There is absolutely no need for unsafe in slotmap. I chose to use unsafe (wrapped in a safe API) to reduce memory usage using intrusive linked freelists.
If done using safe Rust this would involve `enum`s that would take up extra space.
A downside for sure, but one that, at least in this specific example, has limited downsides. If you can button it up into a safe abstraction that you can share with others, then I don't really see what the huge problem is. The fact that you might need to write `unsafe` inside of a well optimized data structure isn't a weakness of Rust, it's the entire point: you use it to encapsulate an unsafe core within a safe interface. The standard library is full of these things.
Now if you're trying to do something that you can't button up into a safe abstraction for others to use, then that's a different story.
