Between 10 thousand and one million lines, flat layout makes the most sense

rust-analyzer/
  Cargo.toml
  Cargo.lock
  crates/
    rust-analyzer/
    hir/
    hir_def/
    hir_ty/
    ...

make the root of the workspace a virtual manifest
write all automation in a dedicated crate (example: cargo xtask)
use version = "0.0.0" for internal crates you don’t want to publish and extract crates you want to publish in a top-level folder libs

See Also

References