Module rustc::middle::infer::bivariateUnstable

Applies the "bivariance relationship" to two types and/or regions. If (A,B) are bivariant then either A <: B or B <: A. It occurs when type/lifetime parameters are unconstrained. Usually this is an error, but we permit it in the specific case where a type parameter is constrained in a where-clause via an associated type.

There are several ways one could implement bivariance. You could just do nothing at all, for example, or you could fully verify that one of the two subtyping relationships hold. We choose to thread a middle line: we relate types up to regions, but ignore all region relationships.

At one point, handling bivariance in this fashion was necessary for inference, but I'm actually not sure if that is true anymore. In particular, it might be enough to say (A,B) are bivariant for all (A,B).

Structs

Bivariate

Functions

Bivariate