I don't especially care about the binary (beyond wanting it to be as small and fast as possible on end-users' machines), but I do want the behavior to be the sa

I don't especially care about the binary (beyond wanting it to be as small and fast as possible on end-users' machines), but I do want the behavior to be the same everywhere. In C, this is difficult because code often accidentally relies on "undefined behavior" that changes depending on the compiler and optimizations you use. But is that still an issue in newer languages that don't have as much undefined behavior? (Is this something Rust programmers struggle with, for example?)