Replace `send` and `recv` flags with a more type-safe API

[dcbishop]

Currently send/recv functions just take them as an int and there are no defines.

At the very least there should be static definitions. But would probably be better as a proper 'Flag' type (enum?).

[dcbishop]

Just noticed there are statics (i was greping for NOBLOCK but it got renamed to DONTWAIT in newer ZMQ). But having a proper type would still seem more rustic (rustonic?).

[erickt]

Hello @dcbishop. Rust proper is exploring a proper solution to passing around flags like these (see rust-lang/rust#6085 and rust-lang/rust#13072). Once something is decided, I'll switch rust-zmq over to a similar approach.

[jdpage]

The bitflags! macro should be able to solve this now.

[rotty]

@jdpage, @dcbishop Indeed, that area of the API is could do with refinement. As the bitflags! macro looks like the currently established way to achieve that, I hope to come up with a PR making use of that, unless anyone beats me to it (which I'd welcome ;-).

[rotty]

This will go in after 0.9, as there seems to be no way to still allow the use of "0" as a flag value using the bitflags crate. I guess for the sake of improved readability, this shouldn't be allowed, either. Thus this requires an API break.

[rotty]

After having pondered about this for a while, I don't think that bitflags is the appropriate solution here. We have different sets of permissible flags, e.g. SNDMORE doesn't make sense with send_multipart() or recv(). Ideally, we want to expose this at the type level, but I'm quite short of a concrete proposal yet, if anyone has ideas about this, I'm all ears!

[rotty]

My current thinking goes in the direction of eliminating, instead of exposing SNDMORE and DONTWAIT. An initial, rough sketch, and some thoughts can be found in the commits on my wip/flagless branch.

[rotty]

Besides SNDMORE and DONTWAIT, which I'm still undecided how to deal with, there is also POLLIN, POLLOUT and POLLERR, which should probably be turned into a bitflags type.

[rotty]

Just a status update: for the poll flags, I now have added a type alias in release/v0.8, see #151, this will be followed up by a switch to bitflags on master. Regarding SNDMORE and DONTWAIT, I've opened an issue to clarify their interaction in zeromq/libzmq#2269. I'll be stalling further "flagless" work waiting for that issue to (hopefully) reach a conclusion.

[BatmanAoD]

@rotty Any status update? Looks like v0.9 has still not been released.