Support for alternative crates.io registries in cargo

[cswindle]

Currently, cargo supports crates.io and local registries, but not non-local non-crates.io registries (e.g. https://github.com/my_awesome_fork/crates.io-index) - see #3917. This PR adds support for such registries to cargo.

There are two main changes:

1. You can now specify a registry index using the registry key in the packages section of Cargo.toml. When cargo publishing, cargo verifies that the host being published to matches the specified registry index.
2. Dependencies can now have a registry specified. Cargo will look in that registry rather than crates.io for the dependencies with that key set.

A Cargo.toml using these features might look like:

[package] 
name = "registry-test" 
version = "0.1.0" 
authors = ["Christopher Swindle <christopher.swindle@metaswitch.com>"] 
registry = "https://github.com/my_awesome_fork/crates.io-index" 
 
[dependencies] 
libc = { version = "*", registry = "https://github.com/my_second_awesome_fork/crates.io-index" } 

We are also planning on pushing changes to the crates.io repo to allow setting up a private registry, but that requires tidying before we do so.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[cswindle]

Is there a problem with continuous-integration/appveyor/branch, it does not seem to have started?

[alexcrichton]

Thanks for the PR! Always exciting to see movement on this. I'm personally a little wary to see this merged, however, before we've got a more complete picture in our heads for what custom registries might look like. I fear that given the discussion on #3917 everyone's got a slightly different perception of what a custom registry will look like.

I personally feel like this feature to Cargo, when viewed from its entirety, may be deserving of an RFC in rust-lang/rfcs. I think that enough of the community would haven an opinion and like to give feedback that it's worth going through the process.

I'm curious, how would you feel about that?

[cswindle]

Thanks for the update. I will sort out an RFC for this over the next few weeks.

[alexcrichton]

Ok let me know if you need any help! I'm sure @carols10cents would be up for assisting here as well!

[carols10cents]

Yes! I have thoughts and use cases, some of which I wrote up on #3917. I'm totally up for bouncing ideas around, thinking through implications of decisions on various workflows, answering questions, etc. However I can be helpful, please let me know!

[phrohdoh]

If I can help (code, discussion, review, etc) move this forward please ping me.
A non-crates.io registry is very relevant to my interests.

[cswindle]

I have now created the RFC and sent in the pull request for review (rust-lang/rfcs#2006) so feel free to review/provide comments.

[alexcrichton]

Thanks @cswindle! I'm gonna close this for now in favor of that RFC