Add missing file

src/engine/operators/empty.rs

@@ -0,0 +1,43 @@
+use crate::engine::*;
+use std::fmt;
+
+pub struct Empty<T> {
+    pub output: BufferRef<T>,
+}
+
+impl<'a, T> VecOperator<'a> for Empty<T> where T: VecData<T> + 'a {
+    fn execute(&mut self, _: bool, _: &mut Scratchpad<'a>) -> Result<(), QueryError> {
+        Ok(())
+    }
+
+    fn init(&mut self, _: usize, _: usize, scratchpad: &mut Scratchpad<'a>) {
+        scratchpad.set(self.output, vec![]);
+    }
+
+    fn inputs(&self) -> Vec<BufferRef<Any>> {
+        vec![]
+    }
+    fn inputs_mut(&mut self) -> Vec<&mut usize> { vec![] }
+    fn outputs(&self) -> Vec<BufferRef<Any>> {
+        vec![self.output.any()]
+    }
+    fn can_stream_input(&self, _: usize) -> bool {
+        false
+    }
+    fn can_stream_output(&self, _: usize) -> bool {
+        false
+    }
+    fn allocates(&self) -> bool {
+        false
+    }
+
+    fn display_op(&self, _: bool) -> String {
+        "Empty".to_string()
+    }
+}
+
+impl<T> fmt::Debug for Empty<T> where T: VecData<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.display_op(false))
+    }
+}