Fixing the toolchain in Cargo.toml

[gnzlbg]

It would be nice for those working with particular snapshots of nightly if it would be possible to fix the toolchain in the Cargo.toml file, e.g., something like:

[toolchain]
channel = nightly
date = 2016-11-13
host = x86_64-apple-darwin

and if cargo would use rustup to get the appropriate toolchain.

[alexcrichton]

This is similar to #2751 which has some more comments: #2751 (comment).

The tl;dr; is that I don't think this'd be too useful to add to Cargo right now. Cargo doesn't manage toolchains, so it'd basically ignore this configuration most of the time.

[gnzlbg]

Indeed, I'll close this and repost there instead. Thanks!