Archetype invariants

rfcs/archetype-invariants.md

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+# Feature Name: `archetype-invariants`
+
+## Summary
+
+**Archetype invariants** are global rules about which archetypes can exist in a Bevy app's ECS.
+By defining which components must or cannot coexist, we can enforce domain-specific rules to ensure correctness.
+Once these rules are encoded into our ECS, we can leverage them to improve performance in a large variety of ways.
+
+## Motivation
+
+TODO: Complete.
+
+Why are we doing this? What use cases does it support?
+
+## Guide-level explanation
+
+TODO: Complete.
+
+Explain the proposal as if it was already included in the engine and you were teaching it to another Bevy user. That generally means:
+
+- Introducing new named concepts.
+- Explaining the feature, ideally through simple examples of solutions to concrete problems.
+- Explaining how Bevy users should *think* about the feature, and how it should impact the way they use Bevy. It should explain the impact as concretely as possible.
+- If applicable, provide sample error messages, deprecation warnings, or migration guidance.
+- If applicable, explain how this feature compares to similar existing features, and in what situations the user would use each one.
+
+## Reference-level explanation
+
+TODO: Complete.
+
+
+This is the technical portion of the RFC. Explain the design in sufficient detail that:
+
+- Its interaction with other features is clear.
+- It is reasonably clear how the feature would be implemented.
+- Corner cases are dissected by example.
+
+The section should return to the examples given in the previous section, and explain more fully how the detailed proposal makes those examples work.
+
+## Drawbacks
+
+TODO: Complete.
+
+Why should we *not* do this?
+
+## Rationale and alternatives
+
+TODO: Complete.
+
+- Why is this design the best in the space of possible designs?
+- What other designs have been considered and what is the rationale for not choosing them?
+- What is the impact of not doing this?
+- Why is this important to implement as a feature of Bevy itself, rather than an ecosystem crate?
+
+## Unresolved questions
+
+TODO: Complete.
+
+- What parts of the design do you expect to resolve through the RFC process before this gets merged?
+- What parts of the design do you expect to resolve through the implementation of this feature before the feature PR is merged?
+- What related issues do you consider out of scope for this RFC that could be addressed in the future independently of the solution that comes out of this RFC?
+
+## \[Optional\] Future possibilities
+
+TODO: Complete.
+
+Think about what the natural extension and evolution of your proposal would
+be and how it would affect Bevy as a whole in a holistic way.
+Try to use this section as a tool to more fully consider other possible
+interactions with the engine in your proposal.
+
+This is also a good place to "dump ideas", if they are out of scope for the
+RFC you are writing but otherwise related.
+
+Note that having something written down in the future-possibilities section
+is not a reason to accept the current or a future RFC; such notes should be
+in the section on motivation or rationale in this or subsequent RFCs.
+If a feature or change has no direct value on its own, expand your RFC to include the first valuable feature that would build on it.