True, those are even more places where C++ can implicitly allocate. The operator overloading also applies to Rust. Rust has no implicit conversions though, which are arguably worse since they are invisible (that's why I usually mark all my expensive single argument constructors as "explicit").

Sure it has, you cannot ensure Deref implementations don't allocate.

While technically true, the documentation makes it very clear that Deref should only be implemented for smart pointers and never fail. So no allocations in practice.

Ah problem solved then, we just need to document very clear that C code should not corrupt memory, how did I never thought of it.