Publish a manifest-only `ci` or `git` channel?

[anp]

I'd really like to be able to install a single merge commit's toolchain with rustup. Right now there are a few tools that do this out of band by downloading releases directly from the CI server without any component/target/signature info (https://github.com/kennytm/rustup-toolchain-install-master and https://github.com/rust-lang-nursery/cargo-bisect-rustc at a minimum), but I am not sure that's appropriate to put into rustup itself. I think it would be much better to rely on rustup's existing understanding and tests w.r.t distribution manifests.

My hope is to find a solution that doesn't show up on any hosting bills but which allows rustup to install recent per-merge toolchains alongside the existing channels. I think that running a modified build-manifest (from rust-lang/rust's tools) + a modified promote-release (from this repo) on successful merges to master should cover my needs. Does this seem reasonable? Strange? I'm happy to do the work, but don't want to jump in if this approach won't work for reasons I'm not seeing.

[alexcrichton]

This seems reasonable to me yeah! I suspect such a change would be pretty easy in terms of making new channels. I forget how everything exactly hooks up with rustup and friends but if we basically just publish a manifest that points to the latest rust-lang-ci2 artifacts that seems quite plausible.

[anp]

OK! I'll try to understand enough of this repo to put a PR together for this. Thanks.