A taste of Rust

This is one the main drawback of having your sentinel value(s) not being a discriminated union with your actual values. If it's not the sentinel, the code can only assume that it is valid. I suppose that in C, you could do some heuristics to check for plausible heap and stack pointers, but then if you see a pointer from some other allocator, you're going to reject perfectly valid pointers. Not to mention it being very not cross-platform.