Having worked on a few Rust projects that interface with C libraries, I think your last question is a good summary: once you introduce a C API boundary it's back to tools like Valgrind.

Maybe you could even say Zig is at an advantage there, because tools like Valgrind are considered part of the game, whereas in Rust they are way less commonly used (pure-Rust codebases don't need anything of the sort, unless you are using `unsafe`).