Cargo should be self-hosting

[bkoropoff]

After updating rust, building cargo by running make fails due to not rebuilding dependencies:

mkdir -p target
rustc  -L libs/hammer.rs/target -L libs/toml-rs/build --out-dir target src/cargo/lib.rs
src/cargo/lib.rs:14:1: 14:21 error: found possibly newer version of crate `std` which `hammer` depends on
src/cargo/lib.rs:14 extern crate hammer;
                    ^~~~~~~~~~~~~~~~~~~~
src/cargo/lib.rs:14:1: 14:21 note: perhaps this crate needs to be recompiled?
src/cargo/lib.rs:14 extern crate hammer;
                    ^~~~~~~~~~~~~~~~~~~~

This seems like a job for cargo!

[alexcrichton]

This is indeed a problem that cargo should likely solve! Cargo is still a bit premature, however, and can't quite deal with building itself. Some steps that need to be taken to get to this point:

• Nightly snapshots of cargo
• Archived snapshots
• Cross compilation

And there are likely more. These are all on the upcoming docket, so I'm going to close this for now as it's not too actionable.