Preserve the order of requested fields

[LegNeato]

Fixes #82

Note: I am new to Rust, please let me know if there is anything I can do better.

[theduke]

Since the spec recommends it, I'm definitely in favor.

Thanks for the PR, @LegNeato!

There are two open questions for me:

• what is the performance impact of linked-hash-map vs Hashmap?

• I'm not so comfortable with bringing in https://github.com/contain-rs/linked-hash-map as a dependency
  The README states:
  WARNING: THIS PROJECT IS IN MAINTENANCE MODE, DUE TO INSUFFICIENT MAINTAINER RESOURCES

Is there maybe another order preserving hashmap crate around which is better maintained?

[LegNeato]

Re: perf, I couldn't get cargo bench to work locally...any pointers would be appreciated.

Re: linked-hash-map, I was hesitant as well. https://github.com/bluss/ordermap looks better maintained but doesn't yet have serde support (indexmap-rs/indexmap#28) which juniper needs. Also, linked-hash-map maintainers said they will merge changes to keep it compatible and building...which I think is all we really need for juniper in the future. I intend to keep an eye on ordermap and switch if/when it has serde support.

[rushmorem]

Can't we switch to OrderMap and use remote derive for now?

[LegNeato]

I just tried and it doesn't look like you can use remote derive for this, as just using

#[derive(Serialize, Deserialize)]
#[serde(remote = "OrderMap")]

Fails as the default derive isn't sufficent and

#[serde(remote = "OrderMap")]

fails without derive().

I'm a Rust n00b, so if there is a way to do this that I am missing please let me know!

[LegNeato]

I got them to do a release, so latest changes use ordermap.

[bluss]

Nice to see that you get some utility out of ordermap. As a point of order, you'll should probably add Ordermap's 1.0-ing on your own 1.0 road map, since it's a “public dependency” when used like this.

OrderMap itself doesn't have an 1.0 road map, for the record, but the future is open ended.

[mhallin]

Thanks for this PR! Bringing us closer to spec compliance :)