nlohmann::json instance and JSON Schema for MemorySourceAccessor

Also do a better JSON and testing for deep and shallow NAR listings.

As documented, this for file system objects themselves, since
`MemorySourceAccessor` is an implementation detail.

doc/manual/package.nix

@@ -37,6 +37,7 @@ mkMesonDerivation (finalAttrs: {
       (fileset.unions [
         ../../.version
         # For example JSON
+        ../../src/libutil-tests/data/memory-source-accessor
         ../../src/libutil-tests/data/hash
         ../../src/libstore-tests/data/content-address
         ../../src/libstore-tests/data/store-path

doc/manual/source/SUMMARY.md.in

@@ -121,6 +121,7 @@
 - [Architecture and Design](architecture/architecture.md)
 - [Formats and Protocols](protocols/index.md)
   - [JSON Formats](protocols/json/index.md)
+    - [File System Object](protocols/json/file-system-object.md)
     - [Hash](protocols/json/hash.md)
     - [Content Address](protocols/json/content-address.md)
     - [Store Path](protocols/json/store-path.md)

doc/manual/source/protocols/json/file-system-object.md

@@ -0,0 +1,21 @@
+{{#include file-system-object-v1-fixed.md}}
+
+## Examples
+
+### Simple
+
+```json
+{{#include schema/file-system-object-v1/simple.json}}
+```
+
+### Complex
+
+```json
+{{#include schema/file-system-object-v1/complex.json}}
+```
+
+<!-- need to convert YAML to JSON first
+## Raw Schema
+
+[JSON Schema for File System Object v1](schema/file-system-object-v1.json)
+-->

doc/manual/source/protocols/json/fixup-json-schema-generated-doc.sed

@@ -11,6 +11,7 @@ s/\\`/`/g
 #
 # As we have more such relative links, more replacements of this nature
 # should appear below.
+s^#/\$defs/\(regular\|symlink\|directory\)^In this schema^g
 s^\(./hash-v1.yaml\)\?#/$defs/algorithm^[JSON format for `Hash`](./hash.html#algorithm)^g
 s^\(./hash-v1.yaml\)^[JSON format for `Hash`](./hash.html)^g
 s^\(./content-address-v1.yaml\)\?#/$defs/method^[JSON format for `ContentAddress`](./content-address.html#method)^g

doc/manual/source/protocols/json/meson.build

@@ -9,6 +9,7 @@ json_schema_for_humans = find_program('generate-schema-doc', required : false)
 json_schema_config = files('json-schema-for-humans-config.yaml')
 
 schemas = [
+  'file-system-object-v1',
   'hash-v1',
   'content-address-v1',
   'store-path-v1',

doc/manual/source/protocols/json/schema/file-system-object-v1

@@ -0,0 +1 @@
+../../../../../../src/libutil-tests/data/memory-source-accessor

doc/manual/source/protocols/json/schema/file-system-object-v1.yaml

@@ -0,0 +1,71 @@
+"$schema": http://json-schema.org/draft-04/schema#
+"$id": https://nix.dev/manual/nix/latest/protocols/json/schema/file-system-object-v1.json
+title: File System Object
+description: |
+  This schema describes the JSON representation of Nix's [File System Object](@docroot@/store/file-system-object.md).
+
+  The schema is recursive because file system objects contain other file system objects.
+type: object
+required: ["type"]
+properties:
+  type:
+    type: string
+    enum: ["regular", "symlink", "directory"]
+
+# Enforce conditional structure based on `type`
+anyOf:
+  - $ref: "#/$defs/regular"
+    required: ["type", "contents"]
+
+  - $ref: "#/$defs/directory"
+    required: ["type", "entries"]
+
+  - $ref: "#/$defs/symlink"
+    required: ["type", "target"]
+
+"$defs":
+  regular:
+    title: Regular File
+    description: |
+      See [Regular File](@docroot@/store/file-system-object.md#regular) in the manual for details.
+    required: ["contents"]
+    properties:
+      type:
+        const: "regular"
+      contents:
+        type: string
+        description: File contents
+      executable:
+        type: boolean
+        description: Whether the file is executable.
+        default: false
+    additionalProperties: false
+
+  directory:
+    title: Directory
+    description: |
+      See [Directory](@docroot@/store/file-system-object.md#directory) in the manual for details.
+    required: ["entries"]
+    properties:
+      type:
+        const: "directory"
+      entries:
+        type: object
+        description: |
+          Map of names to nested file system objects (for type=directory)
+        additionalProperties:
+          $ref: "#"
+    additionalProperties: false
+
+  symlink:
+    title: Symbolic Link
+    description: |
+      See [Symbolic Link](@docroot@/store/file-system-object.md#symlink) in the manual for details.
+    required: ["target"]
+    properties:
+      type:
+        const: "symlink"
+      target:
+        type: string
+        description: Target path of the symlink.
+    additionalProperties: false

doc/manual/source/store/file-system-object.md

@@ -3,19 +3,23 @@
 Nix uses a simplified model of the file system, which consists of file system objects.
 Every file system object is one of the following:
 
- - File
+ - [**Regular File**]{#regular}
 
    - A possibly empty sequence of bytes for contents
    - A single boolean representing the [executable](https://en.m.wikipedia.org/wiki/File-system_permissions#Permissions) permission
 
- - Directory
+ - [**Directory**]{#directory}
 
    Mapping of names to child file system objects
 
- - [Symbolic link](https://en.m.wikipedia.org/wiki/Symbolic_link)
+ - [**Symbolic link**]{#symlink}
 
-   An arbitrary string.
-   Nix does not assign any semantics to symbolic links.
+   An arbitrary string, known as the *target* of the symlink.
+
+   In general, Nix does not assign any semantics to symbolic links.
+   Certain operations however, may make additional assumptions and attempt to use the target to find another file system object.
+
+   > See [the Wikpedia article on symbolic links](https://en.m.wikipedia.org/wiki/Symbolic_link) for background information if you are unfamiliar with this Unix concept.
 
 File system objects and their children form a tree.
 A bare file or symlink can be a root file system object.

src/json-schema-checks/file-system-object

@@ -0,0 +1 @@
+../../src/libutil-tests/data/memory-source-accessor

src/json-schema-checks/meson.build

@@ -20,6 +20,14 @@ schema_dir = meson.current_source_dir() / 'schema'
 
 # Get all example files
 schemas = [
+  {
+    'stem' : 'file-system-object',
+    'schema' : schema_dir / 'file-system-object-v1.yaml',
+    'files' : [
+      'simple.json',
+      'complex.json',
+    ],
+  },
   {
     'stem' : 'hash',
     'schema' : schema_dir / 'hash-v1.yaml',

src/json-schema-checks/package.nix

@@ -20,6 +20,7 @@ mkMesonDerivation (finalAttrs: {
   fileset = lib.fileset.unions [
     ../../.version
     ../../doc/manual/source/protocols/json/schema
+    ../../src/libutil-tests/data/memory-source-accessor
     ../../src/libutil-tests/data/hash
     ../../src/libstore-tests/data/content-address
     ../../src/libstore-tests/data/store-path

src/libutil-tests/data/memory-source-accessor/complex.json

@@ -0,0 +1,24 @@
+{
+  "entries": {
+    "bar": {
+      "entries": {
+        "baz": {
+          "contents": "good day,\n\u0000\n\tworld!",
+          "executable": true,
+          "type": "regular"
+        },
+        "quux": {
+          "target": "/over/there",
+          "type": "symlink"
+        }
+      },
+      "type": "directory"
+    },
+    "foo": {
+      "contents": "hello\n\u0000\n\tworld!",
+      "executable": false,
+      "type": "regular"
+    }
+  },
+  "type": "directory"
+}

src/libutil-tests/data/memory-source-accessor/simple.json

@@ -0,0 +1,5 @@
+{
+  "contents": "asdf",
+  "executable": false,
+  "type": "regular"
+}

src/libutil-tests/data/nar-listing/deep.json

@@ -0,0 +1,23 @@
+{
+  "entries": {
+    "bar": {
+      "entries": {
+        "baz": {
+          "executable": true,
+          "size": 19,
+          "type": "regular"
+        },
+        "quux": {
+          "target": "/over/there",
+          "type": "symlink"
+        }
+      },
+      "type": "directory"
+    },
+    "foo": {
+      "size": 15,
+      "type": "regular"
+    }
+  },
+  "type": "directory"
+}

src/libutil-tests/data/nar-listing/shallow.json

@@ -0,0 +1,7 @@
+{
+  "entries": {
+    "bar": {},
+    "foo": {}
+  },
+  "type": "directory"
+}

src/libutil-tests/git.cc

@@ -224,42 +224,15 @@ TEST_F(GitTest, tree_sha256_write)
     });
 }
 
+namespace memory_source_accessor {
+
+extern ref<MemorySourceAccessor> exampleComplex();
+
+}
+
 TEST_F(GitTest, both_roundrip)
 {
-    using File = MemorySourceAccessor::File;
-
-    auto files = make_ref<MemorySourceAccessor>();
-    files->root = File::Directory{
-        .entries{
-            {
-                "foo",
-                File::Regular{
-                    .contents = "hello\n\0\n\tworld!",
-                },
-            },
-            {
-                "bar",
-                File::Directory{
-                    .entries =
-                        {
-                            {
-                                "baz",
-                                File::Regular{
-                                    .executable = true,
-                                    .contents = "good day,\n\0\n\tworld!",
-                                },
-                            },
-                            {
-                                "quux",
-                                File::Symlink{
-                                    .target = "/over/there",
-                                },
-                            },
-                        },
-                },
-            },
-        },
-    };
+    auto files = memory_source_accessor::exampleComplex();
 
     for (const auto hashAlgo : {HashAlgorithm::SHA1, HashAlgorithm::SHA256}) {
         std::map<Hash, std::string> cas;

src/libutil-tests/memory-source-accessor.cc

@@ -0,0 +1,116 @@
+#include <string_view>
+
+#include "nix/util/memory-source-accessor.hh"
+#include "nix/util/tests/json-characterization.hh"
+
+namespace nix {
+
+namespace memory_source_accessor {
+
+using namespace std::literals;
+using File = MemorySourceAccessor::File;
+
+ref<MemorySourceAccessor> exampleSimple()
+{
+    auto sc = make_ref<MemorySourceAccessor>();
+    sc->root = File{File::Regular{
+        .executable = false,
+        .contents = "asdf",
+    }};
+    return sc;
+}
+
+ref<MemorySourceAccessor> exampleComplex()
+{
+    auto files = make_ref<MemorySourceAccessor>();
+    files->root = File::Directory{
+        .entries{
+            {
+                "foo",
+                File::Regular{
+                    .contents = "hello\n\0\n\tworld!"s,
+                },
+            },
+            {
+                "bar",
+                File::Directory{
+                    .entries =
+                        {
+                            {
+                                "baz",
+                                File::Regular{
+                                    .executable = true,
+                                    .contents = "good day,\n\0\n\tworld!"s,
+                                },
+                            },
+                            {
+                                "quux",
+                                File::Symlink{
+                                    .target = "/over/there",
+                                },
+                            },
+                        },
+                },
+            },
+        },
+    };
+    return files;
+}
+
+} // namespace memory_source_accessor
+
+/* ----------------------------------------------------------------------------
+ * JSON
+ * --------------------------------------------------------------------------*/
+
+class MemorySourceAccessorTest : public virtual CharacterizationTest
+{
+    std::filesystem::path unitTestData = getUnitTestData() / "memory-source-accessor";
+
+public:
+
+    std::filesystem::path goldenMaster(std::string_view testStem) const override
+    {
+        return unitTestData / testStem;
+    }
+};
+
+using nlohmann::json;
+
+struct MemorySourceAccessorJsonTest : MemorySourceAccessorTest,
+                                      JsonCharacterizationTest<MemorySourceAccessor>,
+                                      ::testing::WithParamInterface<std::pair<std::string_view, MemorySourceAccessor>>
+{};
+
+TEST_P(MemorySourceAccessorJsonTest, from_json)
+{
+    auto & [name, expected] = GetParam();
+    /* Cannot use `readJsonTest` because need to compare `root` field of
+       the source accessors for equality. */
+    readTest(Path{name} + ".json", [&](const auto & encodedRaw) {
+        auto encoded = json::parse(encodedRaw);
+        auto decoded = static_cast<MemorySourceAccessor>(encoded);
+        ASSERT_EQ(decoded.root, expected.root);
+    });
+}
+
+TEST_P(MemorySourceAccessorJsonTest, to_json)
+{
+    auto & [name, value] = GetParam();
+    writeJsonTest(name, value);
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    MemorySourceAccessorJSON,
+    MemorySourceAccessorJsonTest,
+    ::testing::Values(
+        std::pair{
+            "simple",
+            *memory_source_accessor::exampleSimple(),
+        },
+        std::pair{
+            "complex",
+            *memory_source_accessor::exampleComplex(),
+        }));
+
+} // namespace nix

src/libutil-tests/meson.build

@@ -63,7 +63,9 @@ sources = files(
   'json-utils.cc',
   'logging.cc',
   'lru-cache.cc',
+  'memory-source-accessor.cc',
   'monitorfdhup.cc',
+  'nar-listing.cc',
   'nix_api_util.cc',
   'nix_api_util_internal.cc',
   'pool.cc',

src/libutil-tests/nar-listing.cc

@@ -0,0 +1,83 @@
+#include <string_view>
+
+#include "nix/util/nar-accessor.hh"
+#include "nix/util/tests/json-characterization.hh"
+
+namespace nix {
+
+// Forward declaration from memory-source-accessor.cc
+namespace memory_source_accessor {
+ref<MemorySourceAccessor> exampleComplex();
+}
+
+/* ----------------------------------------------------------------------------
+ * JSON
+ * --------------------------------------------------------------------------*/
+
+class NarListingTest : public virtual CharacterizationTest
+{
+    std::filesystem::path unitTestData = getUnitTestData() / "nar-listing";
+
+public:
+
+    std::filesystem::path goldenMaster(std::string_view testStem) const override
+    {
+        return unitTestData / testStem;
+    }
+};
+
+using nlohmann::json;
+
+struct NarListingJsonTest : NarListingTest,
+                            JsonCharacterizationTest<NarListing>,
+                            ::testing::WithParamInterface<std::pair<std::string_view, NarListing>>
+{};
+
+TEST_P(NarListingJsonTest, from_json)
+{
+    auto & [name, expected] = GetParam();
+    readJsonTest(name, expected);
+}
+
+TEST_P(NarListingJsonTest, to_json)
+{
+    auto & [name, value] = GetParam();
+    writeJsonTest(name, value);
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    NarListingJSON,
+    NarListingJsonTest,
+    ::testing::Values(
+        std::pair{
+            "deep",
+            listNarDeep(*memory_source_accessor::exampleComplex(), CanonPath::root),
+        }));
+
+struct ShallowNarListingJsonTest : NarListingTest,
+                                   JsonCharacterizationTest<ShallowNarListing>,
+                                   ::testing::WithParamInterface<std::pair<std::string_view, ShallowNarListing>>
+{};
+
+TEST_P(ShallowNarListingJsonTest, from_json)
+{
+    auto & [name, expected] = GetParam();
+    readJsonTest(name, expected);
+}
+
+TEST_P(ShallowNarListingJsonTest, to_json)
+{
+    auto & [name, value] = GetParam();
+    writeJsonTest(name, value);
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    ShallowNarListingJSON,
+    ShallowNarListingJsonTest,
+    ::testing::Values(
+        std::pair{
+            "shallow",
+            listNarShallow(*memory_source_accessor::exampleComplex(), CanonPath::root),
+        }));
+
+} // namespace nix

src/libutil/include/nix/util/json-impls.hh

@@ -6,15 +6,18 @@
 #include "nix/util/experimental-features.hh"
 
 // Following https://github.com/nlohmann/json#how-can-i-use-get-for-non-default-constructiblenon-copyable-types
-#define JSON_IMPL(TYPE)                                   \
-    namespace nlohmann {                                  \
-    using namespace nix;                                  \
-    template<>                                            \
+#define JSON_IMPL_INNER(TYPE)                             \
     struct adl_serializer<TYPE>                           \
     {                                                     \
         static TYPE from_json(const json & json);         \
         static void to_json(json & json, const TYPE & t); \
-    };                                                    \
+    };
+
+#define JSON_IMPL(TYPE)   \
+    namespace nlohmann {  \
+    using namespace nix;  \
+    template<>            \
+    JSON_IMPL_INNER(TYPE) \
     }
 
 #define JSON_IMPL_WITH_XP_FEATURES(TYPE)                                                                            \

src/libutil/include/nix/util/memory-source-accessor.hh

@@ -160,4 +160,53 @@ struct MemorySink : FileSystemObjectSink
     void createSymlink(const CanonPath & path, const std::string & target) override;
 };
 
+template<>
+struct json_avoids_null<MemorySourceAccessor::File::Regular> : std::true_type
+{};
+
+template<>
+struct json_avoids_null<MemorySourceAccessor::File::Directory> : std::true_type
+{};
+
+template<>
+struct json_avoids_null<MemorySourceAccessor::File::Symlink> : std::true_type
+{};
+
+template<>
+struct json_avoids_null<MemorySourceAccessor::File> : std::true_type
+{};
+
+template<>
+struct json_avoids_null<MemorySourceAccessor> : std::true_type
+{};
+
 } // namespace nix
+
+namespace nlohmann {
+
+using namespace nix;
+
+#define ARG fso::Regular<RegularContents>
+template<typename RegularContents>
+JSON_IMPL_INNER(ARG)
+#undef ARG
+
+#define ARG fso::DirectoryT<Child>
+template<typename Child>
+JSON_IMPL_INNER(ARG)
+#undef ARG
+
+template<>
+JSON_IMPL_INNER(fso::Symlink)
+
+template<>
+JSON_IMPL_INNER(fso::Opaque)
+
+#define ARG fso::VariantT<RegularContents, recur>
+template<typename RegularContents, bool recur>
+JSON_IMPL_INNER(ARG)
+#undef ARG
+
+} // namespace nlohmann
+
+JSON_IMPL(MemorySourceAccessor)

src/libutil/include/nix/util/nar-accessor.hh

@@ -75,14 +75,6 @@ NarListing listNarDeep(SourceAccessor & accessor, const CanonPath & path);
  */
 ShallowNarListing listNarShallow(SourceAccessor & accessor, const CanonPath & path);
 
-/**
- * Serialize a NarListing to JSON.
- */
-void to_json(nlohmann::json & j, const NarListing & listing);
-
-/**
- * Serialize a ShallowNarListing to JSON.
- */
-void to_json(nlohmann::json & j, const ShallowNarListing & listing);
+// All json_avoids_null and JSON_IMPL covered by generic templates in memory-source-accessor.hh
 
 } // namespace nix

src/libutil/memory-source-accessor.cc

@@ -1,4 +1,5 @@
 #include "nix/util/memory-source-accessor.hh"
+#include "nix/util/json-utils.hh"
 
 namespace nix {
 

src/libutil/memory-source-accessor/json.cc

@@ -0,0 +1,162 @@
+#include "nix/util/memory-source-accessor.hh"
+#include "nix/util/nar-accessor.hh"
+#include "nix/util/json-utils.hh"
+
+#include <nlohmann/json.hpp>
+
+namespace nlohmann {
+
+using namespace nix;
+
+// fso::Regular<RegularContents>
+template<>
+MemorySourceAccessor::File::Regular adl_serializer<MemorySourceAccessor::File::Regular>::from_json(const json & json)
+{
+    auto & obj = getObject(json);
+    return MemorySourceAccessor::File::Regular{
+        .executable = getBoolean(valueAt(obj, "executable")),
+        .contents = getString(valueAt(obj, "contents")),
+    };
+}
+
+template<>
+void adl_serializer<MemorySourceAccessor::File::Regular>::to_json(
+    json & json, const MemorySourceAccessor::File::Regular & r)
+{
+    json = {
+        {"executable", r.executable},
+        {"contents", r.contents},
+    };
+}
+
+template<>
+NarListing::Regular adl_serializer<NarListing::Regular>::from_json(const json & json)
+{
+    auto & obj = getObject(json);
+    auto * execPtr = optionalValueAt(obj, "executable");
+    auto * sizePtr = optionalValueAt(obj, "size");
+    auto * offsetPtr = optionalValueAt(obj, "narOffset");
+    return NarListing::Regular{
+        .executable = execPtr ? getBoolean(*execPtr) : false,
+        .contents{
+            .fileSize = ptrToOwned<uint64_t>(sizePtr),
+            .narOffset = ptrToOwned<uint64_t>(offsetPtr).and_then(
+                [](auto v) { return v != 0 ? std::optional{v} : std::nullopt; }),
+        },
+    };
+}
+
+template<>
+void adl_serializer<NarListing::Regular>::to_json(json & j, const NarListing::Regular & r)
+{
+    if (r.contents.fileSize)
+        j["size"] = *r.contents.fileSize;
+    if (r.executable)
+        j["executable"] = true;
+    if (r.contents.narOffset)
+        j["narOffset"] = *r.contents.narOffset;
+}
+
+template<typename Child>
+void adl_serializer<fso::DirectoryT<Child>>::to_json(json & j, const fso::DirectoryT<Child> & d)
+{
+    j["entries"] = d.entries;
+}
+
+template<typename Child>
+fso::DirectoryT<Child> adl_serializer<fso::DirectoryT<Child>>::from_json(const json & json)
+{
+    auto & obj = getObject(json);
+    return fso::DirectoryT<Child>{
+        .entries = valueAt(obj, "entries"),
+    };
+}
+
+// fso::Symlink
+fso::Symlink adl_serializer<fso::Symlink>::from_json(const json & json)
+{
+    auto & obj = getObject(json);
+    return fso::Symlink{
+        .target = getString(valueAt(obj, "target")),
+    };
+}
+
+void adl_serializer<fso::Symlink>::to_json(json & json, const fso::Symlink & s)
+{
+    json = {
+        {"target", s.target},
+    };
+}
+
+// fso::Opaque
+fso::Opaque adl_serializer<fso::Opaque>::from_json(const json &)
+{
+    return fso::Opaque{};
+}
+
+void adl_serializer<fso::Opaque>::to_json(json & j, const fso::Opaque &)
+{
+    j = nlohmann::json::object();
+}
+
+// fso::VariantT<RegularContents, recur> - generic implementation
+template<typename RegularContents, bool recur>
+void adl_serializer<fso::VariantT<RegularContents, recur>>::to_json(
+    json & j, const fso::VariantT<RegularContents, recur> & val)
+{
+    using Variant = fso::VariantT<RegularContents, recur>;
+    j = nlohmann::json::object();
+    std::visit(
+        overloaded{
+            [&](const typename Variant::Regular & r) {
+                j = r;
+                j["type"] = "regular";
+            },
+            [&](const typename Variant::Directory & d) {
+                j = d;
+                j["type"] = "directory";
+            },
+            [&](const typename Variant::Symlink & s) {
+                j = s;
+                j["type"] = "symlink";
+            },
+        },
+        val.raw);
+}
+
+template<typename RegularContents, bool recur>
+fso::VariantT<RegularContents, recur>
+adl_serializer<fso::VariantT<RegularContents, recur>>::from_json(const json & json)
+{
+    using Variant = fso::VariantT<RegularContents, recur>;
+    auto & obj = getObject(json);
+    auto type = getString(valueAt(obj, "type"));
+    if (type == "regular")
+        return static_cast<typename Variant::Regular>(json);
+    if (type == "directory")
+        return static_cast<typename Variant::Directory>(json);
+    if (type == "symlink")
+        return static_cast<typename Variant::Symlink>(json);
+    else
+        throw Error("unknown type of file '%s'", type);
+}
+
+// Explicit instantiations for VariantT types we use
+template struct adl_serializer<MemorySourceAccessor::File>;
+template struct adl_serializer<NarListing>;
+template struct adl_serializer<ShallowNarListing>;
+
+// MemorySourceAccessor
+MemorySourceAccessor adl_serializer<MemorySourceAccessor>::from_json(const json & json)
+{
+    MemorySourceAccessor res;
+    res.root = json;
+    return res;
+}
+
+void adl_serializer<MemorySourceAccessor>::to_json(json & json, const MemorySourceAccessor & val)
+{
+    json = val.root;
+}
+
+} // namespace nlohmann

src/libutil/meson.build

@@ -146,6 +146,7 @@ sources = [ config_priv_h ] + files(
   'json-utils.cc',
   'logging.cc',
   'memory-source-accessor.cc',
+  'memory-source-accessor/json.cc',
   'mounted-source-accessor.cc',
   'nar-accessor.cc',
   'pos-table.cc',

src/libutil/nar-accessor.cc

@@ -324,52 +324,4 @@ ShallowNarListing listNarShallow(SourceAccessor & accessor, const CanonPath & pa
     return listNarImpl<false>(accessor, path);
 }
 
-template<typename Listing>
-static void to_json_impl(nlohmann::json & j, const Listing & listing)
-{
-    std::visit(
-        overloaded{
-            [&](const typename Listing::Regular & r) {
-                j = nlohmann::json::object();
-                j["type"] = "regular";
-                if (r.contents.fileSize)
-                    j["size"] = *r.contents.fileSize;
-                if (r.executable)
-                    j["executable"] = true;
-                if (r.contents.narOffset)
-                    j["narOffset"] = *r.contents.narOffset;
-            },
-            [&](const typename Listing::Directory & d) {
-                j = nlohmann::json::object();
-                j["type"] = "directory";
-                j["entries"] = nlohmann::json::object();
-                for (const auto & [name, child] : d.entries) {
-                    if constexpr (std::is_same_v<Listing, NarListing>) {
-                        to_json(j["entries"][name], child);
-                    } else if constexpr (std::is_same_v<Listing, ShallowNarListing>) {
-                        j["entries"][name] = nlohmann::json::object();
-                    } else {
-                        static_assert(false);
-                    }
-                }
-            },
-            [&](const typename Listing::Symlink & s) {
-                j = nlohmann::json::object();
-                j["type"] = "symlink";
-                j["target"] = s.target;
-            },
-        },
-        listing.raw);
-}
-
-void to_json(nlohmann::json & j, const NarListing & listing)
-{
-    to_json_impl(j, listing);
-}
-
-void to_json(nlohmann::json & j, const ShallowNarListing & listing)
-{
-    to_json_impl(j, listing);
-}
-
 } // namespace nix