Found it. The address used as the local endpoint for the socket, in SetupLocalSocket(), used a similarly naive method of getting the address; by resolving the local host and getting the first address. That first address, more often than not, is an IPv6 address, not the IPv4 address that was obviously expected. So, I had it look for the first IPv4 address in the list and use that as the endpoint, and it worked.

In our previous post we introduced the placeholder async<A> syntax to describe the concept of a "function which is generic over its asyncness". We always knew we wanted something that felt lighter weight than that, so in for our current design we've chosen to drop the notion of a generic parameter for the end-user syntax, and instead picked the ?async notation. We've borrowed this from the trait system, where for example + ?Sized indicates that something may or may not implement the Sized trait. Similarly ?async means a function may or may not be async. We also refer to these as "maybe-async" functions.