It's scaffolding for supporting let declarations, if let, and switch statements to make function builders more expressive, although that generalization is still a work-in-progress.
7:58 AM
For example, right now a single-expression closure can infer a closure result type: but a multi-statement closure does not: and one will have to provide some type annotations to make this code compile. We should be able to infer the result type of the closure to avoid the need for extra type annotations.
