Implement new return type for poll_at

[barskern]

Change return type for poll_at for sockets to be a PollAt instead of the former Option<Instant>.

pub(crate) enum PollAt {
    New,
    Time(Instant),
    Ingress,
}

NB! PollAt is for now just an internal type related to the sockets. I did not find a good place to put it if it should be a part of the public API. This means that the signature of the public function smoltcp::iface::ethernet::Interface::poll_at remains at

pub fn poll_at(&self, sockets: &SocketSet, timestamp: Instant) -> Option<Instant>

Maybe it should be considered to change this function to fit with the newly created type, however it would be a breaking change and would, IMO, require moving the implementation of PollAt to a different location.

See also: #215

[whitequark]

@m-labs-homu r+

[m-labs-homu]

📌 Commit 786f043 has been approved by whitequark

[m-labs-homu]

⌛ Testing commit 786f043 with merge 805e11b...

[m-labs-homu]

💔 Test failed - status-travis

[barskern]

The failed tests on Travis are because of the function PollAt::is_ingress, which is currently only being used in the TcpSocket. I was thinking that this function will probably be useful in other contexts aswell to filter out non-interesting sockets. It is just to make that check a bit less verbose. I could add a attribute to conditionally compile it, but wouldn't that require unnecessary changes to the attributes if it will be used in the future?

Conditional compile vs #[allow(unused)]

[whitequark]

Sorry, I misread. We typically use conditional compilation in these cases.

[whitequark]

@m-labs-homu r+

[m-labs-homu]

📌 Commit e035aa5 has been approved by whitequark

[m-labs-homu]

⌛ Testing commit e035aa5 with merge 3fb0c22...

[m-labs-homu]

☀️ Test successful - status-travis
Approved by: whitequark
Pushing 3fb0c22 to master...