When you have variable types (say, you're deserializing foreign data, always fun), typical C code will end up having a huge switch() block do determine the type

When you have variable types (say, you're deserializing foreign data, always fun), typical C code will end up having a huge switch() block do determine the type (expensive, e.g. due to branch prediction) and do something with it (trivial). Ahead-of-time compilation cannot optimize away the switch(), JIT can, by evaluating the data fed to the function at runtime. What OP wants is a way to get the advantage of ahead-of-time compilation for the 99% of the code that benefit from it, and the advantages of JIT for the remaining 1% (or even less). However, I'm not sure how feasible that would be.