compiletest can explode due to cyclic auxiliaries

[jieyouxu]

Right now, compiletest does not forbid auxiliaries from declaring each other as auxiliaries, creating a cycle in the auxiliary build graph. This means that, e.g. for aux A requesting aux build of B, aux B can also request aux build of A.

This will eventually kaboom.

I can't find an existing issue for this, tried searching.

[Shunpoco]

Hi, can I take this ticket?

During the initial investigation, I found that:

• Cyclic auxiliaries may only happen in auxiliary sub-folder, (for example, a.rs <-> auxiliary/b.rs never cause the cycle but auxiliary/a.rs <-> auxiliary/b.rs may cause)
• In TestCx, the cycle may happen between build_auxiliary and build_all_auxiliary because they call each other.

So I think that keeping auxiliaries on a HashSet and return panic if it has duplication is straight forward way.

[jieyouxu]

@Shunpoco you could, but be advised that I haven't dug into this deeply and it may be tricky to fix.

[jieyouxu]

FTR: @Shunpoco had a prototype in #134180, will need to revisit how to incorporate their changes (with attribution) with proper self-test coverage. See this comment for some more consideration.