if using an int* over an int[] changes the memory layout of a struct, that necessarily implies a difference in the ABI.

As an example, C++'s std::array can be implemented as a struct containing a fixed-size C-style array. This can be passed-by-value. This means that returning a std::array can be more performant than returning a std::vector (which might be implemented as an int* that is reallocated when you add too many elements), because a std::array is returned on the stack while a std::vector is returned on the heap.

I was bit by this once when returning a std::unordered_map because I was doing a heap-allocation in a performance-critical section.