Function with_thread_host 

pub fn with_thread_host<T: 'static, R>(
    build: impl FnOnce() -> LuaHost<T>,
    f: impl FnOnce(&mut LuaHost<T>) -> R,
) -> Option<R>

Run f against this thread’s lazily-built LuaHost<T>.

A LuaHost is !Send — luna-core’s Vm holds Rcs and raw GC pointers — so a thread-per-core server cannot park it inside its Send per-shard command value. This slot keeps one host per shard thread instead: identical isolation (thread == shard), owned by the crate that knows why the type can’t travel.

build runs once, on the first call on this thread. Returns None when the slot is already borrowed — a re-entrant eval on the same thread; callers surface their nested-eval error. A parked host of a different T (mixed test harnesses; production uses one T per process) is dropped and rebuilt.