Add some unittests checking parenting functionality

crates/nu-protocol/src/engine/stack.rs

@@ -449,4 +449,76 @@ impl Default for Stack {
 }
 
 #[cfg(test)]
-mod test {}
+mod test {
+    use std::sync::Arc;
+
+    use crate::{Span, Value};
+
+    use super::Stack;
+
+    const ZERO_SPAN: Span = Span { start: 0, end: 0 };
+    fn string_value(s: &str) -> Value {
+        return Value::String {
+            val: s.to_string(),
+            internal_span: ZERO_SPAN,
+        };
+    }
+
+    #[test]
+    fn test_children_see_inner_values() {
+        let mut original = Stack::new();
+        original.add_var(0, string_value("hello"));
+
+        let cloned = Stack::with_parent(Arc::new(original));
+        assert_eq!(cloned.get_var(0, ZERO_SPAN), Ok(string_value("hello")));
+    }
+
+    #[test]
+    fn test_children_dont_see_deleted_values() {
+        let mut original = Stack::new();
+        original.add_var(0, string_value("hello"));
+
+        let mut cloned = Stack::with_parent(Arc::new(original));
+        cloned.remove_var(0);
+
+        assert_eq!(
+            cloned.get_var(0, ZERO_SPAN),
+            Err(crate::ShellError::VariableNotFoundAtRuntime { span: ZERO_SPAN })
+        );
+    }
+
+    #[test]
+    fn test_children_changes_override_parent() {
+        let mut original = Stack::new();
+        original.add_var(0, string_value("hello"));
+
+        let mut cloned = Stack::with_parent(Arc::new(original));
+        cloned.add_var(0, string_value("there"));
+        assert_eq!(cloned.get_var(0, ZERO_SPAN), Ok(string_value("there")));
+
+        cloned.remove_var(0);
+        // the underlying value shouldn't magically re-appear
+        assert_eq!(
+            cloned.get_var(0, ZERO_SPAN),
+            Err(crate::ShellError::VariableNotFoundAtRuntime { span: ZERO_SPAN })
+        );
+    }
+    #[test]
+    fn test_children_changes_persist_in_offspring() {
+        let mut original = Stack::new();
+        original.add_var(0, string_value("hello"));
+
+        let mut cloned = Stack::with_parent(Arc::new(original));
+        cloned.add_var(1, string_value("there"));
+
+        cloned.remove_var(0);
+        let cloned = Stack::with_parent(Arc::new(cloned));
+
+        assert_eq!(
+            cloned.get_var(0, ZERO_SPAN),
+            Err(crate::ShellError::VariableNotFoundAtRuntime { span: ZERO_SPAN })
+        );
+
+        assert_eq!(cloned.get_var(1, ZERO_SPAN), Ok(string_value("there")));
+    }
+}