[lifthrasiir]

Lifetime subtyping is normally not much necessary to ensure the correctness, provided that you are working at the sufficiently high level (so, say, std is available) and do have some freedom in the design. It often indicates that you are excessively avoiding heap allocations.

[LoganDark]

You can't just allocate a Lua coroutine on the heap. Lua coroutines are managed by the Lua virtual machine. So that's one heap allocation "avoided". I don't remember the others off the top of my head but some functions needed to have three lifetimes for various reasons. Not sure if I eventually refactored that out or not, it's been a while. I should get back to it one day when my job isn't nothing but startup crunching