Silence private_bounds lint in KnownLayout derive

[jswrenn]

For reasons not-yet-known, the private_bounds lint flags the output of derive(KnownLayout) on repr(C) structs generated by macros. This is likely a rustc bug, but to preserve a good experience for our users, we nonetheless, allow(private_bounds).

Fixes #2177

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.42%. Comparing base (2c8ef74) to head (83e68d3).

Additional details and impacted files

@@           Coverage Diff           @@
##           v0.8.x    #2182   +/-   ##
=======================================
  Coverage   87.42%   87.42%           
=======================================
  Files          16       16           
  Lines        6115     6115           
=======================================
  Hits         5346     5346           
  Misses        769      769           

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[joshlf]

Can we add a test for this?

[jswrenn]

Hm. It's not straightforward to add it as a trybuild test, because trybuild expects compilation failures (which we avoid by allowing the lint in derive(KnownLayout)). It's not straightforward to add it as a regular integration test, since those tests don't fail on warnings.

And we can't meaningfully pull shenanigans with forbid(private_bounds), because rustc errors out early as soon as it sees the allow(private_bounds), rather than upon the actual triggering of the lint. force-warn might do the trick, but it doesn't exist in attribute form, and trybuild doesn't have a ui-test-style way of specifying per-file rustflags.

My preference would be to merge this PR and cut a release sooner, rather than later, then put it on our roadmap to seek a general solution (or, at least, policy) for the problem of "our consumers might deny lints that trigger in certain invocations of our derives`.

[joshlf]

Approved, but let's file an issue to track either fixing the rustc bug or adding a test to prevent regressions.