UdpSocket: please support vectored send

[joshtriplett]

The UdpSocket type provides send and send_to methods, but does not provide a vectored send operation. Such an operation is particularly helpful for datagrams, where separate send operations result in separate datagrams; a vectored send allows sending data from multiple buffers in a single datagram.

This would require calling sendmsg, which supports supplying an iovec. (This should work on Windows as well.)

[frehberg]

Support for method UdpSocket::send_to_vectored() would be really great

If rustlib would provide this, the feature could be lifted to mio and further to tokio-UdpSocket

[frehberg]

Request for Comments:

I need the functionality UdpSocket::send_to_vectored() in rustlib, then mio and finally in tokio::net

To start with I did a first sketch in rustlib implementing UdpSocket::send_to_vectored() for unix.

Please tell me, if this is heading into the right direction, I would add further code for other target systems, docs and tests
#108120

If this functionality makes it into rustlib, then I will go another layer up in software stack, adding the functionality to mio.

[workingjubilee]

I'm concerned the proposed API has the problem where UDP datagrams tend to resist splitting-up by the protocol, so someone might use send_to_vectored and easily overflow the maximum size of the datagram.

Anyways, please file an API Change Proposal.