Function on_deep_stack 

pub fn on_deep_stack<T, F>(work: F) -> DeepStack<T>
where
    T: Send,
    F: FnOnce() -> T + Send,

Runs work on a dedicated thread with a large reserved stack (DEEP_STACK), so deeply nested input cannot overflow the caller’s stack.

The closure is consumed whether or not a worker starts, so a caller that wants to retry on the current stack expresses the work as a re-callable expression and rebuilds it in the DeepStack::NotSpawned arm.