Generalize DiGraph/UnGraph 📊

[ItsDoot]

Objective

As the first step towards more modular schedule graph structures, we first need to generalize the foundational pieces: the Graph data structure and related algorithms.

One might ask: "what are 'more modular schedule graph structures'?" I've come across quite a few use cases that can almost be fulfilled with a Schedule, but have different-enough requirements that render Schedule incompatible as-is:

• My WIP utility AI library needs a graph of read-only systems while linking system output values as inputs to other systems.
• bevy_render's RenderGraph is already somewhat Schedule-like, but requires all nodes to be read-only and currently only allows a single query to be performed.
• bevy_render's Extract schedule could be statically enforced to be read-only, which would allow its executor to skip access conflict checks.
• Occasionally users in the Discord will show interest in statically-dispatched schedules, where a fork-join sequence can be modeled at compile time.

Therefore, by providing modular and generic building blocks for building Schedule-shaped things, we can satisfy each of the above and more. And since most of these are likely to make use of DiGraph or UnGraph, generalizing them is a good start.

One might ask: "what does this give us over petgraph?" A couple things:

• petgraph is not currently no_std, which is a prerequisite to pull it in now that bevy_ecs is no_std.
  • Add no_std Support petgraph/petgraph#747 has been recently opened to fix this.
• The associated types on GraphNodeId allows us to use memory-efficient forms for edge and neighbor information, which currently saves us 8 bytes for each edge pair and each neighbor+direction.

Solution

Replaces the hardcoded usage of NodeId with an implementable GraphNodeId trait in DiGraph/UnGraph and associated algorithms. Algorithms implemented as part of ScheduleGraph will be tackled in a future PR in order to keep this one small.

Testing

Current tests are being reused.

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Migration Guide

• Bare DiGraph or UnGraph usage must be changed to DiGraph<NodeId> or UnGraph<NodeId>, respectively.

[alice-i-cecile]

Can you say more about why this will be useful for your long-term plans? IIRC we did the opposite thing when vendoring petgraph 😂

[ItsDoot]

Can you say more about why this will be useful for your long-term plans? IIRC we did the opposite thing when vendoring petgraph 😂

Context added.