If you have two packages in a repo and wish to use shared target dirs to avoid rebuilds, you might use relative target-dir specs (one in each package) to do this by setting them to ../target or something.

This causes cargo to rebuild everything, probably because it sees the two target dirs as `/path/to/package_a/../target" and "/path/to/package_b/../target".

Right now you can workaround this by using absolute paths through the CARGO_TARGET_DIR environment var.