Export the #[derive(FromPrimitive)] from rust proper

[erickt]

This changes the name to #[derive(NumFromPrimitive)] in order to not conflict with the libsyntax macro.

If the general direction of this num library will supplant std::num, then I'd instead prefer to do the same trick we did for rustc-serialize, and have the compiler add a #[derive(RustcFromPrimitive)] that uses the trait in this crate instead.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[alexcrichton]

I think it's fine to start this at 0.1.0, and could this be formatted as:

[dev-dependencies]
num-macros = { path = "num-macros", version = "0.1.0" }

[alexcrichton]

Nice! Could you also edit .travis.yml to run the tests for this new crate as well?

Also cc @aturon, how do you feel about this?

[aturon]

Sorry I missed this before. Someone wrote a macro to replace this derive mode, so my preference for the moment would be to remove this altogether.

The rust-lang num crate may eventually be a place to test ideas for std::num, but at the moment it's largely a place that deprecated items have been dropped into.

[nwin]

The macro solution is quite limited. It is difficult to impossible to document your enums (#24189). I would be very much in favor of #[derive(NumFromPrimitive)] as an intermediate solution.

[aturon]

OK, taking a closer look at this, I'm fine with this direction for the moment. (In the long run, I still hope we can provide better built-in support in Rust itself for this kind of conversion of enums).

[alexcrichton]

I renamed num_macros to num-macros and otherwise just tidied up the merge a bit, thanks again @erickt!