Fix accumulator only accumulating direct children

[PhoebeSzmucer]

I noticed that some of the accumulated values were getting dropped, and I traced it to the accumulated_by function only accumulating from direct children.

Let me know if this fix is any good - I'm quite new to Rust and this codebase, and I wasn't sure how to best implement this recursion.

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[nikomatsakis]

Hmm. Ideally we would accumulate in execution order, I think. A queue won't really do it, that will give us a breadth-first search. I think what we want to do is to push the children onto the stack but in reverse order -- something like this. But you also have to remove the hashset insertion I think and instead do the insertion when you pop off the stack. Otherwise this case would print wrong:

A {
    B {
        D
    }
    C,
    D,
}

it would print like A, B, C, D instead of A, B, D, C.

Or you could go back to a recursive function (I wouldn't bother with the generic method, I'd just pull out a simple helper function). I think that would be fine, especially since the recursion depth is reflecting an execution that already took place. =)

[PhoebeSzmucer]

@nikomatsakis is the failure a flake?

[nikomatsakis]

Yeah, the miri failure is #520

[nikomatsakis]

I was thinking @PhoebeSzmucer that it'd be good to add a test for one of the more complex scenarios -- e.g. to test duplicates. Are you up to add something like that?

[PhoebeSzmucer]

@nikomatsakis yes if be happy to add more tests.

I think there are accumulator tests that indirectly test for the lack of duplicates already (the accumulator-dag one, accumulator-execution-order one that I added here), but I can add some more.

[PhoebeSzmucer]

@nikomatsakis I added one more complex test here https://github.com/salsa-rs/salsa/pull/524/commits/a85ac260d3c1587fc5acd0e128a3691d5f940fe2

[nikomatsakis]

@PhoebeSzmucer nice! Approved.