Add API for simple system ordering

[harudagondi]

Objective

• Add API for easy system ordering.
• Fixes Add a straightforward API for simple system ordering #4220

Solution

• Create new traits IntoLinkedSystemSet and IntoSequentialSystemSet.
• Using these trait methods will produce a SystemSet that can be added to App through App::add_system_set.

IntoLinkedSystemSet

• IntoLinkedSystemSet is a trait that provides a method link().
• IntoLinkedSystemSet is implemented on tuples of systems containing 2 to 15 elements.

Syntax:

(system_a, system_b, system_c, system_d).link()

This is equal to:

SystemSet::new()
    .with_system(system_a)
    .with_system(system_b.after(system_a))
    .with_system(system_c.after(system_b))
    .with_system(system_d.after(system_c));

IntoSequentialSystemSet

• IntoSequentialSystemSet is trait that provides a method then(next_system)
• IntoSequentialSystemSet is implemented on a system, and accepts a system.

Syntax:

system_a.then(system_b)

This is equal to:

SystemSet::new()
    .with_system(system_a)
    .with_system(system_b.after(system_a));

Or

(system_a, system_b).link()

Known Limitations

• IntoSequentialSystemSet is not implemented for SystemSet. This means you can't do system_a.then(system_b).then(system_c).
• IntoLinkedSystemSet is not implemented for tuples with more than 15 elements.

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Changelog

Added

• Allow sequential ordering of two systems by using IntoSequentialSystemSet::then and doing system_a.then(system_b).
• Allow sequential ordering of multiple systems (up to 15) by using IntoLinkedSystemSet::link on tuples of systems.

---

P.S.: writing and debugging proc macros is not a very fun experience lol

[Nilirad]

Some doc nits. I'm not confident enough with macros and system internals to do a full review though.

[Nilirad]

Suggested change

	/// A helper method to create system sets with automatic
	/// A helper method to create system sets with sequential

[Nilirad]

Suggested change

	/// A helper method to create system sets with automatic
	/// A helper method to create system sets with sequential

[alice-i-cecile]

Nice work! I'll do a full review soon. Did you experiment with the .then() syntax as well?

[MrGVSV]

I’m not up-to-date on bevyengine/rfcs#45 but isn't the terminology chain instead of link?

[alice-i-cecile]

Yes, but to do that we'll need to first rename the existing system chaining :)

[james7132]

This seems awfully similar to earlier iterations of #2381 before it was extended to support fan-in/fan-out topologies: strictly sequential, no graph ordering. While the then syntax could be logically extended to support those kinds of use cases, the link approach may not.

[harudagondi]

Hello, sorry for the late response. I was not aware of #2381 and bevyengine/rfcs#45, whoops sorry. In hindsight, this is a very naive way of implementing sequential chains of system. I honestly think that there should be an easier way of making a linear sequential order of systems, but with the rfc I probably think it would be for the best that we should wait until stageless is implemented. However, I think the proc-macro I made for this should be useful in implementing system![].chain() or chain![].

[alice-i-cecile]

Closing as this has been done in the stageless rework.