Struct rustc_infer::infer::generalize::Generalization
source · pub struct Generalization<T> {
pub value: T,
pub needs_wf: bool,
}
Expand description
Result from a generalization operation. This includes not only the generalized type, but also a bool flag indicating whether further WF checks are needed.
Fields§
§value: T
§needs_wf: bool
If true, then the generalized type may not be well-formed, even if the source type is well-formed, so we should add an additional check to enforce that it is. This arises in particular around ‘bivariant’ type parameters that are only constrained by a where-clause. As an example, imagine a type:
struct Foo<A, B> where A: Iterator<Item = B> {
data: A
}
here, A
will be covariant, but B
is
unconstrained. However, whatever it is, for Foo
to be WF, it
must be equal to A::Item
. If we have an input Foo<?A, ?B>
,
then after generalization we will wind up with a type like
Foo<?C, ?D>
. When we enforce that Foo<?A, ?B> <: Foo<?C, ?D>
(or >:
), we will wind up with the requirement that ?A <: ?C
, but no particular relationship between ?B
and ?D
(after all, we do not know the variance of the normalized form
of A::Item
with respect to A
). If we do nothing else, this
may mean that ?D
goes unconstrained (as in #41677). So, in
this scenario where we create a new type variable in a
bivariant context, we set the needs_wf
flag to true. This
will force the calling code to check that WF(Foo<?C, ?D>)
holds, which in turn implies that ?C::Item == ?D
. So once
?C
is constrained, that should suffice to restrict ?D
.
Trait Implementations§
Auto Trait Implementations§
impl<T> RefUnwindSafe for Generalization<T>where T: RefUnwindSafe,
impl<T> Send for Generalization<T>where T: Send,
impl<T> Sync for Generalization<T>where T: Sync,
impl<T> Unpin for Generalization<T>where T: Unpin,
impl<T> UnwindSafe for Generalization<T>where T: UnwindSafe,
Blanket Implementations§
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
Layout§
Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.