Add support for package.metadata table

[phil-opp]

Since rust-lang/cargo#5360 cargo metadata outputs the contents of the optional package.metadata table. This PR adds support for the table to this crate. Since the table can contain arbitrary content, the return type is a dynamic serde_json::Map.

Unresolved questions:

• Is it ok to expose types of the serde_json dependency in the public API? Or do we need to make the serde_json dependency public?
• The test doesn't work on stable yet. Is there a way to conditionally execute the test only on nightly and beta?

[oli-obk]

The test doesn't work on stable yet. Is there a way to conditionally execute the test only on nightly and beta?

There should be env vars in travis.

Is it ok to expose types of the serde_json dependency in the public API? Or do we need to make the serde_json dependency public?

Maybe we could make the field generic. If it's generic the user can choose their own metadata type instead of having to get their data from a serde map.

For backwards compat you could rename all generic types to GenericFoo or sth and reexport via type Foo = GenericFoo<()>; or type Foo = GenericFoo<json::Map<String, serde_json::Value>>;

no need to reexport serde_json, bumping to a potential 2.0 would require a version bump anyway.

[oli-obk]

Does this need to be an Option? Can't it simply be an empty table? I don't want to distinguish between no table and an empty table. That feels like noise

[phil-opp]

An empty table doesn't parse if the key is not present. But I pushed a new version that makes the field a serde_json::Value (null if not present).

[oli-obk]

Have you tried #[serde(default)] on the metadata field?

[phil-opp]

It seems like the metadata field is always present and null if not set. serde(default) fails in this case. So I think serde_json::Value is the correct type here.

[phil-opp]

But serde(default) helps that no “missing field metadata” error occurs on stable.

[phil-opp]

There should be env vars in travis.

Ok. Should I add an if condition to the test that checks the environment variable? Or should I create a new nightly/beta only test and put the condition in the travis.yml?

Maybe we could make the field generic. If it's generic the user can choose their own metadata type instead of having to get their data from a serde map.

Sounds like a good idea, but I'm not quite sure if this would work with multiple packages. The metadata struct stores a Vec<Package>, so if we make Package generic all packages must have the same metadata package.

[oli-obk]

Ok. Should I add an if condition to the test that checks the environment variable? Or should I create a new nightly/beta only test and put the condition in the travis.yml?

just put it in the test, maybe with a nice error message if the condition is not set, so users running locally know what's going on.

Sounds like a good idea, but I'm not quite sure if this would work with multiple packages. The metadata struct stores a Vec, so if we make Package generic all packages must have the same metadata package.

oh god. yea let's not do anything here. Just store the json and maybe provide an example in the readme showing how to deserialize that into a custom structure.

[phil-opp]

I added a “NOTE: This test currently only works on the beta and nightly channel.” error message and a short example how to transform a serde_json::Value into a custom struct.

[phil-opp]

Thanks! Could you release a new version to crates.io?

[oli-obk]

I do have a deskop open for it and even ran cargo test then forgot about it -.- Thanks for the reminder

[oli-obk]

published as v0.5.5

[phil-opp]

Thanks!