Macro for selecting from multiple ports

[jruderman]

Could macros build on #1847 to create nice syntax for selecting from differently-typed ports?

#select {
    int_port: n { consume_int(n); }
    str_port: s { consume_str(s); }
}

[brson]

Yes, I think that could work.

[eholk]

I think with @paulstansifer's new macro system, and a lot of the infrastructure in pipes, we can probably make this work now. I'm currently experimenting. It will be a pretty hairy macro.

[eholk]

I've been working on the pipe select macro here: https://github.com/mozilla/rust/blob/incoming/src/test/run-pass/pipe-select-macro.rs

Due to bugs in the macro system, it doesn't work yet, but I don't see any insurmountable problems.

[eholk]

This commit has a version of the macro that works: 4ec1dd9

I'm not sure the problem was so much a bug in the macro system as some surprising restrictions on pattern syntax and less than informative error messages...

[eholk]

This macro seems pretty complete, so I'm going to close this bug. You can import the macro using include!, but clearly to do this right we need #3114.