Miss Dada  🏳️‍⚧️

@ManishEarth re: elsa, I get why the indirection of a Box is needed, so that insert() might move the Box but not its contents, but why is an UnsafeCell needed around the HashMap?

It's UB in rust to mutate something when other &T references exist, even if you verify they're not reading between mutations. This requirement is relaxed for mutations poking within UnsafeCell, they "don't count" as mutating the T from the outside.

and does UB mean "won't compile" or "the compiler will do something unspecified"

The latter. And it's a stronger statement than implementation-defined, implementation-defined is "unspecified but reasonable", UB is "unspecified and potentially catastrophic"

does safe rust have any UB, or is it only unsafe code?

In theory safe rust would not have any UB, that's the main sell. In practice there are ways to trigger UB in safe rust (compiler bugs, etc) These are hard to do by accident though.