Removed NDEBUG-dependent SimTK::StateImpl members

[adamkewley]

Although the members that are affected by this diff (m_qVersion, m_discreteVarVersions, etc.) are unused in release builds, it is dangerous to conditionally expose them.

The reason why is because of ABI breakages. StateImpl.h is a header that can, in principle, be included by downstream code (I know it's internal/, but that won't stop it from being transitively included by, e.g. OpenSim) there is a chance that different compilation units will compile different downstream code based on what their NDEBUG is set to (might be different from simbody's build).

Example: In Linux, I was getting a segfault on a cache variable read from a not-debug part of StateImpl, probably because my downstream code--compiled in Debug mode--compiled assembly code that computes an incorrect offset into the StateImpl struct, because it inlined a function via OpenSim.

That segfault disappears if I apply this patch, and I can then build opensim-creator in Debug mode against Release-mode binaries in Linux. The utility of this is that I'm using debug mode /w libasan to kick out harder-to-detect faults.

I have read in other PRs, issues, etc. on both simbody and opensim-core that downstream projects must be compiled with the same optimization+debug levels. This should not be the case. It is perfectly normal to use a downstream debug binary with an upstream release library (think about graphics bindings, OS APIs, game engines, etc.).

The only exception is maybe MSVC, which has some kind of iterator debug level macro in its standard library, which causes is to cry quite a bit if you mix up flags. I have only seen this in MSVC--specifically for iterators--and both OSX and Linux are usually ok with mixing debug levels.

This patch makes StateImpl slightly bigger but, assuming empty containers cause no allocations, the overhead should be small compared to the rest of the datastructure. If there is genuinely a performance concern then I would recommend heap-allocating a DebugData struct behind a unique_ptr to put a cap (pointer size) on the overhead.

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This change is 

[adamkewley]

@sherm1 , I've had another OpenSim user run into (luckily, compile-time) compilation issues on Linux when compiling debug binaries against Release OpenSim+Simbody binaries because of this bug.

Is there any chance that this could be merged? I know you're busy but it's one of those issues that is super-confusing to any dev that isn't aware of ABI stability etc.

[sherm1]

Sorry for the delay, Adam -- apparently I forgot about this long ago!

Thanks for the fix.

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @adamkewley)