"Memory leaks" would be a mischaracterisation. "Memory leak" typically refers to not freeing heap-allocated data, while I'm talking about data being copied to temporary locations, most commonly on the stack or in registers.

In a nutshell, if you have a function like

    fn secret_func() -> LargeType {
        /* do some secret calculations */
        LargeType::init_with_safe_Data()
    }

More realistically, I'm assuming you're aware that optimizing compilers often simplify `memset(p, 0, size); free(p);` to `free(p);`. A compiler frontend can use things like `memset_s` to force rewrites, but this will only affect the locals created by the frontend. It's entirely possible that the LLVM backend notices that the IR wants to erase some variable, and then decides to just copy the data to another location on the stack and work with that, say to utilize instruction-level parallelism.

I'm partially talking out of my ass here, I don't actually know if LLVM utilizes this. I'm sure it does for small types, but maybe not with aggregates? Either way, this is something that can break very easily as optimizing compilers improve, similarly to how cryptography library authors have found that their "constant-time" hacks are now optimized to conditional jumps.

Of course, this ignores the overall issue that Rust does not have a runtime. If you enter the secret mode, the stack frames of all nested invoked functions needs to be erased, but no information about the size of that stack is accessible. For all you know, memcpy might save some dangerous data to stack (say, spill the vector registers or something), but since it's implemented in libc and linked dynamically, there is simply no information available on the size of the stack allocation.

This is a long yap, but personally, I've found that trying to harden general-purpose languages simply doesn't work well enough. Hopefully everyone realizes by now that a borrow checker is a must if you want to prevent memory unsoundness issues in a low-level language; similarly, I believe an entirely novel concept is needed for cryptographical applications. I don't buy that you can just bolt it onto an existing language.