Comment by jjmarr

"Stable ABI" is a joke in C++ because you can't keep ABI and change the implementation of a templated function, which blocks improvements to the standard library.

In C, ABI = API because the declaration of a function contains the name and arguments, which is all the info needed to use it. You can swap out the definition without affecting callers.

That's why Rust allows a stable C-style ABI; the definition of a function declared in C doesn't have to be in C!

But in a C++-style templated function, the caller needs access to the definition to do template substitution. If you change the definition, you need to recompile calling code i.e. ABI breakage.

If you don't recompile calling code and link with other libraries that are using the new definition, you'll violate the one-definition rule (ODR).

This is bad because duplicate template functions are pruned at link-time for size reasons. So it's a mystery as to what definition you'll get. Your code will break in mysterious ways.

This means the C++ committee can never change the implementation of a standardized templated class or function. The only time they did was a minor optimization to std::string in 2011 and it was such a catastrophe they never did it again.

That is why Rust will not support stable ABIs for any of its features relying on generic types. It is impossible to keep the ABI stable and optimize an implementation.