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You can't have a negative trait bound to say "I take anything that _doesn't_ implement Drop". It's not a stabilized feature in Rust because it would allow addin

You can't have a negative trait bound to say "I take anything that _doesn't_ implement Drop". It's not a stabilized feature in Rust because it would allow adding a trait to a struct to become a breaking API change where it currently is not. Copy guarantees !Drop so it's currently the only way to do it without opting into nightly features. You can remove the requirement for trivially destructible types by maintaining a singly linked list of (ptr to alloc'd object, destructorFn) pairs with the links allocated from the arena itself. The arena can just walk the list and call all the drop functions when it gets reset. You can even specialize for types with trivial destructors so they don't append to the list and so you only pay the extra cost if you allocate Drop types (using std::mem::needs_drop).

LegionMammal978 2y ago
> You can't have a negative trait bound to say "I take anything that _doesn't_ implement Drop". It's not a stabilized feature in Rust because it would allow adding a trait to a struct to become a breaking API change where it currently is not. Copy guarantees !Drop so it's currently the only way to do it without opting into nightly features. Alternatively, you could just do an std::mem::needs_drop() check when inserting an element, and fail if it indicates that it would need to be dropped. (Yeah, its documentation protests that it can have false positives, but IIRC you have to be doing super wacky type-level stuff to trigger that for real.) That's why I said that Copy is sufficient (but restrictive) as a type-level bound, since needs_drop() can't be done on that level. At least, not without the trick of throwing a post-mono error with an associated const.