> Well, trait objects are absolutely dynamically-sized types, but in order to store them on the stack you'd need to stuff them behind a Sized pointer type. I d

> Well, trait objects are absolutely dynamically-sized types, but in order to store them on the stack you'd need to stuff them behind a Sized pointer type. I don't think the question here is about Sized vs. !Sized, but instead about the analyzability of the program's control flow. Aside from layout information, a trait object's vtable is literally just an array of function pointers. So since the target of the dynamic dispatch can only be computed at runtime, proving that it is never recursive can only be done with escape analysis of all trait objects, alongside all function pointers. (Also, you're correct that Rust does not support directly recursive closures; you can use opaque types to get around the naming problem, but the compiler sees that the closure type becomes cyclic. At least one closure in the chain would have to use dynamic dispatch.)