Refactor build_schedule and related errors

[hymm]

Objective

• break up large build_schedule system to make it easier to read
• Clean up related error messages.
• I have a follow up PR that adds the schedule name to the error messages, but wanted to break this up from that.

Changelog

• refactor build_schedule to be easier to read

Sample Error Messages

Dependency Cycle

thread 'main' panicked at 'System dependencies contain cycle(s).
schedule has 1 before/after cycle(s):
cycle 1: system set 'A' must run before itself
system set 'A'
 ... which must run before system set 'B'
 ... which must run before system set 'A'

', crates\bevy_ecs\src\schedule\schedule.rs:228:13

thread 'main' panicked at 'System dependencies contain cycle(s).
schedule has 1 before/after cycle(s):
cycle 1: system 'foo' must run before itself
system 'foo'
 ... which must run before system 'bar'
 ... which must run before system 'foo'

', crates\bevy_ecs\src\schedule\schedule.rs:228:13

Hierarchy Cycle

thread 'main' panicked at 'System set hierarchy contains cycle(s).
schedule has 1 in_set cycle(s):
cycle 1: set 'A' contains itself
set 'A'
 ... which contains set 'B'
 ... which contains set 'A'

', crates\bevy_ecs\src\schedule\schedule.rs:230:13

System Type Set

thread 'main' panicked at 'Tried to order against `SystemTypeSet(fn foo())` in a schedule that has more than one `SystemTypeSet(fn foo())` instance. `SystemTypeSet(fn foo())` is a `SystemTypeSet` and cannot be used for ordering if ambiguous. Use a different set without this restriction.', crates\bevy_ecs\src\schedule\schedule.rs:230:13

Hierarchy Redundancy

thread 'main' panicked at 'System set hierarchy contains redundant edges.
hierarchy contains redundant edge(s) -- system set 'X' cannot be child of set 'A', longer path exists
', crates\bevy_ecs\src\schedule\schedule.rs:230:13

Systems have ordering but interset

thread 'main' panicked at '`A` and `C` have a `before`-`after` relationship (which may be transitive) but share systems.', crates\bevy_ecs\src\schedule\schedule.rs:227:51

Cross Dependency

thread 'main' panicked at '`A` and `B` have both `in_set` and `before`-`after` relationships (these might be transitive). This combination is unsolvable as a system cannot run before or after a set it belongs to.', crates\bevy_ecs\src\schedule\schedule.rs:230:13

Ambiguity

thread 'main' panicked at 'Systems with conflicting access have indeterminate run order.
1 pairs of systems with conflicting data access have indeterminate execution order. Consider adding `before`, `after`, or `ambiguous_with` relationships between these:
 -- res_mut and res_ref
    conflict on: ["bevymark::ambiguity::X"]
', crates\bevy_ecs\src\schedule\schedule.rs:230:13

Migration Guide

ScheduleBuildError now has strings in more of its variants. You may need to adjust code that is handling these variants.

[alice-i-cecile]

I like chunking this out into functions with clear purposes here,and storing all of the error information in strings on the variants is a much more reasonable architecture than only doing this for some variants.

Note that this is breaking though: ScheduleBuildError now has strings in more of its variants.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[hymm]

added a migration guide

[maniwani]

Looks good to me. I'm all for making this code easier to understand.

[JoJoJet]

Really wish we had assert_matches!