The Standard Library as Architecture

• Path and PathBuf distinguish borrowed locations from owned paths moved into workers.
• Arc<T> shares immutable distributions, caches, reporters, and semaphore budgets.
• HashMap/HashSet encode package indexes, installed state, and plan membership.
• BTreeMap supplies deterministic serialization where reproducible lock output matters.
• NonZeroUsize and available_parallelism() connect configuration to a valid machine-derived default.
• Cow<'a, T> avoids cloning unchanged requirements while still allowing marker-adjusted owned variants.
• filesystem handles and RAII lock guards make process-level ownership end on drop.
• Command, ExitStatus, and pipes model Python/build tools as fallible external actors.

Most performance comes from choosing which values are immutable and reusable. An Arc<Dist> can cross many futures; the mutable environment cannot. A PathBuf moved into spawn_blocking is simple proof that the task does not retain a borrowed command context.