Someone recently looked into doing this by default. The problem was the results varied depending on how often your "host dependencies" (proc macro, build.rs) w

Someone recently looked into doing this by default. The problem was the results varied depending on how often your "host dependencies" (proc macro, build.rs) were shared with your "target dependencies" (`dependencies`, `dev-dependencies`). When there was a lot of sharing, it hurt, but otherwise it sped up build times.