commit 09037f2b69820c16dd68b03dd887b54679d69faa
Author: Charlie Swanson <charlie.swanson@mongodb.com>
Date:   Tue Feb 23 19:06:36 2021 -0500

    WIP from meeting

diff --git a/src/mongo/db/matcher/expression_parser.cpp b/src/mongo/db/matcher/expression_parser.cpp
index 7ef37bc2e0..1f49ba64ec 100644
--- a/src/mongo/db/matcher/expression_parser.cpp
+++ b/src/mongo/db/matcher/expression_parser.cpp
@@ -1148,6 +1148,26 @@ StatusWithMatchExpression parseGeo(StringData name,
     }
 }
 
+template <class T>
+StatusWithMatchExpression parseTreeTopLevelUnary(
+    StringData name,
+    BSONElement elem,
+    const boost::intrusive_ptr<ExpressionContext>& expCtx,
+    const ExtensionsCallback* extensionsCallback,
+    MatchExpressionParser::AllowedFeatureSet allowedFeatures,
+    DocumentParseLevel currentLevel) {
+    if (elem.type() != BSONType::Object) {
+        return {Status(ErrorCodes::BadValue, str::stream() << T::kName << " must be an object")};
+    }
+    auto sub = parse(elem.Obj(), expCtx, extensionsCallback, allowedFeatures, currentLevel);
+    if (!sub.isOK())
+        return sub.getStatus();
+
+    return std::make_unique<T>(sub.getValue().release(),
+                               doc_validation_error::createAnnotation(
+                                   expCtx, elem.fieldNameStringData().toString(), BSONObj()));
+}
+
 template <class T>
 StatusWithMatchExpression parseTreeTopLevel(
     StringData name,
@@ -2024,6 +2044,7 @@ MONGO_INITIALIZER(PathlessOperatorMap)(InitializerContext* context) {
             {"id", &parseDBRef},
             {"jsonSchema", &parseJSONSchema},
             {"nor", &parseTreeTopLevel<NorMatchExpression>},
+            {"not", &parseTreeTopLevelUnary<NotMatchExpression>},
             {"or", &parseTreeTopLevel<OrMatchExpression>},
             {"ref", &parseDBRef},
             {"sampleRate", &parseSampleRate},
@@ -2123,4 +2144,4 @@ boost::optional<PathAcceptingKeyword> MatchExpressionParser::parsePathAcceptingK
     }
     return defaultKeyword;
 }
-}  // namespace mongo
+}  // namespace mongo
\ No newline at end of file
diff --git a/src/mongo/db/matcher/expression_tree.h b/src/mongo/db/matcher/expression_tree.h
index 25d71dde78..c8ae59c1eb 100644
--- a/src/mongo/db/matcher/expression_tree.h
+++ b/src/mongo/db/matcher/expression_tree.h
@@ -227,6 +227,7 @@ public:
 
 class NotMatchExpression final : public MatchExpression {
 public:
+    static constexpr StringData kName = "$not"_sd;
     explicit NotMatchExpression(MatchExpression* e,
                                 clonable_ptr<ErrorAnnotation> annotation = nullptr)
         : MatchExpression(NOT, std::move(annotation)), _exp(e) {}