Add the preserve_order feature (rebased version)

[Kerollmops]

Here is the #242 PR from @Keats rebased on master.

Therefore it closes #242.

[alexcrichton]

Thanks!

@ehuss you mentioned that you're working on a toml parser preserving style, I'm curious if it'd be at all related to something like this? Were you thinking that'd be a totally standalone project?

(mostly looking to minimize major version bumps)

[ehuss]

🤔 I'm not sure, I've been meaning to talk to you about that. It could go either way. Right now I'm developing it in tree. If it is a separate crate, toml-rs will need to expose a few low-level methods and data structures which doesn't sound too appealing. However, it's a fairly large amount of code and I don't want to just dump it on the project. There's at least a few weeks of work left to do, so maybe once it's more together we could take a look and discuss the options?

As for the relation to this, I think code-wise it is independent. I'm not really touching the serde or Value parts.

There's maybe a bigger question of sharing code with serde_json. For example, to really support #266 it might be nice to use the Number type from serde_json. There's a lot of good stuff in serde_json, and just copying it seems a little weird.

[alexcrichton]

Ah ok, do you know if the changes you'd need, if it goes in this repository, will require breaking changes? I'm fine evaluating the best location for the work once it's done!

I'm somewhat wary about Number and such as integers in toml are apparently defined as signed 64-bit integers, so while we should handle 128-bit integers I don't think we can handle values outside the range of a 64-bit signed integer.

[ehuss]

The only changes I see so far is adding some new pub items. In particular:

• Deserializer::line — this is the main parsing function that will return everything including whitespace and comments.
• Line enum (returned by line)
• The "raw" key and value types from Deserializer that will contain whitespace/comments.
• Deserializer::value and Deserializer::dotted_key for parsing just a key or value.

Right now I have them pub(crate) which would unfortunately require a minimum bump to 1.18. Otherwise I don't see any other fundamental changes.

Good point on the integers.

[alexcrichton]

Ok! In that case I'm gonna go ahead and merge this as it sounds like we probably won't need another major version bump