Introduce Credentials

[nox]

This new struct reads from .cargo/credentials.toml

[nox]

I read through Cargo's code and AFAICT .cargo/credentials.toml takes precedence over whatever is found in .cargo/config.toml, but env variables still have the strongest preference. I'm not sure whether we want to directly read .cargo/credentials.toml when using Config::load so I just introduced new structs without changing how config files are read.

[taiki-e]

Thanks for the PR!

Is it possible to add a test?

I read through Cargo's code and AFAICT .cargo/credentials.toml takes precedence over whatever is found in .cargo/config.toml, but env variables still have the strongest preference. I'm not sure whether we want to directly read .cargo/credentials.toml when using Config::load so I just introduced new structs without changing how config files are read.

I think not reading the .cargo/credentials.toml by default is a reasonable decision considering users who do not need credentials.

That said, it might be a bit annoying for users who need credentials, as they would have to manually emulate Cargo's processing there. I think the more preferred option here would be one of the following (although it would require a breaking change).

• Put the credentials related fields behind the Cargo feature (i.e., make them optional) and do the complete process including loading .cargo/credentials.toml when the feature is enabled.
• Change load_with_options to take a struct and add an option to choose whether to read .cargo/credentials.toml.

The breaking change to load_with_options is something that being considered for #3 anyway, and given that it has been over 2 years since the 0.1.0 release, there is no problem to release 0.2.0 with breaking changes.