> If a function is given some input and is supposed to work with it, how can it avoid if/else and how can we move this logic one level up to the caller to ask the caller to verify every parameter before calling the function?

The usual way in idiomatic Rust would be to use type safety for this purpose: have the function accept special types for its input, and provide the caller secondary interfaces to construct these types. The constructors would then be responsible for inspecting and rejecting invalid input. This way, the caller can continue pushing the construction, and thus the if/else statements for validation errors, upward to the ultimate source of the possibly-invalid values.

(This is also possible in C/C++/Java/C#/..., if not so idiomatic.)