Interesting. What happens if the native memory layout doesn't support a feature that the extension code requires? For example, what if a JRuby+Truffle script calls into a C library, which allocates a struct and invokes a Ruby callback with it, which then wants to access the fields via reflection? C doesn't normally include type information with its structs, so how would the Ruby code know the memory layout of the object?

What if it's not the C extension in your project that does the allocation, but a closed-source third-party library that your C code calls into? Do you need to recompile all source code to generate the appropriate typemaps?