It's not clear to me what the alternative would be here. The common case is that the parent calls `join` and handles the result there. This needs to work even i

It's not clear to me what the alternative would be here. The common case is that the parent calls `join` and handles the result there. This needs to work even if the child thread returns quickly, before `join` is called in the parent. If we switched to a strategy where a panic in the child immediately injected a panic into the parent, or aborted the program, that would break the common case. I don't think there's any way for Rust to know the difference between "the parent is doing some work and hasn't joined yet" and "the parent has no intention of ever joining", if the parent isn't going to be explicit about that.