Inability to specify non-nullable pointers, or truly immutable data, or ownership, or thread-safety of objects on the type system level means I have to remember

Inability to specify non-nullable pointers, or truly immutable data, or ownership, or thread-safety of objects on the type system level means I have to remember these unwritten invariants in every case, and/or code defensively. Libraries can't specify these things in a compiler-enforced way either, so they remain "RTFM" problems for programmers to deal with manually.