Prevent infinite recursion and OOM in project_and_unify_term

[BloodStainedCrow]

Prevents an infinite recursion when using the old trait solver.

Fixes #156615.

Note that the example in #156615 encounters this infinite recursion in report_ambiguity_errors.
The overflow abort introduced here shadows the original error for which we are generating a report, which is unfortunate but preferable to hangs/OOMs:

# Notably no info about "type ambiguity"
error[E0275]: overflow evaluating the requirement `<Family as DistributionFamily>::Distribution<_> == _`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0275`.

Ideally, we could report the original error AND report the overflow while generating the report, something like this:

error[E0283]: type annotations needed
  --> non-termination-while-reporting-ambiguity-error-issue-1156615.rs:28:10
  --> error: overflow evaluating the requirement `<Family as DistributionFamily>::Distribution<_> == _` while generating error report

[rustbot]

r? @dingxiangfei2009

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[BloodStainedCrow]

I have added the recursion check to the "lowest" function which added the new obligation,
according to this comment "process_projection_obligation should do its own recursion depth checking" but from what I can tell, nothing in there does any checking? Is that intentional?

So it might make sense to move this check further up to cover more of process_projection_obligation but I do not have enough understanding of the type checking being done here to know which functions could theoretically recurse infinitely.

[BloodStainedCrow]

@rustbot reroll

[mati865]

Sorry for the long wait. I'm not familiar with the code here enough to suggest anything here, but I might be able to find somebody more knowledgeable.

r? project-trait-system-refactor

BTW amazing work on your FactoryGame project ;)

[mati865]

cjgillot you have 20 PRs assigned, so let me know if I should look for somebody else

[rust-bors]

☔ The latest upstream changes (presumably #157894) made this pull request unmergeable. Please resolve the merge conflicts.