diff --git a/lix/libstore/outputs-spec.cc b/lix/libstore/outputs-spec.cc
index 41b92a1ba..aadc1420d 100644
--- a/lix/libstore/outputs-spec.cc
+++ b/lix/libstore/outputs-spec.cc
@@ -148,11 +148,7 @@ bool OutputsSpec::isSubsetOf(const OutputsSpec & that) const
     }, that.raw);
 }
 
-}
-
-namespace nix::json {
-
-OutputsSpec adl_serializer<OutputsSpec>::from_json(const JSON & json) {
+OutputsSpec OutputsSpec::from_json(const JSON & json) {
     auto names = json.get<StringSet>();
     if (names == StringSet({"*"}))
         return OutputsSpec::All {};
@@ -160,7 +156,7 @@ OutputsSpec adl_serializer<OutputsSpec>::from_json(const JSON & json) {
         return OutputsSpec::Names { std::move(names) };
 }
 
-void adl_serializer<OutputsSpec>::to_json(JSON & json, OutputsSpec t) {
+void OutputsSpec::to_json(JSON & json, const OutputsSpec & t) {
     std::visit(overloaded {
         [&](const OutputsSpec::All &) {
             json = std::vector<std::string>({"*"});
@@ -172,7 +168,7 @@ void adl_serializer<OutputsSpec>::to_json(JSON & json, OutputsSpec t) {
 }
 
 
-ExtendedOutputsSpec adl_serializer<ExtendedOutputsSpec>::from_json(const JSON & json) {
+ExtendedOutputsSpec ExtendedOutputsSpec::from_json(const JSON & json) {
     if (json.is_null())
         return ExtendedOutputsSpec::Default {};
     else {
@@ -180,13 +176,13 @@ ExtendedOutputsSpec adl_serializer<ExtendedOutputsSpec>::from_json(const JSON &
     }
 }
 
-void adl_serializer<ExtendedOutputsSpec>::to_json(JSON & json, ExtendedOutputsSpec t) {
+void ExtendedOutputsSpec::to_json(JSON & json, const ExtendedOutputsSpec & t) {
     std::visit(overloaded {
         [&](const ExtendedOutputsSpec::Default &) {
             json = nullptr;
         },
         [&](const ExtendedOutputsSpec::Explicit & e) {
-            adl_serializer<OutputsSpec>::to_json(json, e);
+            OutputsSpec::to_json(json, e);
         },
     }, t.raw);
 }
diff --git a/lix/libstore/outputs-spec.hh b/lix/libstore/outputs-spec.hh
index 2e2170c33..c889f7810 100644
--- a/lix/libstore/outputs-spec.hh
+++ b/lix/libstore/outputs-spec.hh
@@ -7,8 +7,8 @@
 #include <variant>
 
 #include "lix/libutil/comparator.hh"
-#include "lix/libutil/json-impls.hh"
 #include "lix/libutil/comparator.hh"
+#include "lix/libutil/json-fwd.hh"
 #include "lix/libutil/variant-wrapper.hh"
 
 namespace nix {
@@ -89,6 +89,9 @@ struct OutputsSpec {
     static std::optional<OutputsSpec> parseOpt(std::string_view s);
 
     std::string to_string() const;
+
+    static void to_json(JSON & json, const OutputsSpec & t);
+    static OutputsSpec from_json(const JSON & json);
 };
 
 struct ExtendedOutputsSpec {
@@ -116,9 +119,9 @@ struct ExtendedOutputsSpec {
     static std::optional<std::pair<std::string_view, ExtendedOutputsSpec>> parseOpt(std::string_view s);
 
     std::string to_string() const;
+
+    static void to_json(JSON & json, const ExtendedOutputsSpec & spec);
+    static ExtendedOutputsSpec from_json(const JSON & t);
 };
 
 }
-
-JSON_IMPL(OutputsSpec)
-JSON_IMPL(ExtendedOutputsSpec)
diff --git a/lix/libutil/json-impls.hh b/lix/libutil/json-impls.hh
deleted file mode 100644
index d53b99087..000000000
--- a/lix/libutil/json-impls.hh
+++ /dev/null
@@ -1,14 +0,0 @@
-#pragma once
-///@file
-
-#include "lix/libutil/json-fwd.hh"
-
-// Following https://github.com/nlohmann/json#how-can-i-use-get-for-non-default-constructiblenon-copyable-types
-#define JSON_IMPL(TYPE)                                                \
-    namespace nix::json {                                              \
-        template <>                                                    \
-        struct adl_serializer<TYPE> {                                  \
-            static TYPE from_json(const JSON & json);                  \
-            static void to_json(JSON & json, TYPE t);                  \
-        };                                                             \
-    }
diff --git a/lix/libutil/json.hh b/lix/libutil/json.hh
index e2cd81574..9ecee6e20 100644
--- a/lix/libutil/json.hh
+++ b/lix/libutil/json.hh
@@ -6,6 +6,7 @@
 /// deal with `std::optional<T>` types until 3.11.3 at, and we need those.
 
 #include "lix/libutil/json-fwd.hh" // IWYU pragma: keep
+#include <concepts>
 #include <nlohmann/json.hpp> // IWYU pragma: keep
 #include <list>
 #include <type_traits>
@@ -84,6 +85,25 @@ struct adl_serializer<T, void>
     {
         detail::call_from_json(std::forward<Json>(j), value);
     }
+
+    // Following https://github.com/nlohmann/json#how-can-i-use-get-for-non-default-constructiblenon-copyable-types
+    template<typename Json>
+        requires requires(Json & j, T value) { T::to_json(j, value); }
+    static void to_json(Json & j, const T & value)
+    {
+        T::to_json(j, value);
+    }
+
+    template<typename Json>
+        requires requires(Json && j) {
+            {
+                T::from_json(std::forward<Json>(j))
+            } -> std::same_as<T>;
+        }
+    static auto from_json(Json && j)
+    {
+        return T::from_json(std::forward<Json>(j));
+    }
 };
 }
 
diff --git a/lix/libutil/meson.build b/lix/libutil/meson.build
index d35e65d7f..63ce56905 100644
--- a/lix/libutil/meson.build
+++ b/lix/libutil/meson.build
@@ -95,7 +95,6 @@ libutil_headers = files(
   'input-accessor.hh',
   'json.hh',
   'json-fwd.hh',
-  'json-impls.hh',
   'json-utils.hh',
   'logging.hh',
   'logging-json.hh',