If you do the compilation locally, you know the structure of the ABI for the compiled code. You know that regardless of the backend you are using. At this stage

If you do the compilation locally, you know the structure of the ABI for the compiled code. You know that regardless of the backend you are using. At this stage, no functions with generics are compiled, only what is non-generic. What is left is to account for the ABI in any monomorphizations that occur at the boundary, i.e. when your own structures monomorphize generic functions in the dependency. When the compiler creates this monomorphic variant, and lowers down, it can provide the necessary details of the ABI.