Function record_checkout 

pub fn record_checkout(
    store: &Path,
    identity: &str,
    workspace_root: &Path,
    target_dir: &Path,
) -> Result<()>

Record that workspace_root built identity.

Identities are shared by every checkout of one dependency graph, so this is what later tells the collector whether anything still needs what a build cached: one file per checkout under the identity it built. Writing a whole file per checkout rather than merging a list into one keeps concurrent builds out of each other’s way – there is nothing to merge, so there is no lock and no lost update.

A build with no target directory of its own passes workspace_root for target_dir, which reads as “none” rather than as a tree to measure.