Fixing layering order

[wdanilo]

Pull Request Description

This might fix #6874 . I can't reproduce the issue, but I understand why it might happen, as the ordering was not defined explicitly.

Checklist

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[kazcw]

I wonder if we have other undeclared dependencies that could break at any time. It's hard to be sure every needed dependency is declared, but that wouldn't be as much of a problem if ties in the layer item toposort were always resolved the same way. The DependencyGraph toposort looks deterministic per se, but the items its sorting are not: SymbolIds are sequentially-assigned (timing-dependent); ShapeSystemIds are TypeIds (ordering is completely up to the compiler). If we could either make these both have a deterministic total order, or assign stable keys for the toposort, we could ensure that the order we're testing is the same order as everyone will see. For the ShapeSystemIds, we could use their definition_path as a stable ID.

[farmaazon]

Another approval for your impressive collection

[wdanilo]

@kazcw I was thinking exactly about that yesterday and I think we should emit warnings in case we try to render things on the same layer without explicit ordering. This would catch such bugs way earlier and we already had a few regressions exactly because of this issue.

Of course, if we could assign automatically some stable IDs, that would be an amazing idea, however, they could not change when refactoring a code. For example, after moving files around, the ordering of the layers should not change, which makes it a little bit harder. To you see any way of doing it, or do you feel that requiring explicit ordering is the best solution here?