Method add_reference

src/scope.rs:302–304

Adds an additional reference to the scope's reference counter. Every call to this should have a matching call to `Scope::remove_reference`, or the scope will block forever on completion.

(&self)

            //   to `remove_reference()`. Each job holds its increment until
            //   after it has called `func` and dropped its `Arc` handle, so
            //   neither the `Scope` nor anything `f` borrows can be freed
            //   while a job might still use (or drop) them.
            //
            // * If `op` is `!Send` then `JobRef::execute` will only be called
            //   on this thread.
            //