Rust Workspace and Project Topology

Authority: Cargo Book — Workspaces, The Book ch7 and ch14-03, Rust Reference ch7, RFC 1525, matklad — Large Rust Workspaces.

This skill decides how many crates a project should have and how they relate. Its companion rust-module-layout decides what lives inside one crate's src/.

Capability Boundaries

✅ Strengths

1. Single-crate vs multi-crate workspace decision (decision tree with five concrete triggers)
2. Four workspace layout patterns: flat crates/, grouped crates/<category>/, nested sub-workspaces, root package
3. Virtual manifest vs root package — trade-offs and migration
4. Workspace-level shared configuration: [workspace.package], [workspace.dependencies], [workspace.lints]
5. Dependency direction DAGs — types → core → sdk → server → binary
6. Diagnosing and refactoring the "mixed root package" anti-pattern
7. Crate naming, publishing, and version coordination across members