Function create_interface 

pub(crate) unsafe fn create_interface<D: DomTypes>(
    cx: &mut JSContext,
    init: InterfaceInit,
    global: HandleObject<'_>,
    cache: *mut ProtoOrIfaceArray,
) -> *mut JSObject

SAFETY: cache is a non-null pointer to a valid ProtoOrIfaceArray object. The returned object needs to be rooted.