Using CXX in multiple DLLs always results in undefined behavior?

[camio]

We're investigating using CXX to create multiple DLLs for incorporation into a large C++ project.

Non-inline functions with multiple definitions living in separate DLLs are okay when they aren't exposed directly or indirectly (e.g. via. exceptions) over the DLLs' public interfaces. Consequently, cbindgen, in exposing only functions particular to the crate, effectively insulates Rust runtimes within the DLLs.

CXX when used in multiple DLLs, on the other hand, will expose non-inline functions with multiple definitions, such as rust::String's destructor. This is fine as long as these functions's implementations do not access an object with multiple definitions (namely, global state in the DLL's data section). The implementation of rust::String::~String calls ManuallyDrop::drop which then calls drop_in_place. Do we have any guarantees that drop_in_place will not access global state?

If the answer is "no" it seems like linking a single executable to multiple DLLs that use CXX results in undefined behavior.