The Standard Library as Architecture

• TypeId identifies component, resource, system, and schedule types at runtime.
• HashMap and dense/sparse tables connect those IDs to storage and metadata.
• FixedBitSet makes dependency, readiness, conflict, and completion sets cheap to combine.
• Mutex protects executor state shared by scoped worker tasks, while the world itself is accessed through scheduler-proven disjointness.
• Arc makes services such as AssetServer cheaply cloneable across background tasks.
• PhantomData preserves generic type/lifetime relationships in zero-sized parameter and handle machinery.
• Any and downcasting support heterogeneous registries at deliberate runtime boundaries.
• Result lets systems and commands participate in configurable error policy.

The striking point is that &T versus &mut T is not just local syntax. Bevy turns that standard borrowing distinction into global scheduling metadata.