Struct rustc_infer::infer::combine::Generalization

struct Generalization<'tcx> {
    ty: Ty<'tcx>,
    needs_wf: bool,
}

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

ty: Ty<'tcx>

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.