Implement playground-specific custom metadata to control enabled features

[dtolnay]

This is based on the custom metadata configurable for docs.rs -- see https://docs.rs/about. Ours looks like:

# Cargo.toml
[package]
name = "test"

[package.metadata.playground]
default-features = true
features = ["std", "extra-traits"]
all-features = false

This PR includes a bump of the Cargo dependency to pick up rust-lang/cargo#5360 which has not been included in a release yet.

Motivation

I want the "extra-traits" feature of Syn so that people can debug print a syntax tree. This comes up often in IRC.

extern crate syn;

fn main() {
    println!("{:#?}", syn::parse_str::<syn::Type>("u8").unwrap());
}

[shepmaster]

heh

[shepmaster]

As when you added the original feature support: wow, thanks!

Do you think the metadata should completely replace the playground feature in time?

[dtolnay]

Yes I think custom metadata completely replaces our earlier scheme of looking for a "playground" feature. That was implemented in #100 in February 2017 and custom metadata only became available for published crates in rust-lang/cargo#4167 in June 2017. If it had been available earlier we would have used that. (I think preserving custom metadata for published crates was blocked on upgrading Cargo from rustc-serialize to Serde because rustc-serialize cannot handle deserializing data of unspecified structure.)

For now the uuid crate still uses a playground feature: https://github.com/uuid-rs/uuid/blob/0.6.3/Cargo.toml#L46. If you are on board with the approach in this PR, I would land this, then send a PR to uuid to switch to custom metadata, cut a uuid release, and finally remove the playground feature here.

[shepmaster]

Yeah, I agree with everything you said. I'll wait just a while to see what the UUID devs have to say; I think they added that comparatively recently.

[dtolnay]

In uuid-rs/uuid#224 it looks like they are good with it. That person reviewed the original change in uuid-rs/uuid#189.

[kinggoesgaming]

Yes, I would love this change...

Just a note: the original uuid PR for this was uuid-rs/uuid#155

[shepmaster]

Forward to the future!