What is the box operator? This doesn't sound familiar to me, but I also don't use nightly.

The box operator is basically how Box::new operates. One of the big problems if you were to DIY it, is to prevent stack allocation of heap data. The naive approach will result in your data first having to be allocated on the stack, passed to Box::new and then moved into the heap.

The box operator allows moving the initialization of a datastructure into the heap entirely (and the compiler also auto-insert the malloc calls).

It is largely a one-trick pony that only exists to help out the standard library, nobody should be using it ever. But it is the one instance where the rust compiler will deploy an allocation on the heap without any obvious way to tell that it did that.

Box operator isn't actually used in Box::new anymore. It's been replaced with a magic attribute, #[rustc_box]