl<'tcx, T> ArenaAllocatable<'tcx, IsCopy> for Twhere
T: Copy,
impl<'tcx, T> ArenaAllocatable<'tcx, IsCopy> for Twhere
T: Copy,
fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut T
fn allocate_from_iter<'a>( arena: &'a Arena<'tcx>, iter: impl IntoIterator<Item = T>, ) -> &'a mut [T]
source§impl<'tcx, T> ArenaAllocatable<'tcx, IsCopy> for Twhere
T: Copy,
impl<'tcx, T> ArenaAllocatable<'tcx, IsCopy> for Twhere
T: Copy,
fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut T
fn allocate_from_iter<'a>( arena: &'a Arena<'tcx>, iter: impl IntoIterator<Item = T>, ) -> &'a mut [T]
source§impl<T> BorrowMut<T> for Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere
T: ?Sized,
source§fn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
source§impl<T> CloneToUninit for Twhere
T: Clone,
impl<T> CloneToUninit for Twhere
T: Clone,
source§unsafe fn clone_to_uninit(&self, dst: *mut T)
unsafe fn clone_to_uninit(&self, dst: *mut T)
clone_to_uninit
)source§impl<T, R> CollectAndApply<T, R> for T
impl<T, R> CollectAndApply<T, R> for T
source§impl<Tcx, T> DepNodeParams<Tcx> for T
impl<Tcx, T> DepNodeParams<Tcx> for T
default fn fingerprint_style() -> FingerprintStyle
source§default fn to_fingerprint(&self, tcx: Tcx) -> Fingerprint
default fn to_fingerprint(&self, tcx: Tcx) -> Fingerprint
default fn to_debug_str(&self, _: Tcx) -> String
source§default fn recover(_: Tcx, _: &DepNode) -> Option<T>
default fn recover(_: Tcx, _: &DepNode) -> Option<T>
DepNode
,
something which is needed when forcing DepNode
s during red-green
evaluation. The query system will only call this method if
fingerprint_style()
is not FingerprintStyle::Opaque
.
It is always valid to return None
here, in which case incremental
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impl<Q, K> Equivalent<K> for Q
source§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
source§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
source§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
key
and return true
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impl<T> Filterable for T
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self,
filter_name: &'static str,
) -> RequestFilterDataProvider<T, fn(_: DataRequest<'_>) -> bool>
fn filterable( self, filter_name: &'static str, ) -> RequestFilterDataProvider<T, fn(_: DataRequest<'_>) -> bool>
source§impl<T> Instrument for T
impl<T> Instrument for T
source§fn instrument(self, span: Span) -> Instrumented<Self>
fn instrument(self, span: Span) -> Instrumented<Self>
source§fn in_current_span(self) -> Instrumented<Self>
fn in_current_span(self) -> Instrumented<Self>
source§impl<T> IntoEither for T
impl<T> IntoEither for T
source§fn into_either(self, into_left: bool) -> Either<Self, Self>
fn into_either(self, into_left: bool) -> Either<Self, Self>
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variant of Either<Self, Self>
if into_left
is true
.
Converts self
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fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
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into a Left
variant of Either<Self, Self>
if into_left(&self)
returns true
.
Converts self
into a Right
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impl<P> IntoQueryParam<P> for P
fn into_query_param(self) -> P
source§impl<'tcx, T> IsSuggestable<'tcx> for T
impl<'tcx, T> IsSuggestable<'tcx> for T
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impl<T> MaybeResult<T> for T
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I: Interner,
T: TypeVisitable<I>,
impl<I, T> TypeVisitableExt<I> for Twhere
I: Interner,
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fn has_type_flags(&self, flags: TypeFlags) -> bool
source§fn has_vars_bound_at_or_above(&self, binder: DebruijnIndex) -> bool
fn has_vars_bound_at_or_above(&self, binder: DebruijnIndex) -> bool
true
if self
has any late-bound regions that are either
bound by binder
or bound by some binder outside of binder
.
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, this indicates whether
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true
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fn has_escaping_bound_vars(&self) -> bool
true
if this type has regions that are not a part of the type.
For example, for<'a> fn(&'a i32)
return false
, while fn(&'a i32)
would return true
. The latter can occur when traversing through the
former. Read morefn has_aliases(&self) -> bool
fn has_opaque_types(&self) -> bool
fn has_coroutines(&self) -> bool
fn references_error(&self) -> bool
fn has_non_region_param(&self) -> bool
fn has_infer_regions(&self) -> bool
fn has_infer_types(&self) -> bool
fn has_non_region_infer(&self) -> bool
fn has_infer(&self) -> bool
fn has_placeholders(&self) -> bool
fn has_non_region_placeholders(&self) -> bool
fn has_param(&self) -> bool
source§fn has_free_regions(&self) -> bool
fn has_free_regions(&self) -> bool
fn has_erased_regions(&self) -> bool
source§fn has_erasable_regions(&self) -> bool
fn has_erasable_regions(&self) -> bool
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fn is_global(&self) -> bool
source§fn has_bound_regions(&self) -> bool
fn has_bound_regions(&self) -> bool
source§fn has_non_region_bound_vars(&self) -> bool
fn has_non_region_bound_vars(&self) -> bool
source§fn has_bound_vars(&self) -> bool
fn has_bound_vars(&self) -> bool
source§fn still_further_specializable(&self) -> bool
fn still_further_specializable(&self) -> bool
impl
specialization.source§impl<I, T, U> Upcast<I, U> for Twhere
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source§impl<Tcx, T> Value<Tcx> for Twhere
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source§impl<T> WithSubscriber for T
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Layout§
Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...)
attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.
Size: 1 byte
Size for each variant:
Covariant
: 0 bytesInvariant
: 0 bytesContravariant
: 0 bytesBivariant
: 0 bytes