If I do cargo publish --help, I see this documentation for the --host flag:

--host HOST              Host to upload the package to

If I wanted to upload to the staging instance of crates.io at https://staging-crates-io.herokuapp.com/, the name of this flag and the docs make me think I should specify --host staging-crates-io.herokuapp.com here. This does not publish to the staging instance, however.

By digging into the code, I've figured out that what this flag actually wants is the location of the index to which I want to publish:

• In the publish binary, host gets used in the PublishOpts.index field
• This code calls index.to_url(), which means it doesn't just want a host, it wants something that parses as a valid URL
• The URL eventually gets used as a git index, and the config.json file gets read to extract the api_host.

When I'm running crates.io locally, i'm able to publish to my local instance by giving cargo --host=file:///path/to/crates.io/tmp/index-co/. I'm able to get to a sort-of-working point with staging crates.io by specifying --host=https://github.com/alexcrichton/cargo-registry-index.

So I think that --host should be renamed to --index-url, or at the very least, the documentation in all the commands that use --host should explain that it should be a URL to the index you want to publish to.