Propagating closure constraints
When we are checking the type tests and universal regions, we may come across a constraint that we can't prove yet if we are in a closure body! However, the necessary constraints may actually hold (we just don't know it yet). Thus, if we are inside a closure, we just collect all the constraints we can't prove yet and return them. Later, when we are borrow check the MIR node that created the closure, we can also check that these constraints hold. At that time, if we can't prove they hold, we report an error.