A common method to organize a multi-crate project is to have one repository which contains all of the crates. Each crate has a corresponding subdirectory along with a Cargo.toml describing how to build it. There are a number of downsides to this approach, however:
Each sub-crate will have its own Cargo.lock, so it's difficult to ensure that the entire project is using the same version of all dependencies. This is desired as the main crate (often a binary) is often the one that has the Cargo.lock "which counts", but it needs to be kept in sync with all dependencies.
When building or testing sub-crates, all dependencies will be recompiled as the target directory will be changing as you move around the source tree. This can be overridden with build.target-dir or CARGO_TARGET_DIR, but this isn't always convenient to set.
Solving these two problems should help ease the development of large Rust projects by ensuring that all dependencies remain in sync and builds by default use already-built artifacts if available.
RFC: