SERVER-49043 Begin CST -> Pipeline translation

16 files changed, 1001 insertions, 428 deletions

diff --git a/src/mongo/db/cst/SConscript b/src/mongo/db/cst/SConscript
index 52a88dc0406..281631992f4 100644
--- a/src/mongo/db/cst/SConscript
+++ b/src/mongo/db/cst/SConscript
@@ -8,11 +8,13 @@ env.Library(
     target='cst',
     source=[
         'bson_lexer.cpp',
+        'cst_pipeline_translation.cpp',
         'c_node.cpp',
         'pipeline_parser_gen.cpp',
     ],
     LIBDEPS=[
         "$BUILD_DIR/mongo/base",
+        '$BUILD_DIR/mongo/db/pipeline/pipeline',
         '$BUILD_DIR/mongo/db/query/datetime/date_time_support',
     ]
 )
@@ -26,3 +28,14 @@ env.CppUnitTest(
         'cst',
     ]
 )
+
+env.CppUnitTest(
+    target='cst_pipeline_translation_test',
+    source=[
+        'cst_pipeline_translation_test.cpp',
+    ],
+    LIBDEPS=[
+        'cst',
+        '$BUILD_DIR/mongo/db/query/query_test_service_context',
+    ]
+)
diff --git a/src/mongo/db/cst/c_node.cpp b/src/mongo/db/cst/c_node.cpp
index 84fb55d04b2..b677797deff 100644
--- a/src/mongo/db/cst/c_node.cpp
+++ b/src/mongo/db/cst/c_node.cpp
@@ -57,60 +57,33 @@ auto printFieldname(const CNode::Fieldname& fieldname) {
         fieldname);
 }
 
-}  // namespace
-
-std::string CNode::toStringHelper(int numTabs) const {
+template <typename T>
+auto printValue(const T& payload) {
     return stdx::visit(
         visit_helper::Overloaded{
-            [numTabs](const Children& children) {
-                if (auto keyFieldname = stdx::get_if<KeyFieldname>(&children[0].first);
-                    keyFieldname != nullptr && *keyFieldname == KeyFieldname::arrayMarker)
-                    return std::accumulate(children.cbegin(),
-                                           children.cend(),
-                                           tabs(numTabs) + "[\n",
-                                           [numTabs](auto&& string, auto&& childpair) {
-                                               return string +
-                                                   childpair.second.toStringHelper(numTabs + 1) +
-                                                   "\n";
-                                           }) +
-                        tabs(numTabs) + "]";
-                else
-                    return std::accumulate(children.cbegin(),
-                                           children.cend(),
-                                           tabs(numTabs) + "{\n",
-                                           [numTabs](auto&& string, auto&& childpair) {
-                                               return string + tabs(numTabs) +
-                                                   printFieldname(childpair.first) + " :\n" +
-                                                   childpair.second.toStringHelper(numTabs + 1) +
-                                                   "\n";
-                                           }) +
-                        tabs(numTabs) + "}";
-            },
-            [numTabs](const KeyValue& value) {
-                return tabs(numTabs) + "<KeyValue " +
+            [](const CNode::ArrayChildren& children) { return "<Array>"s; },
+            [](const CNode::ObjectChildren& children) { return "<Object>"s; },
+            [](const KeyValue& value) {
+                return "<KeyValue "s +
                     key_value::toString[static_cast<std::underlying_type_t<KeyValue>>(value)] + ">";
             },
-            [numTabs](const UserDouble& userDouble) {
-                return tabs(numTabs) + "<UserDouble " + std::to_string(userDouble) + ">";
+            [](const UserDouble& userDouble) {
+                return "<UserDouble "s + std::to_string(userDouble) + ">";
             },
-            [numTabs](const UserString& userString) {
-                return tabs(numTabs) + "<UserString " + userString + ">";
-            },
-            [numTabs](const UserBinary& userBinary) {
-                return tabs(numTabs) + "<UserBinary " + typeName(userBinary.type) + ", " +
+            [](const UserString& userString) { return "<UserString "s + userString + ">"; },
+            [](const UserBinary& userBinary) {
+                return "<UserBinary "s + typeName(userBinary.type) + ", " +
                     toHex(userBinary.data, userBinary.length) + ">";
             },
-            [numTabs](const UserUndefined& userUndefined) {
-                return tabs(numTabs) + "<UserUndefined>";
-            },
-            [numTabs](const UserObjectId& userObjectId) {
-                return tabs(numTabs) + "<UserObjectId " + userObjectId.toString() + ">";
+            [](const UserUndefined& userUndefined) { return "<UserUndefined>"s; },
+            [](const UserObjectId& userObjectId) {
+                return "<UserObjectId "s + userObjectId.toString() + ">";
             },
-            [numTabs](const UserBoolean& userBoolean) {
-                return tabs(numTabs) + "<UserBoolean " + std::to_string(userBoolean) + ">";
+            [](const UserBoolean& userBoolean) {
+                return "<UserBoolean "s + std::to_string(userBoolean) + ">";
             },
-            [numTabs](const UserDate& userDate) {
-                return tabs(numTabs) + "<UserDate " +
+            [](const UserDate& userDate) {
+                return "<UserDate "s +
                     [&] {
                         if (auto string = TimeZoneDatabase::utcZone().formatDate(
                                 "%Y-%m-%dT%H:%M:%S.%LZ", userDate);
@@ -121,40 +94,131 @@ std::string CNode::toStringHelper(int numTabs) const {
                     }() +
                     ">";
             },
-            [numTabs](const UserNull& userNull) { return tabs(numTabs) + "<UserNull>"; },
-            [numTabs](const UserRegex& userRegex) {
-                return tabs(numTabs) + "<UserRegex " + "/" + userRegex.pattern + "/" +
-                    userRegex.flags + ">";
-            },
-            [numTabs](const UserDBPointer& userDBPointer) {
-                return tabs(numTabs) + "<UserDBPointer " + userDBPointer.ns + ", " +
-                    userDBPointer.oid.toString() + ">";
+            [](const UserNull& userNull) { return "<UserNull>"s; },
+            [](const UserRegex& userRegex) {
+                return "<UserRegex "s + "/" + userRegex.pattern + "/" + userRegex.flags + ">";
             },
-            [numTabs](const UserJavascript& userJavascript) {
-                return tabs(numTabs) + "<UserJavascript " + userJavascript.code + ">";
+            [](const UserDBPointer& userDBPointer) {
+                return "<UserDBPointer "s + userDBPointer.ns + ", " + userDBPointer.oid.toString() +
+                    ">";
             },
-            [numTabs](const UserSymbol& userSymbol) {
-                return tabs(numTabs) + "<UserSymbol " + userSymbol.symbol + ">";
+            [](const UserJavascript& userJavascript) {
+                return "<UserJavascript "s + userJavascript.code + ">";
             },
-            [numTabs](const UserJavascriptWithScope& userJavascriptWithScope) {
-                return tabs(numTabs) + "<UserJavascriptWithScope " + userJavascriptWithScope.code +
-                    ", ";
+            [](const UserSymbol& userSymbol) { return "<UserSymbol "s + userSymbol.symbol + ">"; },
+            [](const UserJavascriptWithScope& userJavascriptWithScope) {
+                return "<UserJavascriptWithScope "s + userJavascriptWithScope.code + ", ";
                 userJavascriptWithScope.scope.toString() + ">";
             },
-            [numTabs](const UserInt& userInt) {
-                return tabs(numTabs) + "<UserInt " + std::to_string(userInt) + ">";
-            },
-            [numTabs](const UserTimestamp& userTimestamp) {
-                return tabs(numTabs) + "<UserTimestamp " + userTimestamp.toString() + ">";
-            },
-            [numTabs](const UserLong& userLong) {
-                return tabs(numTabs) + "<UserLong " + std::to_string(userLong) + ">";
+            [](const UserInt& userInt) { return "<UserInt "s + std::to_string(userInt) + ">"; },
+            [](const UserTimestamp& userTimestamp) {
+                return "<UserTimestamp "s + userTimestamp.toString() + ">";
             },
-            [numTabs](const UserDecimal& userDecimal) {
-                return tabs(numTabs) + "<UserDecimal " + userDecimal.toString() + ">";
+            [](const UserLong& userLong) { return "<UserLong "s + std::to_string(userLong) + ">"; },
+            [](const UserDecimal& userDecimal) {
+                return "<UserDecimal "s + userDecimal.toString() + ">";
             },
-            [numTabs](const UserMinKey& userMinKey) { return tabs(numTabs) + "<UserMinKey>"; },
-            [numTabs](const UserMaxKey& userMaxKey) { return tabs(numTabs) + "<UserMaxKey>"; }},
+            [](const UserMinKey& userMinKey) { return "<UserMinKey>"s; },
+            [](const UserMaxKey& userMaxKey) { return "<UserMaxKey>"s; }},
+        payload);
+}
+
+}  // namespace
+
+std::string CNode::toStringHelper(int numTabs) const {
+    return stdx::visit(
+        visit_helper::Overloaded{
+            [numTabs](const ArrayChildren& children) {
+                return std::accumulate(children.cbegin(),
+                                       children.cend(),
+                                       tabs(numTabs) + "[\n",
+                                       [numTabs](auto&& string, auto&& child) {
+                                           return string + child.toStringHelper(numTabs + 1) + "\n";
+                                       }) +
+                    tabs(numTabs) + "]";
+            },
+            [numTabs](const ObjectChildren& children) {
+                return std::accumulate(children.cbegin(),
+                                       children.cend(),
+                                       tabs(numTabs) + "{\n",
+                                       [numTabs](auto&& string, auto&& childpair) {
+                                           return string + tabs(numTabs) +
+                                               printFieldname(childpair.first) + " :\n" +
+                                               childpair.second.toStringHelper(numTabs + 1) + "\n";
+                                       }) +
+                    tabs(numTabs) + "}";
+            },
+            [this, numTabs](auto&&) { return tabs(numTabs) + printValue(payload); }},
+        payload);
+}
+
+std::pair<BSONObj, bool> CNode::toBsonWithArrayIndicator() const {
+    return stdx::visit(
+        visit_helper::Overloaded{
+            // Build an array which will lose its identity and appear as a BSONObj
+            [](const ArrayChildren& children) {
+                return std::pair{
+                    std::accumulate(
+                        children.cbegin(),
+                        children.cend(),
+                        BSONObj{},
+                        [fieldCount = 0u](auto&& bson, auto&& child) mutable {
+                            // This is a non-compound field. pull the BSONElement out of it's
+                            // BSONObj shell and add it.
+                            if (auto [childBson, isArray] = child.toBsonWithArrayIndicator();
+                                childBson.nFields() > 0 &&
+                                childBson.firstElementFieldNameStringData() == "")
+                                return bson.addField(
+                                    childBson
+                                        .replaceFieldNames(BSON(std::to_string(fieldCount++) << ""))
+                                        .firstElement());
+                            // This field is an array. Reconstruct with BSONArray and add it.
+                            else if (isArray)
+                                return bson.addField(
+                                    BSON(std::to_string(fieldCount++) << BSONArray{childBson})
+                                        .firstElement());
+                            // This field is an object. Add it directly.
+                            else
+                                return bson.addField(
+                                    BSON(std::to_string(fieldCount++) << childBson).firstElement());
+                        }),
+                    true};
+            },
+            // Build an object in a BSONObj.
+            [](const ObjectChildren& children) {
+                return std::pair{
+                    std::accumulate(
+                        children.cbegin(),
+                        children.cend(),
+                        BSONObj{},
+                        [](auto&& bson, auto&& childPair) {
+                            // This is a non-compound field. pull the BSONElement out of it's
+                            // BSONObj shell and add it.
+                            if (auto [childBson, isArray] =
+                                    childPair.second.toBsonWithArrayIndicator();
+                                childBson.nFields() > 0 &&
+                                childBson.firstElementFieldNameStringData() == "")
+                                return bson.addField(childBson
+                                                         .replaceFieldNames(BSON(
+                                                             printFieldname(childPair.first) << ""))
+                                                         .firstElement());
+                            // This field is an array. Reconstruct with BSONArray and add it.
+                            else if (isArray)
+                                return bson.addField(
+                                    BSON(printFieldname(childPair.first) << BSONArray{childBson})
+                                        .firstElement());
+                            // This field is an object. Add it directly.
+                            else
+                                return bson.addField(
+                                    BSON(printFieldname(childPair.first) << childBson)
+                                        .firstElement());
+                        }),
+                    false};
+            },
+            // Build a non-compound field in a BSONObj shell.
+            [this](auto&&) {
+                return std::pair{BSON("" << printValue(payload)), false};
+            }},
         payload);
 }
 
diff --git a/src/mongo/db/cst/c_node.h b/src/mongo/db/cst/c_node.h
index 773eb49e801..47bbbcab05d 100644
--- a/src/mongo/db/cst/c_node.h
+++ b/src/mongo/db/cst/c_node.h
@@ -31,11 +31,13 @@
 
 #include "mongo/platform/basic.h"
 
+#include <memory>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "mongo/bson/bsonmisc.h"
+#include "mongo/bson/bsonobj.h"
 #include "mongo/bson/oid.h"
 #include "mongo/bson/timestamp.h"
 #include "mongo/db/cst/key_fieldname.h"
@@ -70,11 +72,80 @@ struct UserMaxKey {};
 
 struct CNode {
     static auto noopLeaf() {
-        return CNode{Children{}};
+        return CNode{ObjectChildren{}};
     }
 
+    /**
+     * Produce a string formatted with tabs and endlines that describes the CST underneath this
+     * CNode.
+     */
     auto toString() const {
-        return toStringHelper(0);
+        return toStringHelper(0) + "\n";
+    }
+
+    /**
+     * Produce BSON representing this CST. This is for debugging and testing with structured output,
+     * not for decompiling to the input query. The produced BSON will consist of arrays, objects,
+     * and descriptive strings only. This version also returns bool that indicates if the returned
+     * BSON is a BSONArray.
+     */
+    std::pair<BSONObj, bool> toBsonWithArrayIndicator() const;
+    /**
+     * Produce BSON representing this CST. This is for debugging and testing with structured output,
+     * not for decompiling to the input query. The produced BSON will consist of arrays, objects,
+     * and descriptive strings only.
+     */
+    BSONObj toBson() const {
+        return toBsonWithArrayIndicator().first;
+    }
+
+    /*
+     * Produce the children of this CNode representing an array. Throws a fatal exception if this
+     * CNode does not represent an array. Const version.
+     */
+    auto& arrayChildren() const {
+        return stdx::get<ArrayChildren>(payload);
+    }
+    /*
+     * Produce the children of this CNode representing an array. Throws a fatal exception if this
+     * CNode does not represent an array. Non-const version.
+     */
+    auto& arrayChildren() {
+        return stdx::get<ArrayChildren>(payload);
+    }
+
+    /*
+     * Produce the children of this CNode representing an object. Throws a fatal exception if this
+     * CNode does not represent an object. Const version.
+     */
+    auto& objectChildren() const {
+        return stdx::get<ObjectChildren>(payload);
+    }
+    /*
+     * Produce the children of this CNode representing an object. Throws a fatal exception if this
+     * CNode does not represent an object. Non-const version.
+     */
+    auto& objectChildren() {
+        return stdx::get<ObjectChildren>(payload);
+    }
+
+    /*
+     * Produce the KeyFieldname of the first element of this CNode representing an object. Throws a
+     * fatal exception if this CNode does not represent an object, if it is an empty object or if
+     * the first element does not have a KeyFieldname. Const version.
+     */
+    auto& firstKeyFieldname() const {
+        dassert(objectChildren().size() > 0);
+        return stdx::get<KeyFieldname>(objectChildren().begin()->first);
+    }
+    /*
+     * Produce the KeyFieldname of the first element of this CNode representing an object. Throws a
+     * fatal exception if this CNode does not represent an object, if it is an empty object or if
+     * the first element does not have a KeyFieldname. Non-const version.
+     */
+    auto& firstKeyFieldname() {
+        dassert(objectChildren().size() > 0);
+        return stdx::get<KeyFieldname>(objectChildren().begin()->first);
     }
 
 private:
@@ -82,8 +153,10 @@ private:
 
 public:
     using Fieldname = stdx::variant<KeyFieldname, UserFieldname>;
-    using Children = std::vector<std::pair<Fieldname, CNode>>;
-    stdx::variant<Children,
+    using ArrayChildren = std::vector<CNode>;
+    using ObjectChildren = std::vector<std::pair<Fieldname, CNode>>;
+    stdx::variant<ArrayChildren,
+                  ObjectChildren,
                   KeyValue,
                   UserDouble,
                   UserString,
diff --git a/src/mongo/db/cst/cst_pipeline_translation.cpp b/src/mongo/db/cst/cst_pipeline_translation.cpp
new file mode 100644
index 00000000000..85602fe5d64
--- /dev/null
+++ b/src/mongo/db/cst/cst_pipeline_translation.cpp
@@ -0,0 +1,110 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+
+#include <algorithm>
+#include <boost/intrusive_ptr.hpp>
+#include <iterator>
+
+#include "mongo/db/cst/c_node.h"
+#include "mongo/db/cst/cst_pipeline_translation.h"
+#include "mongo/db/cst/key_fieldname.h"
+#include "mongo/db/cst/key_value.h"
+#include "mongo/db/exec/inclusion_projection_executor.h"
+#include "mongo/db/pipeline/document_source.h"
+#include "mongo/db/pipeline/document_source_project.h"
+#include "mongo/db/pipeline/expression_context.h"
+#include "mongo/db/query/projection.h"
+#include "mongo/db/query/projection_ast.h"
+#include "mongo/db/query/projection_parser.h"
+#include "mongo/util/intrusive_counter.h"
+
+namespace mongo::cst_pipeline_translation {
+namespace {
+/**
+ * Walk a projection CNode and produce a ProjectionASTNode.
+ */
+auto translateProjection(const CNode& cst, const boost::intrusive_ptr<ExpressionContext>& expCtx) {
+    using namespace projection_ast;
+    // TODO SERVER-48834: Support more than inclusion projection.
+    return std::make_unique<BooleanConstantASTNode>(true);
+}
+
+/**
+ * Walk a project stage object CNode and produce a DocumentSourceSingleDocumentTransformation.
+ */
+auto translateProject(const CNode& cst, const boost::intrusive_ptr<ExpressionContext>& expCtx) {
+    using namespace projection_ast;
+    auto root = ProjectionPathASTNode{};
+    bool sawId = false;
+
+    for (auto&& [name, child] : cst.objectChildren()) {
+        if (stdx::get<UserFieldname>(name) == UserFieldname{"_id"})
+            sawId = true;
+        addNodeAtPath(&root, stdx::get<UserFieldname>(name), translateProjection(child, expCtx));
+    }
+
+    if (!sawId)
+        addNodeAtPath(&root, "_id", std::make_unique<BooleanConstantASTNode>(true));
+
+    return DocumentSourceProject::create(
+        // TODO SERVER-48834: Support more than inclusion projection.
+        Projection{root, ProjectType::kInclusion},
+        expCtx,
+        "$project");
+}
+
+/**
+ * Walk an aggregation pipeline stage object CNode and produce a DocumentSource.
+ */
+auto translateSource(const CNode& cst, const boost::intrusive_ptr<ExpressionContext>& expCtx) {
+    switch (cst.firstKeyFieldname()) {
+        case KeyFieldname::project:
+            return translateProject(cst.objectChildren()[0].second, expCtx);
+        default:
+            MONGO_UNREACHABLE;
+    }
+}
+
+}  // namespace
+
+/**
+ * Walk a pipeline array CNode and produce a Pipeline.
+ */
+std::unique_ptr<Pipeline, PipelineDeleter> translatePipeline(
+    const CNode& cst, const boost::intrusive_ptr<ExpressionContext>& expCtx) {
+    auto sources = Pipeline::SourceContainer{};
+    static_cast<void>(std::transform(cst.arrayChildren().begin(),
+                                     cst.arrayChildren().end(),
+                                     std::back_inserter(sources),
+                                     [&](auto&& elem) { return translateSource(elem, expCtx); }));
+    return Pipeline::create(std::move(sources), expCtx);
+}
+
+}  // namespace mongo::cst_pipeline_translation
diff --git a/src/mongo/db/cst/cst_pipeline_translation.h b/src/mongo/db/cst/cst_pipeline_translation.h
new file mode 100644
index 00000000000..bac68db91da
--- /dev/null
+++ b/src/mongo/db/cst/cst_pipeline_translation.h
@@ -0,0 +1,46 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/platform/basic.h"
+
+#include <memory>
+
+#include "mongo/db/pipeline/pipeline.h"
+
+namespace mongo::cst_pipeline_translation {
+
+/**
+ * Walk a pipeline array CNode and produce a Pipeline.
+ */
+std::unique_ptr<Pipeline, PipelineDeleter> translatePipeline(
+    const CNode& cst, const boost::intrusive_ptr<ExpressionContext>& expCtx);
+
+}  // namespace mongo::cst_pipeline_translation
diff --git a/src/mongo/db/cst/cst_pipeline_translation_test.cpp b/src/mongo/db/cst/cst_pipeline_translation_test.cpp
new file mode 100644
index 00000000000..bfcc11ef12d
--- /dev/null
+++ b/src/mongo/db/cst/cst_pipeline_translation_test.cpp
@@ -0,0 +1,170 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#include "mongo/platform/basic.h"
+
+#include <boost/intrusive_ptr.hpp>
+#include <iostream>
+#include <string>
+
+#include "mongo/bson/bsonobj.h"
+#include "mongo/bson/unordered_fields_bsonobj_comparator.h"
+#include "mongo/db/cst/c_node.h"
+#include "mongo/db/cst/cst_pipeline_translation.h"
+#include "mongo/db/cst/key_fieldname.h"
+#include "mongo/db/cst/key_value.h"
+#include "mongo/db/exec/document_value/document.h"
+#include "mongo/db/namespace_string.h"
+#include "mongo/db/pipeline/document_source_single_document_transformation.h"
+#include "mongo/db/pipeline/expression_context_for_test.h"
+#include "mongo/unittest/unittest.h"
+
+namespace mongo {
+namespace {
+using namespace std::string_literals;
+
+TEST(CstTest, TranslatesEmpty) {
+    auto nss = NamespaceString{"db", "coll"};
+    const auto cst = CNode{CNode::ArrayChildren{}};
+    boost::intrusive_ptr<ExpressionContextForTest> expCtx(new ExpressionContextForTest(nss));
+    auto pipeline = cst_pipeline_translation::translatePipeline(cst, expCtx);
+    auto& sources = pipeline->getSources();
+    ASSERT_EQ(0u, sources.size());
+}
+
+TEST(CstTest, TranslatesEmptyProject) {
+    auto nss = NamespaceString{"db", "coll"};
+    const auto cst = CNode{CNode::ArrayChildren{
+        CNode{CNode::ObjectChildren{{KeyFieldname::project, CNode{CNode::ObjectChildren{}}}}}}};
+    boost::intrusive_ptr<ExpressionContextForTest> expCtx(new ExpressionContextForTest(nss));
+    auto pipeline = cst_pipeline_translation::translatePipeline(cst, expCtx);
+    auto& sources = pipeline->getSources();
+    ASSERT_EQ(1u, sources.size());
+    auto iter = sources.begin();
+    ASSERT(typeid(DocumentSourceSingleDocumentTransformation) == typeid(**iter));
+}
+
+TEST(CstTest, TranslatesEmptyProjects) {
+    auto nss = NamespaceString{"db", "coll"};
+    const auto cst = CNode{CNode::ArrayChildren{
+        CNode{CNode::ObjectChildren{{KeyFieldname::project, CNode{CNode::ObjectChildren{}}}}},
+        CNode{CNode::ObjectChildren{{KeyFieldname::project, CNode{CNode::ObjectChildren{}}}}},
+        CNode{CNode::ObjectChildren{{KeyFieldname::project, CNode{CNode::ObjectChildren{}}}}}}};
+    boost::intrusive_ptr<ExpressionContextForTest> expCtx(new ExpressionContextForTest(nss));
+    auto pipeline = cst_pipeline_translation::translatePipeline(cst, expCtx);
+    auto& sources = pipeline->getSources();
+    ASSERT_EQ(3u, sources.size());
+    auto iter = sources.begin();
+    ASSERT(typeid(DocumentSourceSingleDocumentTransformation) == typeid(**iter++));
+    ASSERT(typeid(DocumentSourceSingleDocumentTransformation) == typeid(**iter++));
+    ASSERT(typeid(DocumentSourceSingleDocumentTransformation) == typeid(**iter));
+}
+
+TEST(CstTest, TranslatesOneFieldInclusionProjectionStage) {
+    auto nss = NamespaceString{"db", "coll"};
+    const auto cst = CNode{CNode::ArrayChildren{CNode{CNode::ObjectChildren{
+        {KeyFieldname::project,
+         CNode{CNode::ObjectChildren{{UserFieldname{"a"}, CNode{KeyValue::trueKey}}}}}}}}};
+    boost::intrusive_ptr<ExpressionContextForTest> expCtx(new ExpressionContextForTest(nss));
+    auto pipeline = cst_pipeline_translation::translatePipeline(cst, expCtx);
+    auto& sources = pipeline->getSources();
+    ASSERT_EQ(1u, sources.size());
+    auto iter = sources.begin();
+    auto& singleDoc = dynamic_cast<DocumentSourceSingleDocumentTransformation&>(**iter);
+    // DocumenSourceSingleDoucmentTransformation reorders fields so we need to be insensitive.
+    ASSERT(UnorderedFieldsBSONObjComparator{}.evaluate(
+        BSON("_id" << true << "a" << true) ==
+        singleDoc.getTransformer().serializeTransformation(boost::none).toBson()));
+}
+
+TEST(CstTest, TranslatesMultifieldInclusionProjection) {
+    auto nss = NamespaceString{"db", "coll"};
+    const auto cst = CNode{CNode::ArrayChildren{CNode{CNode::ObjectChildren{
+        {KeyFieldname::project,
+         CNode{CNode::ObjectChildren{{UserFieldname{"_id"}, CNode{KeyValue::trueKey}},
+                                     {UserFieldname{"a"}, CNode{KeyValue::trueKey}},
+                                     {UserFieldname{"b"}, CNode{KeyValue::trueKey}}}}}}}}};
+    boost::intrusive_ptr<ExpressionContextForTest> expCtx(new ExpressionContextForTest(nss));
+    auto pipeline = cst_pipeline_translation::translatePipeline(cst, expCtx);
+    auto& sources = pipeline->getSources();
+    ASSERT_EQ(1u, sources.size());
+    auto iter = sources.begin();
+    auto& singleDoc = dynamic_cast<DocumentSourceSingleDocumentTransformation&>(**iter);
+    // DocumenSourceSingleDoucmentTransformation reorders fields so we need to be insensitive.
+    ASSERT(UnorderedFieldsBSONObjComparator{}.evaluate(
+        BSON("_id" << true << "a" << true << "b" << true) ==
+        singleDoc.getTransformer().serializeTransformation(boost::none).toBson()));
+}
+
+TEST(CstTest, TranslatesMultipleInclusionProjectionStages) {
+    auto nss = NamespaceString{"db", "coll"};
+    const auto cst = CNode{CNode::ArrayChildren{
+        CNode{CNode::ObjectChildren{
+            {KeyFieldname::project,
+             CNode{CNode::ObjectChildren{{UserFieldname{"a"}, CNode{KeyValue::trueKey}}}}}}},
+        CNode{CNode::ObjectChildren{
+            {KeyFieldname::project,
+             CNode{CNode::ObjectChildren{{UserFieldname{"b"}, CNode{KeyValue::trueKey}}}}}}},
+        CNode{CNode::ObjectChildren{
+            {KeyFieldname::project,
+             CNode{CNode::ObjectChildren{{UserFieldname{"c"}, CNode{KeyValue::trueKey}}}}}}},
+    }};
+    boost::intrusive_ptr<ExpressionContextForTest> expCtx(new ExpressionContextForTest(nss));
+    auto pipeline = cst_pipeline_translation::translatePipeline(cst, expCtx);
+    auto& sources = pipeline->getSources();
+    ASSERT_EQ(3u, sources.size());
+    auto iter = sources.begin();
+    {
+        auto& singleDoc = dynamic_cast<DocumentSourceSingleDocumentTransformation&>(**iter++);
+        // DocumenSourceSingleDoucmentTransformation reorders fields so we need to be
+        // insensitive.
+        ASSERT(UnorderedFieldsBSONObjComparator{}.evaluate(
+            BSON("_id" << true << "a" << true) ==
+            singleDoc.getTransformer().serializeTransformation(boost::none).toBson()));
+    }
+    {
+        auto& singleDoc = dynamic_cast<DocumentSourceSingleDocumentTransformation&>(**iter++);
+        // DocumenSourceSingleDoucmentTransformation reorders fields so we need to be
+        // insensitive.
+        ASSERT(UnorderedFieldsBSONObjComparator{}.evaluate(
+            BSON("_id" << true << "b" << true) ==
+            singleDoc.getTransformer().serializeTransformation(boost::none).toBson()));
+    }
+    {
+        auto& singleDoc = dynamic_cast<DocumentSourceSingleDocumentTransformation&>(**iter);
+        // DocumenSourceSingleDoucmentTransformation reorders fields so we need to be
+        // insensitive.
+        ASSERT(UnorderedFieldsBSONObjComparator{}.evaluate(
+            BSON("_id" << true << "c" << true) ==
+            singleDoc.getTransformer().serializeTransformation(boost::none).toBson()));
+    }
+}
+
+}  // namespace
+}  // namespace mongo
diff --git a/src/mongo/db/cst/cst_test.cpp b/src/mongo/db/cst/cst_test.cpp
index ce4a20f3f81..0d4181c55aa 100644
--- a/src/mongo/db/cst/cst_test.cpp
+++ b/src/mongo/db/cst/cst_test.cpp
@@ -29,7 +29,6 @@
 
 #include "mongo/platform/basic.h"
 
-#include <iostream>
 #include <string>
 
 #include "mongo/bson/json.h"
@@ -38,30 +37,29 @@
 #include "mongo/db/cst/key_fieldname.h"
 #include "mongo/db/cst/key_value.h"
 #include "mongo/db/cst/pipeline_parser_gen.hpp"
-#include "mongo/db/query/util/make_data_structure.h"
+#include "mongo/unittest/bson_test_util.h"
 #include "mongo/unittest/unittest.h"
 
 namespace mongo {
 namespace {
-using namespace std::string_literals;
 
 TEST(CstTest, BuildsAndPrints) {
     {
-        const auto cst = CNode{CNode::Children{
+        const auto cst = CNode{CNode::ObjectChildren{
             {KeyFieldname::atan2,
-             CNode{CNode::Children{{KeyFieldname::arrayMarker, CNode{UserDouble{3.0}}},
-                                   {KeyFieldname::arrayMarker, CNode{UserDouble{2.0}}}}}}}};
-        ASSERT_EQ("{\natan2 :\n\t[\n\t\t<UserDouble 3.000000>\n\t\t<UserDouble 2.000000>\n\t]\n}"s,
-                  cst.toString());
+             CNode{CNode::ArrayChildren{CNode{UserDouble{3.0}}, CNode{UserDouble{2.0}}}}}}};
+        ASSERT_BSONOBJ_EQ(
+            fromjson("{atan2: [\"<UserDouble 3.000000>\", \"<UserDouble 2.000000>\"]}"),
+            cst.toBson());
     }
     {
-        const auto cst = CNode{CNode::Children{
+        const auto cst = CNode{CNode::ObjectChildren{
             {KeyFieldname::project,
-             CNode{CNode::Children{{UserFieldname{"a"}, CNode{KeyValue::trueKey}},
-                                   {KeyFieldname::id, CNode{KeyValue::falseKey}}}}}}};
-        ASSERT_EQ("{\nproject :\n\t{\n\ta :\n\t\t<KeyValue trueKey>\n"s +
-                      "\tid :\n\t\t<KeyValue falseKey>\n\t}\n}",
-                  cst.toString());
+             CNode{CNode::ObjectChildren{{UserFieldname{"a"}, CNode{KeyValue::trueKey}},
+                                         {KeyFieldname::id, CNode{KeyValue::falseKey}}}}}}};
+        ASSERT_BSONOBJ_EQ(
+            fromjson("{project : {a: \"<KeyValue trueKey>\", id: \"<KeyValue falseKey>\"}}"),
+            cst.toBson());
     }
 }
 
@@ -71,8 +69,8 @@ TEST(CstGrammarTest, EmptyPipeline) {
     BSONLexer lexer(input["pipeline"].Array());
     auto parseTree = PipelineParserGen(lexer, &output);
     ASSERT_EQ(0, parseTree.parse());
-    ASSERT_TRUE(stdx::get_if<CNode::Children>(&output.payload));
-    ASSERT_EQ(0, stdx::get_if<CNode::Children>(&output.payload)->size());
+    ASSERT_TRUE(stdx::get_if<CNode::ArrayChildren>(&output.payload));
+    ASSERT_EQ(0, stdx::get_if<CNode::ArrayChildren>(&output.payload)->size());
 }
 
 TEST(CstGrammarTest, InvalidPipelineSpec) {
@@ -109,9 +107,9 @@ TEST(CstGrammarTest, ParsesInternalInhibitOptimization) {
         BSONLexer lexer(input["pipeline"].Array());
         auto parseTree = PipelineParserGen(lexer, &output);
         ASSERT_EQ(0, parseTree.parse());
-        auto stages = stdx::get<CNode::Children>(output.payload);
+        auto stages = stdx::get<CNode::ArrayChildren>(output.payload);
         ASSERT_EQ(1, stages.size());
-        ASSERT(KeyFieldname::inhibitOptimization == stdx::get<KeyFieldname>(stages[0].first));
+        ASSERT(KeyFieldname::inhibitOptimization == stages[0].firstKeyFieldname());
     }
     {
         CNode output;
diff --git a/src/mongo/db/cst/key_fieldname.h b/src/mongo/db/cst/key_fieldname.h
index 2ddb692b6b0..edde3699941 100644
--- a/src/mongo/db/cst/key_fieldname.h
+++ b/src/mongo/db/cst/key_fieldname.h
@@ -34,7 +34,6 @@
 #include "mongo/util/printable_enum.h"
 
 #define KEYFIELDNAMES(ENUMIFY) \
-    ENUMIFY(arrayMarker)       \
     ENUMIFY(atan2)             \
     ENUMIFY(id)                \
     ENUMIFY(project)           \
diff --git a/src/mongo/db/cst/location_gen.h b/src/mongo/db/cst/location_gen.h
index 7a3a880671c..c1a0c8cac32 100644
--- a/src/mongo/db/cst/location_gen.h
+++ b/src/mongo/db/cst/location_gen.h
@@ -1,4 +1,4 @@
-// A Bison parser, made by GNU Bison 3.5.4.
+// A Bison parser, made by GNU Bison 3.6.3.
 
 // Locations for Bison parsers in C++
 
@@ -31,12 +31,12 @@
 // version 2.2 of Bison.
 
 /**
- ** \file src/mongo/db/cst/location_gen.h
+ ** \file location_gen.h
  ** Define the mongo::location class.
  */
 
-#ifndef YY_YY_SRC_MONGO_DB_CST_LOCATION_GEN_H_INCLUDED
-#define YY_YY_SRC_MONGO_DB_CST_LOCATION_GEN_H_INCLUDED
+#ifndef YY_YY_LOCATION_GEN_H_INCLUDED
+#define YY_YY_LOCATION_GEN_H_INCLUDED
 
 #include <iostream>
 #include <string>
@@ -53,9 +53,9 @@
 #endif
 #endif
 
-#line 52 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 52 "pipeline_grammar.yy"
 namespace mongo {
-#line 59 "src/mongo/db/cst/location_gen.h"
+#line 59 "location_gen.h"
 
 /// A point in a source file.
 class position {
@@ -260,8 +260,8 @@ std::basic_ostream<YYChar>& operator<<(std::basic_ostream<YYChar>& ostr, const l
     return ostr;
 }
 
-#line 52 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 52 "pipeline_grammar.yy"
 }  // namespace mongo
-#line 333 "src/mongo/db/cst/location_gen.h"
+#line 333 "location_gen.h"
 
-#endif  // !YY_YY_SRC_MONGO_DB_CST_LOCATION_GEN_H_INCLUDED
+#endif  // !YY_YY_LOCATION_GEN_H_INCLUDED
diff --git a/src/mongo/db/cst/pipeline_grammar.yy b/src/mongo/db/cst/pipeline_grammar.yy
index 3d8bff404d5..9616e113443 100644
--- a/src/mongo/db/cst/pipeline_grammar.yy
+++ b/src/mongo/db/cst/pipeline_grammar.yy
@@ -118,8 +118,7 @@
 //
 // Semantic values (aka the C++ types produced by the actions).
 //
-%type <CNode> stageList
-%type <std::pair<KeyFieldname, CNode>> stage
+%nterm <CNode> stageList stage
 
 //
 // Grammar rules
@@ -135,14 +134,14 @@ stageList[result]:
     %empty { }
     | START_OBJECT stage END_OBJECT stageList[stagesArg] { 
         $result = std::move($stagesArg);
-        auto& children = stdx::get<CNode::Children>($result.payload);
+        auto& children = stdx::get<CNode::ArrayChildren>($result.payload);
         children.emplace_back(std::move($stage));
     }
 ;
 
 stage:
     STAGE_INHIBIT_OPTIMIZATION START_OBJECT END_OBJECT { 
-        $stage = std::pair{KeyFieldname::inhibitOptimization, CNode::noopLeaf()};
+        $stage = CNode{CNode::ObjectChildren{std::pair{KeyFieldname::inhibitOptimization, CNode::noopLeaf()}}};
     }
 ;
 
diff --git a/src/mongo/db/cst/pipeline_parser_gen.cpp b/src/mongo/db/cst/pipeline_parser_gen.cpp
index d463f5e782f..83f31a004e2 100644
--- a/src/mongo/db/cst/pipeline_parser_gen.cpp
+++ b/src/mongo/db/cst/pipeline_parser_gen.cpp
@@ -1,4 +1,4 @@
-// A Bison parser, made by GNU Bison 3.5.4.
+// A Bison parser, made by GNU Bison 3.6.3.
 
 // Skeleton implementation for Bison LALR(1) parsers in C++
 
@@ -30,15 +30,16 @@
 // This special exception was added by the Free Software Foundation in
 // version 2.2 of Bison.
 
-// Undocumented macros, especially those whose name start with YY_,
-// are private implementation details.  Do not rely on them.
+// DO NOT RELY ON FEATURES THAT ARE NOT DOCUMENTED in the manual,
+// especially those whose name start with YY_ or yy_.  They are
+// private implementation details that can be changed or removed.
 
 
 #include "pipeline_parser_gen.hpp"
 
 
 // Unqualified %code blocks.
-#line 77 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 77 "pipeline_grammar.yy"
 
 #include "mongo/db/cst/bson_lexer.h"
 
@@ -49,7 +50,7 @@ void PipelineParserGen::error(const PipelineParserGen::location_type& loc, const
 }
 }  // namespace mongo
 
-#line 57 "src/mongo/db/cst/pipeline_parser_gen.cpp"
+#line 58 "pipeline_parser_gen.cpp"
 
 
 #ifndef YY_
@@ -64,6 +65,7 @@ void PipelineParserGen::error(const PipelineParserGen::location_type& loc, const
 #endif
 #endif
 
+
 // Whether we are compiled with exception support.
 #ifndef YY_EXCEPTIONS
 #if defined __GNUC__ && !defined __EXCEPTIONS
@@ -114,10 +116,10 @@ void PipelineParserGen::error(const PipelineParserGen::location_type& loc, const
             yy_reduce_print_(Rule); \
     } while (false)
 
-#define YY_STACK_PRINT()      \
-    do {                      \
-        if (yydebug_)         \
-            yystack_print_(); \
+#define YY_STACK_PRINT()       \
+    do {                       \
+        if (yydebug_)          \
+            yy_stack_print_(); \
     } while (false)
 
 #else  // !YYDEBUG
@@ -139,10 +141,9 @@ void PipelineParserGen::error(const PipelineParserGen::location_type& loc, const
 #define YYERROR goto yyerrorlab
 #define YYRECOVERING() (!!yyerrstatus_)
 
-#line 52 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 52 "pipeline_grammar.yy"
 namespace mongo {
-#line 149 "src/mongo/db/cst/pipeline_parser_gen.cpp"
-
+#line 151 "pipeline_parser_gen.cpp"
 
 /// Build a parser object.
 PipelineParserGen::PipelineParserGen(BSONLexer& driver_yyarg, CNode* cst_yyarg)
@@ -161,7 +162,7 @@ PipelineParserGen::~PipelineParserGen() {}
 PipelineParserGen::syntax_error::~syntax_error() YY_NOEXCEPT YY_NOTHROW {}
 
 /*---------------.
-| Symbol types.  |
+| symbol kinds.  |
 `---------------*/
 
 
@@ -181,19 +182,20 @@ void PipelineParserGen::by_state::move(by_state& that) {
 
 PipelineParserGen::by_state::by_state(state_type s) YY_NOEXCEPT : state(s) {}
 
-PipelineParserGen::symbol_number_type PipelineParserGen::by_state::type_get() const YY_NOEXCEPT {
+PipelineParserGen::symbol_kind_type PipelineParserGen::by_state::kind() const YY_NOEXCEPT {
     if (state == empty_state)
-        return empty_symbol;
+        return symbol_kind::S_YYEMPTY;
     else
-        return yystos_[+state];
+        return YY_CAST(symbol_kind_type, yystos_[+state]);
 }
 
 PipelineParserGen::stack_symbol_type::stack_symbol_type() {}
 
 PipelineParserGen::stack_symbol_type::stack_symbol_type(YY_RVREF(stack_symbol_type) that)
     : super_type(YY_MOVE(that.state), YY_MOVE(that.location)) {
-    switch (that.type_get()) {
-        case 17:  // stageList
+    switch (that.kind()) {
+        case 16:  // stageList
+        case 17:  // stage
             value.YY_MOVE_OR_COPY<CNode>(YY_MOVE(that.value));
             break;
 
@@ -213,10 +215,6 @@ PipelineParserGen::stack_symbol_type::stack_symbol_type(YY_RVREF(stack_symbol_ty
             value.YY_MOVE_OR_COPY<long long>(YY_MOVE(that.value));
             break;
 
-        case 18:  // stage
-            value.YY_MOVE_OR_COPY<std::pair<KeyFieldname, CNode>>(YY_MOVE(that.value));
-            break;
-
         case 10:  // STRING
             value.YY_MOVE_OR_COPY<std::string>(YY_MOVE(that.value));
             break;
@@ -233,8 +231,9 @@ PipelineParserGen::stack_symbol_type::stack_symbol_type(YY_RVREF(stack_symbol_ty
 
 PipelineParserGen::stack_symbol_type::stack_symbol_type(state_type s, YY_MOVE_REF(symbol_type) that)
     : super_type(s, YY_MOVE(that.location)) {
-    switch (that.type_get()) {
-        case 17:  // stageList
+    switch (that.kind()) {
+        case 16:  // stageList
+        case 17:  // stage
             value.move<CNode>(YY_MOVE(that.value));
             break;
 
@@ -254,10 +253,6 @@ PipelineParserGen::stack_symbol_type::stack_symbol_type(state_type s, YY_MOVE_RE
             value.move<long long>(YY_MOVE(that.value));
             break;
 
-        case 18:  // stage
-            value.move<std::pair<KeyFieldname, CNode>>(YY_MOVE(that.value));
-            break;
-
         case 10:  // STRING
             value.move<std::string>(YY_MOVE(that.value));
             break;
@@ -267,15 +262,16 @@ PipelineParserGen::stack_symbol_type::stack_symbol_type(state_type s, YY_MOVE_RE
     }
 
     // that is emptied.
-    that.type = empty_symbol;
+    that.kind_ = symbol_kind::S_YYEMPTY;
 }
 
 #if YY_CPLUSPLUS < 201103L
 PipelineParserGen::stack_symbol_type& PipelineParserGen::stack_symbol_type::operator=(
     const stack_symbol_type& that) {
     state = that.state;
-    switch (that.type_get()) {
-        case 17:  // stageList
+    switch (that.kind()) {
+        case 16:  // stageList
+        case 17:  // stage
             value.copy<CNode>(that.value);
             break;
 
@@ -295,10 +291,6 @@ PipelineParserGen::stack_symbol_type& PipelineParserGen::stack_symbol_type::oper
             value.copy<long long>(that.value);
             break;
 
-        case 18:  // stage
-            value.copy<std::pair<KeyFieldname, CNode>>(that.value);
-            break;
-
         case 10:  // STRING
             value.copy<std::string>(that.value);
             break;
@@ -314,8 +306,9 @@ PipelineParserGen::stack_symbol_type& PipelineParserGen::stack_symbol_type::oper
 PipelineParserGen::stack_symbol_type& PipelineParserGen::stack_symbol_type::operator=(
     stack_symbol_type& that) {
     state = that.state;
-    switch (that.type_get()) {
-        case 17:  // stageList
+    switch (that.kind()) {
+        case 16:  // stageList
+        case 17:  // stage
             value.move<CNode>(that.value);
             break;
 
@@ -335,10 +328,6 @@ PipelineParserGen::stack_symbol_type& PipelineParserGen::stack_symbol_type::oper
             value.move<long long>(that.value);
             break;
 
-        case 18:  // stage
-            value.move<std::pair<KeyFieldname, CNode>>(that.value);
-            break;
-
         case 10:  // STRING
             value.move<std::string>(that.value);
             break;
@@ -365,18 +354,15 @@ template <typename Base>
 void PipelineParserGen::yy_print_(std::ostream& yyo, const basic_symbol<Base>& yysym) const {
     std::ostream& yyoutput = yyo;
     YYUSE(yyoutput);
-    symbol_number_type yytype = yysym.type_get();
-#if defined __GNUC__ && !defined __clang__ && !defined __ICC && \
-    __GNUC__ * 100 + __GNUC_MINOR__ <= 408
-    // Avoid a (spurious) G++ 4.8 warning about "array subscript is
-    // below array bounds".
     if (yysym.empty())
-        std::abort();
-#endif
-    yyo << (yytype < yyntokens_ ? "token" : "nterm") << ' ' << yytname_[yytype] << " ("
-        << yysym.location << ": ";
-    YYUSE(yytype);
-    yyo << ')';
+        yyo << "empty symbol";
+    else {
+        symbol_kind_type yykind = yysym.kind();
+        yyo << (yykind < YYNTOKENS ? "token" : "nterm") << ' ' << yysym.name() << " ("
+            << yysym.location << ": ";
+        YYUSE(yykind);
+        yyo << ')';
+    }
 }
 #endif
 
@@ -419,11 +405,11 @@ void PipelineParserGen::set_debug_level(debug_level_type l) {
 #endif  // YYDEBUG
 
 PipelineParserGen::state_type PipelineParserGen::yy_lr_goto_state_(state_type yystate, int yysym) {
-    int yyr = yypgoto_[yysym - yyntokens_] + yystate;
+    int yyr = yypgoto_[yysym - YYNTOKENS] + yystate;
     if (0 <= yyr && yyr <= yylast_ && yycheck_[yyr] == yystate)
         return yytable_[yyr];
     else
-        return yydefgoto_[yysym - yyntokens_];
+        return yydefgoto_[yysym - YYNTOKENS];
 }
 
 bool PipelineParserGen::yy_pact_value_is_default_(int yyvalue) {
@@ -475,6 +461,7 @@ int PipelineParserGen::parse() {
     `-----------------------------------------------*/
     yynewstate:
         YYCDEBUG << "Entering state " << int(yystack_[0].state) << '\n';
+        YY_STACK_PRINT();
 
         // Accept?
         if (yystack_[0].state == yyfinal_)
@@ -494,7 +481,7 @@ int PipelineParserGen::parse() {
 
         // Read a lookahead token.
         if (yyla.empty()) {
-            YYCDEBUG << "Reading a token: ";
+            YYCDEBUG << "Reading a token\n";
 #if YY_EXCEPTIONS
             try
 #endif  // YY_EXCEPTIONS
@@ -512,10 +499,19 @@ int PipelineParserGen::parse() {
         }
         YY_SYMBOL_PRINT("Next token is", yyla);
 
+        if (yyla.kind() == symbol_kind::S_YYerror) {
+            // The scanner already issued an error message, process directly
+            // to error recovery.  But do not keep the error token as
+            // lookahead, it is too special and may lead us to an endless
+            // loop in error recovery. */
+            yyla.kind_ = symbol_kind::S_YYUNDEF;
+            goto yyerrlab1;
+        }
+
         /* If the proper action on seeing token YYLA.TYPE is to reduce or
            to detect an error, take that action.  */
-        yyn += yyla.type_get();
-        if (yyn < 0 || yylast_ < yyn || yycheck_[yyn] != yyla.type_get()) {
+        yyn += yyla.kind();
+        if (yyn < 0 || yylast_ < yyn || yycheck_[yyn] != yyla.kind()) {
             goto yydefault;
         }
 
@@ -559,7 +555,8 @@ int PipelineParserGen::parse() {
                correct type. The default '$$ = $1' action is NOT applied
                when using variants.  */
             switch (yyr1_[yyn]) {
-                case 17:  // stageList
+                case 16:  // stageList
+                case 17:  // stage
                     yylhs.value.emplace<CNode>();
                     break;
 
@@ -579,10 +576,6 @@ int PipelineParserGen::parse() {
                     yylhs.value.emplace<long long>();
                     break;
 
-                case 18:  // stage
-                    yylhs.value.emplace<std::pair<KeyFieldname, CNode>>();
-                    break;
-
                 case 10:  // STRING
                     yylhs.value.emplace<std::string>();
                     break;
@@ -607,43 +600,42 @@ int PipelineParserGen::parse() {
             {
                 switch (yyn) {
                     case 2:
-#line 130 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 129 "pipeline_grammar.yy"
                     {
                         *cst = std::move(yystack_[1].value.as<CNode>());
                     }
-#line 678 "src/mongo/db/cst/pipeline_parser_gen.cpp"
+#line 673 "pipeline_parser_gen.cpp"
                     break;
 
                     case 3:
-#line 135 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 134 "pipeline_grammar.yy"
                     {
                     }
-#line 684 "src/mongo/db/cst/pipeline_parser_gen.cpp"
+#line 679 "pipeline_parser_gen.cpp"
                     break;
 
                     case 4:
-#line 136 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 135 "pipeline_grammar.yy"
                     {
                         yylhs.value.as<CNode>() = std::move(yystack_[0].value.as<CNode>());
                         auto& children =
-                            stdx::get<CNode::Children>(yylhs.value.as<CNode>().payload);
-                        children.emplace_back(
-                            std::move(yystack_[2].value.as<std::pair<KeyFieldname, CNode>>()));
+                            stdx::get<CNode::ArrayChildren>(yylhs.value.as<CNode>().payload);
+                        children.emplace_back(std::move(yystack_[2].value.as<CNode>()));
                     }
-#line 694 "src/mongo/db/cst/pipeline_parser_gen.cpp"
+#line 689 "pipeline_parser_gen.cpp"
                     break;
 
                     case 5:
-#line 144 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 143 "pipeline_grammar.yy"
                     {
-                        yylhs.value.as<std::pair<KeyFieldname, CNode>>() =
-                            std::pair{KeyFieldname::inhibitOptimization, CNode::noopLeaf()};
+                        yylhs.value.as<CNode>() = CNode{CNode::ObjectChildren{
+                            std::pair{KeyFieldname::inhibitOptimization, CNode::noopLeaf()}}};
                     }
-#line 702 "src/mongo/db/cst/pipeline_parser_gen.cpp"
+#line 697 "pipeline_parser_gen.cpp"
                     break;
 
 
-#line 706 "src/mongo/db/cst/pipeline_parser_gen.cpp"
+#line 701 "pipeline_parser_gen.cpp"
 
                     default:
                         break;
@@ -659,7 +651,6 @@ int PipelineParserGen::parse() {
             YY_SYMBOL_PRINT("-> $$ =", yylhs);
             yypop_(yylen);
             yylen = 0;
-            YY_STACK_PRINT();
 
             // Shift the result of the reduction.
             yypush_(YY_NULLPTR, YY_MOVE(yylhs));
@@ -674,7 +665,8 @@ int PipelineParserGen::parse() {
         // If not already recovering from an error, report this error.
         if (!yyerrstatus_) {
             ++yynerrs_;
-            error(yyla.location, yysyntax_error_(yystack_[0].state, yyla));
+            std::string msg = YY_("syntax error");
+            error(yyla.location, YY_MOVE(msg));
         }
 
 
@@ -684,7 +676,7 @@ int PipelineParserGen::parse() {
                error, discard it.  */
 
             // Return failure if at end of input.
-            if (yyla.type_get() == yyeof_)
+            if (yyla.kind() == symbol_kind::S_YYEOF)
                 YYABORT;
             else if (!yyla.empty()) {
                 yy_destroy_("Error: discarding", yyla);
@@ -709,6 +701,7 @@ int PipelineParserGen::parse() {
            this YYERROR.  */
         yypop_(yylen);
         yylen = 0;
+        YY_STACK_PRINT();
         goto yyerrlab1;
 
 
@@ -717,28 +710,29 @@ int PipelineParserGen::parse() {
     `-------------------------------------------------------------*/
     yyerrlab1:
         yyerrstatus_ = 3;  // Each real token shifted decrements this.
-        {
-            stack_symbol_type error_token;
-            for (;;) {
-                yyn = yypact_[+yystack_[0].state];
-                if (!yy_pact_value_is_default_(yyn)) {
-                    yyn += yy_error_token_;
-                    if (0 <= yyn && yyn <= yylast_ && yycheck_[yyn] == yy_error_token_) {
-                        yyn = yytable_[yyn];
-                        if (0 < yyn)
-                            break;
-                    }
+        // Pop stack until we find a state that shifts the error token.
+        for (;;) {
+            yyn = yypact_[+yystack_[0].state];
+            if (!yy_pact_value_is_default_(yyn)) {
+                yyn += symbol_kind::S_YYerror;
+                if (0 <= yyn && yyn <= yylast_ && yycheck_[yyn] == symbol_kind::S_YYerror) {
+                    yyn = yytable_[yyn];
+                    if (0 < yyn)
+                        break;
                 }
+            }
 
-                // Pop the current state because it cannot handle the error token.
-                if (yystack_.size() == 1)
-                    YYABORT;
+            // Pop the current state because it cannot handle the error token.
+            if (yystack_.size() == 1)
+                YYABORT;
 
-                yyerror_range[1].location = yystack_[0].location;
-                yy_destroy_("Error: popping", yystack_[0]);
-                yypop_();
-                YY_STACK_PRINT();
-            }
+            yyerror_range[1].location = yystack_[0].location;
+            yy_destroy_("Error: popping", yystack_[0]);
+            yypop_();
+            YY_STACK_PRINT();
+        }
+        {
+            stack_symbol_type error_token;
 
             yyerror_range[2].location = yyla.location;
             YYLLOC_DEFAULT(error_token.location, yyerror_range, 2);
@@ -776,6 +770,7 @@ int PipelineParserGen::parse() {
         /* Do not reclaim the symbols of the rule whose action triggered
            this YYABORT or YYACCEPT.  */
         yypop_(yylen);
+        YY_STACK_PRINT();
         while (1 < yystack_.size()) {
             yy_destroy_("Cleanup: popping", yystack_[0]);
             yypop_();
@@ -804,10 +799,11 @@ void PipelineParserGen::error(const syntax_error& yyexc) {
     error(yyexc.location, yyexc.what());
 }
 
-// Generate an error message.
-std::string PipelineParserGen::yysyntax_error_(state_type, const symbol_type&) const {
-    return YY_("syntax error");
+#if YYDEBUG || 0
+const char* PipelineParserGen::symbol_name(symbol_kind_type yysymbol) {
+    return yytname_[yysymbol];
 }
+#endif  // #if YYDEBUG || 0
 
 
 const signed char PipelineParserGen::yypact_ninf_ = -8;
@@ -818,27 +814,27 @@ const signed char PipelineParserGen::yypact_[] = {-7, -2, 2, -6, -4, -8, 3, 1, -
 
 const signed char PipelineParserGen::yydefact_[] = {0, 3, 0, 0, 0, 1, 0, 0, 2, 0, 3, 5, 4};
 
-const signed char PipelineParserGen::yypgoto_[] = {-8, -8, -3, -8};
+const signed char PipelineParserGen::yypgoto_[] = {-8, -3, -8, -8};
 
-const signed char PipelineParserGen::yydefgoto_[] = {-1, 2, 4, 7};
+const signed char PipelineParserGen::yydefgoto_[] = {-1, 4, 7, 2};
 
 const signed char PipelineParserGen::yytable_[] = {1, 3, 5, 6, 8, 10, 9, 12, 11};
 
 const signed char PipelineParserGen::yycheck_[] = {7, 3, 0, 9, 8, 4, 3, 10, 4};
 
-const signed char PipelineParserGen::yystos_[] = {0, 7, 16, 3, 17, 0, 9, 18, 8, 3, 4, 4, 17};
+const signed char PipelineParserGen::yystos_[] = {0, 7, 18, 3, 16, 0, 9, 17, 8, 3, 4, 4, 16};
 
-const signed char PipelineParserGen::yyr1_[] = {0, 15, 16, 17, 17, 18};
+const signed char PipelineParserGen::yyr1_[] = {0, 15, 18, 16, 16, 17};
 
 const signed char PipelineParserGen::yyr2_[] = {0, 2, 3, 0, 4, 3};
 
 
 #if YYDEBUG
 // YYTNAME[SYMBOL-NUM] -- String name of the symbol SYMBOL-NUM.
-// First, the terminals, then, starting at \a yyntokens_, nonterminals.
+// First, the terminals, then, starting at \a YYNTOKENS, nonterminals.
 const char* const PipelineParserGen::yytname_[] = {"\"EOF\"",
                                                    "error",
-                                                   "$undefined",
+                                                   "\"invalid token\"",
                                                    "START_OBJECT",
                                                    "END_OBJECT",
                                                    "START_ORDERED_OBJECT",
@@ -852,24 +848,24 @@ const char* const PipelineParserGen::yytname_[] = {"\"EOF\"",
                                                    "NUMBER_DOUBLE",
                                                    "BOOL",
                                                    "$accept",
-                                                   "pipeline",
                                                    "stageList",
                                                    "stage",
+                                                   "pipeline",
                                                    YY_NULLPTR};
+#endif
 
 
-const unsigned char PipelineParserGen::yyrline_[] = {0, 130, 130, 135, 136, 144};
+#if YYDEBUG
+const unsigned char PipelineParserGen::yyrline_[] = {0, 129, 129, 134, 135, 143};
 
-// Print the state stack on the debug stream.
-void PipelineParserGen::yystack_print_() {
+void PipelineParserGen::yy_stack_print_() const {
     *yycdebug_ << "Stack now";
     for (stack_type::const_iterator i = yystack_.begin(), i_end = yystack_.end(); i != i_end; ++i)
         *yycdebug_ << ' ' << int(i->state);
     *yycdebug_ << '\n';
 }
 
-// Report on the debug stream that the rule \a yyrule is going to be reduced.
-void PipelineParserGen::yy_reduce_print_(int yyrule) {
+void PipelineParserGen::yy_reduce_print_(int yyrule) const {
     int yylno = yyrline_[yyrule];
     int yynrhs = yyr2_[yyrule];
     // Print the symbols being reduced, and their result.
@@ -881,8 +877,8 @@ void PipelineParserGen::yy_reduce_print_(int yyrule) {
 #endif  // YYDEBUG
 
 
-#line 52 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 52 "pipeline_grammar.yy"
 }  // namespace mongo
-#line 1000 "src/mongo/db/cst/pipeline_parser_gen.cpp"
+#line 1003 "pipeline_parser_gen.cpp"
 
-#line 149 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 148 "pipeline_grammar.yy"
diff --git a/src/mongo/db/cst/pipeline_parser_gen.hpp b/src/mongo/db/cst/pipeline_parser_gen.hpp
index bc191a87041..c2f2e24a314 100644
--- a/src/mongo/db/cst/pipeline_parser_gen.hpp
+++ b/src/mongo/db/cst/pipeline_parser_gen.hpp
@@ -1,4 +1,4 @@
-// A Bison parser, made by GNU Bison 3.5.4.
+// A Bison parser, made by GNU Bison 3.6.3.
 
 // Skeleton interface for Bison LALR(1) parsers in C++
 
@@ -32,19 +32,20 @@
 
 
 /**
- ** \file src/mongo/db/cst/pipeline_parser_gen.hpp
+ ** \file pipeline_parser_gen.hpp
  ** Define the mongo::parser class.
  */
 
 // C++ LALR(1) parser skeleton written by Akim Demaille.
 
-// Undocumented macros, especially those whose name start with YY_,
-// are private implementation details.  Do not rely on them.
+// DO NOT RELY ON FEATURES THAT ARE NOT DOCUMENTED in the manual,
+// especially those whose name start with YY_ or yy_.  They are
+// private implementation details that can be changed or removed.
 
-#ifndef YY_YY_SRC_MONGO_DB_CST_PIPELINE_PARSER_GEN_HPP_INCLUDED
-#define YY_YY_SRC_MONGO_DB_CST_PIPELINE_PARSER_GEN_HPP_INCLUDED
+#ifndef YY_YY_PIPELINE_PARSER_GEN_HPP_INCLUDED
+#define YY_YY_PIPELINE_PARSER_GEN_HPP_INCLUDED
 // "%code requires" blocks.
-#line 60 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 60 "pipeline_grammar.yy"
 
 #include "mongo/db/cst/c_node.h"
 #include "mongo/db/cst/key_fieldname.h"
@@ -60,7 +61,7 @@ class BSONLexer;
 #pragma warning(disable : 4065)
 #endif
 
-#line 64 "src/mongo/db/cst/pipeline_parser_gen.hpp"
+#line 65 "pipeline_parser_gen.hpp"
 
 #include <cassert>
 #include <cstdlib>  // std::abort
@@ -189,9 +190,9 @@ class BSONLexer;
 #define YYDEBUG 0
 #endif
 
-#line 52 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 52 "pipeline_grammar.yy"
 namespace mongo {
-#line 199 "src/mongo/db/cst/pipeline_parser_gen.hpp"
+#line 200 "pipeline_parser_gen.hpp"
 
 
 /// A Bison parser.
@@ -218,6 +219,13 @@ public:
             new (yyas_<T>()) T(YY_MOVE(t));
         }
 
+#if 201103L <= YY_CPLUSPLUS
+        /// Non copyable.
+        semantic_type(const self_type&) = delete;
+        /// Non copyable.
+        self_type& operator=(const self_type&) = delete;
+#endif
+
         /// Destruction, allowed only if empty.
         ~semantic_type() YY_NOEXCEPT {
             YY_ASSERT(!yytypeid_);
@@ -336,9 +344,12 @@ public:
         }
 
     private:
-        /// Prohibit blind copies.
-        self_type& operator=(const self_type&);
+#if YY_CPLUSPLUS < 201103L
+        /// Non copyable.
         semantic_type(const self_type&);
+        /// Non copyable.
+        self_type& operator=(const self_type&);
+#endif
 
         /// Accessor to raw memory as \a T.
         template <typename T>
@@ -357,6 +368,7 @@ public:
         /// An auxiliary type to compute the largest semantic type.
         union union_type {
             // stageList
+            // stage
             char dummy1[sizeof(CNode)];
 
             // BOOL
@@ -371,11 +383,8 @@ public:
             // NUMBER_LONG
             char dummy5[sizeof(long long)];
 
-            // stage
-            char dummy6[sizeof(std::pair<KeyFieldname, CNode>)];
-
             // STRING
-            char dummy7[sizeof(std::string)];
+            char dummy6[sizeof(std::string)];
         };
 
         /// The size of the largest semantic type.
@@ -411,41 +420,73 @@ public:
         location_type location;
     };
 
-    /// Tokens.
+    /// Token kinds.
     struct token {
-        enum yytokentype {
-            END_OF_FILE = 0,
-            START_OBJECT = 3,
-            END_OBJECT = 4,
-            START_ORDERED_OBJECT = 5,
-            END_ORDERED_OBJECT = 6,
-            START_ARRAY = 7,
-            END_ARRAY = 8,
-            STAGE_INHIBIT_OPTIMIZATION = 9,
-            STRING = 10,
-            NUMBER_INT = 11,
-            NUMBER_LONG = 12,
-            NUMBER_DOUBLE = 13,
-            BOOL = 14
+        enum token_kind_type {
+            YYEMPTY = -2,
+            END_OF_FILE = 0,                 // "EOF"
+            YYerror = 1,                     // error
+            YYUNDEF = 2,                     // "invalid token"
+            START_OBJECT = 3,                // START_OBJECT
+            END_OBJECT = 4,                  // END_OBJECT
+            START_ORDERED_OBJECT = 5,        // START_ORDERED_OBJECT
+            END_ORDERED_OBJECT = 6,          // END_ORDERED_OBJECT
+            START_ARRAY = 7,                 // START_ARRAY
+            END_ARRAY = 8,                   // END_ARRAY
+            STAGE_INHIBIT_OPTIMIZATION = 9,  // STAGE_INHIBIT_OPTIMIZATION
+            STRING = 10,                     // STRING
+            NUMBER_INT = 11,                 // NUMBER_INT
+            NUMBER_LONG = 12,                // NUMBER_LONG
+            NUMBER_DOUBLE = 13,              // NUMBER_DOUBLE
+            BOOL = 14                        // BOOL
         };
+        /// Backward compatibility alias (Bison 3.6).
+        typedef token_kind_type yytokentype;
     };
 
-    /// (External) token type, as returned by yylex.
-    typedef token::yytokentype token_type;
-
-    /// Symbol type: an internal symbol number.
-    typedef int symbol_number_type;
+    /// Token kind, as returned by yylex.
+    typedef token::yytokentype token_kind_type;
+
+    /// Backward compatibility alias (Bison 3.6).
+    typedef token_kind_type token_type;
+
+    /// Symbol kinds.
+    struct symbol_kind {
+        enum symbol_kind_type {
+            YYNTOKENS = 15,  ///< Number of tokens.
+            S_YYEMPTY = -2,
+            S_YYEOF = 0,                       // "EOF"
+            S_YYerror = 1,                     // error
+            S_YYUNDEF = 2,                     // "invalid token"
+            S_START_OBJECT = 3,                // START_OBJECT
+            S_END_OBJECT = 4,                  // END_OBJECT
+            S_START_ORDERED_OBJECT = 5,        // START_ORDERED_OBJECT
+            S_END_ORDERED_OBJECT = 6,          // END_ORDERED_OBJECT
+            S_START_ARRAY = 7,                 // START_ARRAY
+            S_END_ARRAY = 8,                   // END_ARRAY
+            S_STAGE_INHIBIT_OPTIMIZATION = 9,  // STAGE_INHIBIT_OPTIMIZATION
+            S_STRING = 10,                     // STRING
+            S_NUMBER_INT = 11,                 // NUMBER_INT
+            S_NUMBER_LONG = 12,                // NUMBER_LONG
+            S_NUMBER_DOUBLE = 13,              // NUMBER_DOUBLE
+            S_BOOL = 14,                       // BOOL
+            S_YYACCEPT = 15,                   // $accept
+            S_stageList = 16,                  // stageList
+            S_stage = 17,                      // stage
+            S_pipeline = 18                    // pipeline
+        };
+    };
 
-    /// The symbol type number to denote an empty symbol.
-    enum { empty_symbol = -2 };
+    /// (Internal) symbol kind.
+    typedef symbol_kind::symbol_kind_type symbol_kind_type;
 
-    /// Internal symbol number for tokens (subsumed by symbol_number_type).
-    typedef signed char token_number_type;
+    /// The number of tokens.
+    static const symbol_kind_type YYNTOKENS = symbol_kind::YYNTOKENS;
 
     /// A complete symbol.
     ///
-    /// Expects its Base type to provide access to the symbol type
-    /// via type_get ().
+    /// Expects its Base type to provide access to the symbol kind
+    /// via kind ().
     ///
     /// Provide access to semantic value and location.
     template <typename Base>
@@ -458,7 +499,38 @@ public:
 
 #if 201103L <= YY_CPLUSPLUS
         /// Move constructor.
-        basic_symbol(basic_symbol&& that);
+        basic_symbol(basic_symbol&& that)
+            : Base(std::move(that)), value(), location(std::move(that.location)) {
+            switch (this->kind()) {
+                case 16:  // stageList
+                case 17:  // stage
+                    value.move<CNode>(std::move(that.value));
+                    break;
+
+                case 14:  // BOOL
+                    value.move<bool>(std::move(that.value));
+                    break;
+
+                case 13:  // NUMBER_DOUBLE
+                    value.move<double>(std::move(that.value));
+                    break;
+
+                case 11:  // NUMBER_INT
+                    value.move<int>(std::move(that.value));
+                    break;
+
+                case 12:  // NUMBER_LONG
+                    value.move<long long>(std::move(that.value));
+                    break;
+
+                case 10:  // STRING
+                    value.move<std::string>(std::move(that.value));
+                    break;
+
+                default:
+                    break;
+            }
+        }
 #endif
 
         /// Copy constructor.
@@ -507,17 +579,6 @@ public:
             : Base(t), value(v), location(l) {}
 #endif
 #if 201103L <= YY_CPLUSPLUS
-        basic_symbol(typename Base::kind_type t,
-                     std::pair<KeyFieldname, CNode>&& v,
-                     location_type&& l)
-            : Base(t), value(std::move(v)), location(std::move(l)) {}
-#else
-        basic_symbol(typename Base::kind_type t,
-                     const std::pair<KeyFieldname, CNode>& v,
-                     const location_type& l)
-            : Base(t), value(v), location(l) {}
-#endif
-#if 201103L <= YY_CPLUSPLUS
         basic_symbol(typename Base::kind_type t, std::string&& v, location_type&& l)
             : Base(t), value(std::move(v)), location(std::move(l)) {}
 #else
@@ -533,17 +594,18 @@ public:
         /// Destroy contents, and record that is empty.
         void clear() {
             // User destructor.
-            symbol_number_type yytype = this->type_get();
+            symbol_kind_type yykind = this->kind();
             basic_symbol<Base>& yysym = *this;
             (void)yysym;
-            switch (yytype) {
+            switch (yykind) {
                 default:
                     break;
             }
 
-            // Type destructor.
-            switch (yytype) {
-                case 17:  // stageList
+            // Value type destructor.
+            switch (yykind) {
+                case 16:  // stageList
+                case 17:  // stage
                     value.template destroy<CNode>();
                     break;
 
@@ -563,10 +625,6 @@ public:
                     value.template destroy<long long>();
                     break;
 
-                case 18:  // stage
-                    value.template destroy<std::pair<KeyFieldname, CNode>>();
-                    break;
-
                 case 10:  // STRING
                     value.template destroy<std::string>();
                     break;
@@ -578,6 +636,17 @@ public:
             Base::clear();
         }
 
+#if YYDEBUG || 0
+        /// The user-facing name of this symbol.
+        const char* name() const YY_NOEXCEPT {
+            return PipelineParserGen::symbol_name(this->kind());
+        }
+#endif  // #if YYDEBUG || 0
+
+
+        /// Backward compatibility (Bison 3.6).
+        symbol_kind_type type_get() const YY_NOEXCEPT;
+
         /// Whether empty.
         bool empty() const YY_NOEXCEPT;
 
@@ -598,44 +667,49 @@ public:
     };
 
     /// Type access provider for token (enum) based symbols.
-    struct by_type {
+    struct by_kind {
         /// Default constructor.
-        by_type();
+        by_kind();
 
 #if 201103L <= YY_CPLUSPLUS
         /// Move constructor.
-        by_type(by_type&& that);
+        by_kind(by_kind&& that);
 #endif
 
         /// Copy constructor.
-        by_type(const by_type& that);
+        by_kind(const by_kind& that);
 
-        /// The symbol type as needed by the constructor.
-        typedef token_type kind_type;
+        /// The symbol kind as needed by the constructor.
+        typedef token_kind_type kind_type;
 
         /// Constructor from (external) token numbers.
-        by_type(kind_type t);
+        by_kind(kind_type t);
 
         /// Record that this symbol is empty.
         void clear();
 
-        /// Steal the symbol type from \a that.
-        void move(by_type& that);
+        /// Steal the symbol kind from \a that.
+        void move(by_kind& that);
 
         /// The (internal) type number (corresponding to \a type).
         /// \a empty when empty.
-        symbol_number_type type_get() const YY_NOEXCEPT;
+        symbol_kind_type kind() const YY_NOEXCEPT;
 
-        /// The symbol type.
-        /// \a empty_symbol when empty.
-        /// An int, not token_number_type, to be able to store empty_symbol.
-        int type;
+        /// Backward compatibility (Bison 3.6).
+        symbol_kind_type type_get() const YY_NOEXCEPT;
+
+        /// The symbol kind.
+        /// \a S_YYEMPTY when empty.
+        symbol_kind_type kind_;
     };
 
+    /// Backward compatibility for a private implementation detail (Bison 3.6).
+    typedef by_kind by_type;
+
     /// "External" symbols: returned by the scanner.
-    struct symbol_type : basic_symbol<by_type> {
+    struct symbol_type : basic_symbol<by_kind> {
         /// Superclass.
-        typedef basic_symbol<by_type> super_type;
+        typedef basic_symbol<by_kind> super_type;
 
         /// Empty symbol.
         symbol_type() {}
@@ -643,17 +717,19 @@ public:
         /// Constructor for valueless symbols, and symbols from each type.
 #if 201103L <= YY_CPLUSPLUS
         symbol_type(int tok, location_type l) : super_type(token_type(tok), std::move(l)) {
-            YY_ASSERT(tok == token::END_OF_FILE || tok == token::START_OBJECT ||
-                      tok == token::END_OBJECT || tok == token::START_ORDERED_OBJECT ||
-                      tok == token::END_ORDERED_OBJECT || tok == token::START_ARRAY ||
-                      tok == token::END_ARRAY || tok == token::STAGE_INHIBIT_OPTIMIZATION);
+            YY_ASSERT(tok == token::END_OF_FILE || tok == token::YYerror || tok == token::YYUNDEF ||
+                      tok == token::START_OBJECT || tok == token::END_OBJECT ||
+                      tok == token::START_ORDERED_OBJECT || tok == token::END_ORDERED_OBJECT ||
+                      tok == token::START_ARRAY || tok == token::END_ARRAY ||
+                      tok == token::STAGE_INHIBIT_OPTIMIZATION);
         }
 #else
         symbol_type(int tok, const location_type& l) : super_type(token_type(tok), l) {
-            YY_ASSERT(tok == token::END_OF_FILE || tok == token::START_OBJECT ||
-                      tok == token::END_OBJECT || tok == token::START_ORDERED_OBJECT ||
-                      tok == token::END_ORDERED_OBJECT || tok == token::START_ARRAY ||
-                      tok == token::END_ARRAY || tok == token::STAGE_INHIBIT_OPTIMIZATION);
+            YY_ASSERT(tok == token::END_OF_FILE || tok == token::YYerror || tok == token::YYUNDEF ||
+                      tok == token::START_OBJECT || tok == token::END_OBJECT ||
+                      tok == token::START_ORDERED_OBJECT || tok == token::END_ORDERED_OBJECT ||
+                      tok == token::START_ARRAY || tok == token::END_ARRAY ||
+                      tok == token::STAGE_INHIBIT_OPTIMIZATION);
         }
 #endif
 #if 201103L <= YY_CPLUSPLUS
@@ -717,6 +793,13 @@ public:
     PipelineParserGen(BSONLexer& driver_yyarg, CNode* cst_yyarg);
     virtual ~PipelineParserGen();
 
+#if 201103L <= YY_CPLUSPLUS
+    /// Non copyable.
+    PipelineParserGen(const PipelineParserGen&) = delete;
+    /// Non copyable.
+    PipelineParserGen& operator=(const PipelineParserGen&) = delete;
+#endif
+
     /// Parse.  An alias for parse ().
     /// \returns  0 iff parsing succeeded.
     int operator()();
@@ -747,6 +830,13 @@ public:
     /// Report a syntax error.
     void error(const syntax_error& err);
 
+#if YYDEBUG || 0
+    /// The user-facing name of the symbol whose (internal) number is
+    /// YYSYMBOL.  No bounds checking.
+    static const char* symbol_name(symbol_kind_type yysymbol);
+#endif  // #if YYDEBUG || 0
+
+
     // Implementation of make_symbol for each symbol type.
 #if 201103L <= YY_CPLUSPLUS
     static symbol_type make_END_OF_FILE(location_type l) {
@@ -758,6 +848,24 @@ public:
     }
 #endif
 #if 201103L <= YY_CPLUSPLUS
+    static symbol_type make_YYerror(location_type l) {
+        return symbol_type(token::YYerror, std::move(l));
+    }
+#else
+    static symbol_type make_YYerror(const location_type& l) {
+        return symbol_type(token::YYerror, l);
+    }
+#endif
+#if 201103L <= YY_CPLUSPLUS
+    static symbol_type make_YYUNDEF(location_type l) {
+        return symbol_type(token::YYUNDEF, std::move(l));
+    }
+#else
+    static symbol_type make_YYUNDEF(const location_type& l) {
+        return symbol_type(token::YYUNDEF, l);
+    }
+#endif
+#if 201103L <= YY_CPLUSPLUS
     static symbol_type make_START_OBJECT(location_type l) {
         return symbol_type(token::START_OBJECT, std::move(l));
     }
@@ -868,18 +976,17 @@ public:
 
 
 private:
-    /// This class is not copyable.
+#if YY_CPLUSPLUS < 201103L
+    /// Non copyable.
     PipelineParserGen(const PipelineParserGen&);
+    /// Non copyable.
     PipelineParserGen& operator=(const PipelineParserGen&);
+#endif
+
 
     /// Stored state numbers (used for stacks).
     typedef signed char state_type;
 
-    /// Generate an error message.
-    /// \param yystate   the state where the error occurred.
-    /// \param yyla      the lookahead token.
-    virtual std::string yysyntax_error_(state_type yystate, const symbol_type& yyla) const;
-
     /// Compute post-reduction state.
     /// \param yystate   the current state
     /// \param yysym     the nonterminal to push on the stack
@@ -896,10 +1003,16 @@ private:
     static const signed char yypact_ninf_;
     static const signed char yytable_ninf_;
 
-    /// Convert a scanner token number \a t to a symbol number.
-    /// In theory \a t should be a token_type, but character literals
+    /// Convert a scanner token kind \a t to a symbol kind.
+    /// In theory \a t should be a token_kind_type, but character literals
     /// are valid, yet not members of the token_type enum.
-    static token_number_type yytranslate_(int t);
+    static symbol_kind_type yytranslate_(int t);
+
+#if YYDEBUG || 0
+    /// For a symbol, its name in clear.
+    static const char* const yytname_[];
+#endif  // #if YYDEBUG || 0
+
 
     // Tables.
     // YYPACT[STATE-NUM] -- Index in YYTABLE of the portion describing
@@ -936,22 +1049,19 @@ private:
 
 
 #if YYDEBUG
-    /// For a symbol, its name in clear.
-    static const char* const yytname_[];
-
     // YYRLINE[YYN] -- Source line where rule number YYN was defined.
     static const unsigned char yyrline_[];
     /// Report on the debug stream that the rule \a r is going to be reduced.
-    virtual void yy_reduce_print_(int r);
+    virtual void yy_reduce_print_(int r) const;
     /// Print the state stack on the debug stream.
-    virtual void yystack_print_();
+    virtual void yy_stack_print_() const;
 
     /// Debugging level.
     int yydebug_;
     /// Debug stream.
     std::ostream* yycdebug_;
 
-    /// \brief Display a symbol type, value and location.
+    /// \brief Display a symbol kind, value and location.
     /// \param yyo    The output stream.
     /// \param yysym  The symbol.
     template <typename Base>
@@ -971,7 +1081,7 @@ private:
         /// Default constructor.
         by_state() YY_NOEXCEPT;
 
-        /// The symbol type as needed by the constructor.
+        /// The symbol kind as needed by the constructor.
         typedef state_type kind_type;
 
         /// Constructor.
@@ -983,12 +1093,12 @@ private:
         /// Record that this symbol is empty.
         void clear() YY_NOEXCEPT;
 
-        /// Steal the symbol type from \a that.
+        /// Steal the symbol kind from \a that.
         void move(by_state& that);
 
-        /// The (internal) type number (corresponding to \a state).
-        /// \a empty_symbol when empty.
-        symbol_number_type type_get() const YY_NOEXCEPT;
+        /// The symbol kind (corresponding to \a state).
+        /// \a S_YYEMPTY when empty.
+        symbol_kind_type kind() const YY_NOEXCEPT;
 
         /// The state number used to denote an empty symbol.
         /// We use the initial state, as it does not have a value.
@@ -1025,13 +1135,20 @@ private:
     class stack {
     public:
         // Hide our reversed order.
-        typedef typename S::reverse_iterator iterator;
-        typedef typename S::const_reverse_iterator const_iterator;
+        typedef typename S::iterator iterator;
+        typedef typename S::const_iterator const_iterator;
         typedef typename S::size_type size_type;
         typedef typename std::ptrdiff_t index_type;
 
         stack(size_type n = 200) : seq_(n) {}
 
+#if 201103L <= YY_CPLUSPLUS
+        /// Non copyable.
+        stack(const stack&) = delete;
+        /// Non copyable.
+        stack& operator=(const stack&) = delete;
+#endif
+
         /// Random access.
         ///
         /// Index 0 returns the topmost element.
@@ -1070,18 +1187,14 @@ private:
             return index_type(seq_.size());
         }
 
-        std::ptrdiff_t ssize() const YY_NOEXCEPT {
-            return std::ptrdiff_t(size());
-        }
-
         /// Iterator on top of the stack (going downwards).
         const_iterator begin() const YY_NOEXCEPT {
-            return seq_.rbegin();
+            return seq_.begin();
         }
 
         /// Bottom of the stack.
         const_iterator end() const YY_NOEXCEPT {
-            return seq_.rend();
+            return seq_.end();
         }
 
         /// Present a slice of the top of a stack.
@@ -1099,8 +1212,12 @@ private:
         };
 
     private:
+#if YY_CPLUSPLUS < 201103L
+        /// Non copyable.
         stack(const stack&);
+        /// Non copyable.
         stack& operator=(const stack&);
+#endif
         /// The wrapped container.
         S seq_;
     };
@@ -1130,17 +1247,11 @@ private:
     /// Pop \a n symbols from the stack.
     void yypop_(int n = 1);
 
-    /// Some specific tokens.
-    static const token_number_type yy_error_token_ = 1;
-    static const token_number_type yy_undef_token_ = 2;
-
     /// Constants.
     enum {
-        yyeof_ = 0,
-        yylast_ = 8,     ///< Last index in yytable_.
-        yynnts_ = 4,     ///< Number of nonterminal symbols.
-        yyfinal_ = 5,    ///< Termination state number.
-        yyntokens_ = 15  ///< Number of tokens.
+        yylast_ = 8,  ///< Last index in yytable_.
+        yynnts_ = 4,  ///< Number of nonterminal symbols.
+        yyfinal_ = 5  ///< Termination state number.
     };
 
 
@@ -1149,55 +1260,17 @@ private:
     CNode* cst;
 };
 
-inline PipelineParserGen::token_number_type PipelineParserGen::yytranslate_(int t) {
-    return static_cast<token_number_type>(t);
+inline PipelineParserGen::symbol_kind_type PipelineParserGen::yytranslate_(int t) {
+    return static_cast<symbol_kind_type>(t);
 }
 
 // basic_symbol.
-#if 201103L <= YY_CPLUSPLUS
-template <typename Base>
-PipelineParserGen::basic_symbol<Base>::basic_symbol(basic_symbol&& that)
-    : Base(std::move(that)), value(), location(std::move(that.location)) {
-    switch (this->type_get()) {
-        case 17:  // stageList
-            value.move<CNode>(std::move(that.value));
-            break;
-
-        case 14:  // BOOL
-            value.move<bool>(std::move(that.value));
-            break;
-
-        case 13:  // NUMBER_DOUBLE
-            value.move<double>(std::move(that.value));
-            break;
-
-        case 11:  // NUMBER_INT
-            value.move<int>(std::move(that.value));
-            break;
-
-        case 12:  // NUMBER_LONG
-            value.move<long long>(std::move(that.value));
-            break;
-
-        case 18:  // stage
-            value.move<std::pair<KeyFieldname, CNode>>(std::move(that.value));
-            break;
-
-        case 10:  // STRING
-            value.move<std::string>(std::move(that.value));
-            break;
-
-        default:
-            break;
-    }
-}
-#endif
-
 template <typename Base>
 PipelineParserGen::basic_symbol<Base>::basic_symbol(const basic_symbol& that)
     : Base(that), value(), location(that.location) {
-    switch (this->type_get()) {
-        case 17:  // stageList
+    switch (this->kind()) {
+        case 16:  // stageList
+        case 17:  // stage
             value.copy<CNode>(YY_MOVE(that.value));
             break;
 
@@ -1217,10 +1290,6 @@ PipelineParserGen::basic_symbol<Base>::basic_symbol(const basic_symbol& that)
             value.copy<long long>(YY_MOVE(that.value));
             break;
 
-        case 18:  // stage
-            value.copy<std::pair<KeyFieldname, CNode>>(YY_MOVE(that.value));
-            break;
-
         case 10:  // STRING
             value.copy<std::string>(YY_MOVE(that.value));
             break;
@@ -1232,15 +1301,22 @@ PipelineParserGen::basic_symbol<Base>::basic_symbol(const basic_symbol& that)
 
 
 template <typename Base>
+PipelineParserGen::symbol_kind_type PipelineParserGen::basic_symbol<Base>::type_get() const
+    YY_NOEXCEPT {
+    return this->kind();
+}
+
+template <typename Base>
 bool PipelineParserGen::basic_symbol<Base>::empty() const YY_NOEXCEPT {
-    return Base::type_get() == empty_symbol;
+    return this->kind() == symbol_kind::S_YYEMPTY;
 }
 
 template <typename Base>
 void PipelineParserGen::basic_symbol<Base>::move(basic_symbol& s) {
     super_type::move(s);
-    switch (this->type_get()) {
-        case 17:  // stageList
+    switch (this->kind()) {
+        case 16:  // stageList
+        case 17:  // stage
             value.move<CNode>(YY_MOVE(s.value));
             break;
 
@@ -1260,10 +1336,6 @@ void PipelineParserGen::basic_symbol<Base>::move(basic_symbol& s) {
             value.move<long long>(YY_MOVE(s.value));
             break;
 
-        case 18:  // stage
-            value.move<std::pair<KeyFieldname, CNode>>(YY_MOVE(s.value));
-            break;
-
         case 10:  // STRING
             value.move<std::string>(YY_MOVE(s.value));
             break;
@@ -1275,35 +1347,40 @@ void PipelineParserGen::basic_symbol<Base>::move(basic_symbol& s) {
     location = YY_MOVE(s.location);
 }
 
-// by_type.
-inline PipelineParserGen::by_type::by_type() : type(empty_symbol) {}
+// by_kind.
+inline PipelineParserGen::by_kind::by_kind() : kind_(symbol_kind::S_YYEMPTY) {}
 
 #if 201103L <= YY_CPLUSPLUS
-inline PipelineParserGen::by_type::by_type(by_type&& that) : type(that.type) {
+inline PipelineParserGen::by_kind::by_kind(by_kind&& that) : kind_(that.kind_) {
     that.clear();
 }
 #endif
 
-inline PipelineParserGen::by_type::by_type(const by_type& that) : type(that.type) {}
+inline PipelineParserGen::by_kind::by_kind(const by_kind& that) : kind_(that.kind_) {}
 
-inline PipelineParserGen::by_type::by_type(token_type t) : type(yytranslate_(t)) {}
+inline PipelineParserGen::by_kind::by_kind(token_kind_type t) : kind_(yytranslate_(t)) {}
 
-inline void PipelineParserGen::by_type::clear() {
-    type = empty_symbol;
+inline void PipelineParserGen::by_kind::clear() {
+    kind_ = symbol_kind::S_YYEMPTY;
 }
 
-inline void PipelineParserGen::by_type::move(by_type& that) {
-    type = that.type;
+inline void PipelineParserGen::by_kind::move(by_kind& that) {
+    kind_ = that.kind_;
     that.clear();
 }
 
-inline int PipelineParserGen::by_type::type_get() const YY_NOEXCEPT {
-    return type;
+inline PipelineParserGen::symbol_kind_type PipelineParserGen::by_kind::kind() const YY_NOEXCEPT {
+    return kind_;
+}
+
+inline PipelineParserGen::symbol_kind_type PipelineParserGen::by_kind::type_get() const
+    YY_NOEXCEPT {
+    return this->kind();
 }
 
-#line 52 "src/mongo/db/cst/pipeline_grammar.yy"
+#line 52 "pipeline_grammar.yy"
 }  // namespace mongo
-#line 1569 "src/mongo/db/cst/pipeline_parser_gen.hpp"
+#line 1658 "pipeline_parser_gen.hpp"
 
 
-#endif  // !YY_YY_SRC_MONGO_DB_CST_PIPELINE_PARSER_GEN_HPP_INCLUDED
+#endif  // !YY_YY_PIPELINE_PARSER_GEN_HPP_INCLUDED
diff --git a/src/mongo/db/pipeline/document_source_project.cpp b/src/mongo/db/pipeline/document_source_project.cpp
index 657350dc656..e5f048a8e3d 100644
--- a/src/mongo/db/pipeline/document_source_project.cpp
+++ b/src/mongo/db/pipeline/document_source_project.cpp
@@ -37,7 +37,6 @@
 #include "mongo/db/exec/projection_executor.h"
 #include "mongo/db/exec/projection_executor_builder.h"
 #include "mongo/db/pipeline/lite_parsed_document_source.h"
-#include "mongo/db/query/projection_parser.h"
 
 namespace mongo {
 
@@ -62,7 +61,9 @@ BSONObj buildExclusionProjectionSpecification(const std::vector<BSONElement>& un
 }  // namespace
 
 intrusive_ptr<DocumentSource> DocumentSourceProject::create(
-    BSONObj projectSpec, const intrusive_ptr<ExpressionContext>& expCtx, StringData specifiedName) {
+    projection_ast::Projection projection,
+    const intrusive_ptr<ExpressionContext>& expCtx,
+    StringData specifiedName) {
     const bool isIndependentOfAnyCollection = false;
     intrusive_ptr<DocumentSource> project(new DocumentSourceSingleDocumentTransformation(
         expCtx,
@@ -72,8 +73,6 @@ intrusive_ptr<DocumentSource> DocumentSourceProject::create(
             // here so we can add the name that was actually specified by the user, be it $project
             // or an alias.
             try {
-                auto policies = ProjectionPolicies::aggregateProjectionPolicies();
-                auto projection = projection_ast::parse(expCtx, projectSpec, policies);
                 // We won't optimize the executor on creation, and will do it as part of the
                 // pipeline optimization process when requested via the 'optimize()' method on
                 // 'DocumentSourceSingleDocumentTransformation', so we won't pass the
@@ -87,7 +86,10 @@ intrusive_ptr<DocumentSource> DocumentSourceProject::create(
                     projection_executor::kDefaultBuilderParams};
                 builderParams.reset(projection_executor::kOptimizeExecutor);
                 return projection_executor::buildProjectionExecutor(
-                    expCtx, &projection, policies, builderParams);
+                    expCtx,
+                    &projection,
+                    ProjectionPolicies::aggregateProjectionPolicies(),
+                    builderParams);
             } catch (DBException& ex) {
                 ex.addContext("Invalid " + specifiedName.toString());
                 throw;
diff --git a/src/mongo/db/pipeline/document_source_project.h b/src/mongo/db/pipeline/document_source_project.h
index 43ee90fce0f..13d45282a75 100644
--- a/src/mongo/db/pipeline/document_source_project.h
+++ b/src/mongo/db/pipeline/document_source_project.h
@@ -30,6 +30,7 @@
 #pragma once
 
 #include "mongo/db/pipeline/document_source_single_document_transformation.h"
+#include "mongo/db/query/projection_parser.h"
 
 namespace mongo {
 
@@ -45,12 +46,28 @@ public:
     static constexpr StringData kAliasNameUnset = "$unset"_sd;
 
     /**
+     * Method to create a $project stage from a Projection AST.
+     */
+    static boost::intrusive_ptr<DocumentSource> create(
+        projection_ast::Projection projection,
+        const boost::intrusive_ptr<ExpressionContext>& expCtx,
+        StringData specifiedName);
+
+    /**
      * Convenience method to create a $project stage from 'projectSpec'.
      */
     static boost::intrusive_ptr<DocumentSource> create(
         BSONObj projectSpec,
         const boost::intrusive_ptr<ExpressionContext>& expCtx,
-        StringData specifiedName);
+        StringData specifiedName) try {
+        return create(projection_ast::parse(
+                          expCtx, projectSpec, ProjectionPolicies::aggregateProjectionPolicies()),
+                      expCtx,
+                      specifiedName);
+    } catch (DBException& ex) {
+        ex.addContext("Invalid " + specifiedName.toString());
+        throw;
+    }
 
     /**
      * Parses a $project stage from the user-supplied BSON.
diff --git a/src/mongo/db/query/projection_parser.cpp b/src/mongo/db/query/projection_parser.cpp
index 54f050b7353..bc0f6944dd4 100644
--- a/src/mongo/db/query/projection_parser.cpp
+++ b/src/mongo/db/query/projection_parser.cpp
@@ -186,12 +186,6 @@ void addNodeAtPathHelper(ProjectionPathASTNode* root,
     addNodeAtPathHelper(childPathNode, path, componentIndex + 1, std::move(newChild));
 }
 
-void addNodeAtPath(ProjectionPathASTNode* root,
-                   const FieldPath& path,
-                   std::unique_ptr<ASTNode> newChild) {
-    addNodeAtPathHelper(root, path, 0, std::move(newChild));
-}
-
 bool hasPositionalOperator(StringData path) {
     return path.endsWith(".$");
 }
@@ -641,5 +635,12 @@ Projection parse(boost::intrusive_ptr<ExpressionContext> expCtx,
                  ProjectionPolicies policies) {
     return parse(std::move(expCtx), obj, nullptr, BSONObj(), std::move(policies));
 }
+
+void addNodeAtPath(ProjectionPathASTNode* root,
+                   const FieldPath& path,
+                   std::unique_ptr<ASTNode> newChild) {
+    addNodeAtPathHelper(root, path, 0, std::move(newChild));
+}
+
 }  // namespace projection_ast
 }  // namespace mongo
diff --git a/src/mongo/db/query/projection_parser.h b/src/mongo/db/query/projection_parser.h
index bee70317f9b..11d36846e72 100644
--- a/src/mongo/db/query/projection_parser.h
+++ b/src/mongo/db/query/projection_parser.h
@@ -53,5 +53,13 @@ Projection parse(boost::intrusive_ptr<ExpressionContext> expCtx,
 Projection parse(boost::intrusive_ptr<ExpressionContext> expCtx,
                  const BSONObj& obj,
                  ProjectionPolicies policies);
+
+/**
+ * Adds a node to the projection AST rooted at 'root' to the path specified by 'path'.
+ */
+void addNodeAtPath(ProjectionPathASTNode* root,
+                   const FieldPath& path,
+                   std::unique_ptr<ASTNode> newChild);
+
 }  // namespace projection_ast
 }  // namespace mongo