Uplift rustc_mir_transform::coverage::counters::union_find to rustc_data_structures.

[cjgillot]

I was wondering if we had some union-find implementation in the compiler. We do. So put it where we can find it.

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in coverage instrumentation.

cc @Zalathar

[Zalathar]

I had another look through my implementation, and it does seem worth uplifting.

I feel tempted to write some more thorough unit tests, but that can just as easily be done after the uplift.

@bors r+ rollup

[bors]

📌 Commit 689171d has been approved by Zalathar

It is now in the queue for this repository.

[lqd]

We do have ena, but the ergonomics aren’t that great for regular union-find (e.g. it creates the keys or something IIRC).

[Zalathar]

Yeah, I found that the learning curve and amount of boilerplate required to use ena for basic union-find stuff is prohibitively high.