pub enum Variance {
Covariant,
Invariant,
Contravariant,
Bivariant,
}
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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.
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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
.
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Performs copy-assignment from source
. Read more
This method tests for self
and other
values to be equal, and is used by ==
. Read more
This method tests for !=
.
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Formats the value using the given formatter. Read more
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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
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The type returned in the event of a conversion error.
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Immutably borrows from an owned value. Read more
type Error = <U as TryFrom<T>>::Error
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The type returned in the event of a conversion error.
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Mutably borrows from an owned value. Read more
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this method will likely be replaced by an associated static
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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