Sum types are fantastic and I’ll join you singing their praises all day long. But this article is about rust’s memory model & memory safety - which is independent of sum types.

Unsafe blocks in rust are the only place you can dereference pointers. There are whole classes of memory bugs which are impossible to code up in safe rust code because of this.

As an example of how this helps, a couple years ago I was working on a complex & clever rope data structure. My code segfaulted - and I can count on one hand the number of times my rust code has ever done that. A segfault like that can only come from memory corruption or dereferencing invalid memory - both of which are only possible in unsafe code. And the bottom of the stack trace was a safe function, so I knew I’d corrupted memory somehow. These bugs are really hard to find in C - basically the worst case for debugging because the problem could be literally anywhere. But in rust I knew the bug was in one of only two unsafe blocks in the ~thousand or so lines of my library. And one of those blocks was executed not long before the crash.

5 minutes later I had found and fixed the bug. In C, I’ve had similar problems take hours to days to fix.

Bugs like this aren’t possible at all in Go or C# because of their memory model. But in those languages you pay for safety by sacrificing performance. The rust I’ve written & benchmarked so far somehow faster than the equivalent C code. Which is pretty wild!