CargoMetadata variant on enum Error is not enough structured

[Stargateur]

CargoMetadata {
  stderr: String,
}

CargoMetadata is not usable to give a precise error to the user, notably on program like bacon that end up doing a random parsing on stderr.

[oli-obk]

Hmm... the stderr is unstructured, so I'm not sure how we could emit something better. Do you have any suggestions on how to resolve this issue?

[Stargateur]

I don't know much about cargo metadata maybe by doing the parsing trying a "best guess" of the underline error and fallback to a raw error if the parsing didn't guess the error type, doing the parsing here have the big advantage to not implement the parsing in every crate that use cargo_metadata:

enum Error {
    CargoMetadata(CargoMetadataError),
    // ..
}

enum CargoMetadataError {
  ConfigNotFound {
    file: BufPath,
    dir: BufPath,
  },
  OtherError,
  Unknown{
    stderr: String,
  },
}

This is clearly not perfect but I don't have any better since this is just a wrapper other cargo metadata subcommand. But I think most users of this crate will gain a lot to maintains a common parsing of stderr and the fallback allow to be flexible if cargo change the stderr output.

The first thing to have would be to have a list of every current error cargo metadata can produce. The error can implement Display and Error to reverse back to original output making this breaking change painless.

[oli-obk]

Hmm... I dimly remember that there is a way to make cargo generally emit json errors, maybe that works for cargo metadata, too?

[Stargateur]

you know better than me but I search a little when you ask me a solution and didn't find anything. But I think we could make a request for it #63. And this would help much more than just this crate so that would be very nice. Having a error output format, and have error structure public. To everyone that wrap cargo command. I don't see why cargo is not just a library for everyone could use