It's not about static contracts at all, it's about keeping performance of high-level APIs high. It's all just about templates and generics, as far as I'm aware -- the same problem that plagues C++, except that it's worse in Rust because it's more ergonomic to expose templates in public library APIs in Rust than C++. Well, and also the trait solver might be quite slow, but again, it has nothing to do with memory safety.