Servo can't use 1/5 Rust nightly with Cargo (unknown crate type: proc-macro)

[jdm]

Using 508d66b, building Servo with the 1/5 Rust nightly yields:

cargo build -v --features debugmozjs
error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `rustc - --crate-name ___ --print=file-names -W unused-extern-crates --crate-type bin --crate-type dylib --crate-type proc-macro --crate-type rlib --target x86_64-apple-darwin` (exit code: 101)

--- stderr
error: unknown crate type: `proc-macro`

The 12/29 Rust nightly works fine, but it's not clear to me whether rustc or Cargo is causing the problem.

[alexcrichton]

Was Cargo accidentally paired with an older compiler? The error message:

error: unknown crate type: `proc-macro`

doesn't seem to make much sense here...

[jdm]

Lol, I used 2016-01-05. Carry on.