HOT SALE: 15% off your build times!

This was achieved by running maintainers/buildtime_report.sh on the
build directory of a meson build, then asking "why the heck is json
eating our build times", and strategically moving the json using bits
out of widely included headers.

It turns out that putting literally any metrics whatsoever into the
build had immediate and predictable results.

Results are 1382.5s frontend time -> 1175.4s frontend time, back end
time approximately invariant.

Related: https://git.lix.systems/lix-project/lix/issues/159

Change-Id: I7edea95c8536203325c8bb4dae5f32d727a21b2d

8 files changed, 59 insertions, 34 deletions

diff --git a/src/libutil/abstract-setting-to-json.hh b/src/libutil/abstract-setting-to-json.hh
index eea687d8a..60807412a 100644
--- a/src/libutil/abstract-setting-to-json.hh
+++ b/src/libutil/abstract-setting-to-json.hh
@@ -4,6 +4,8 @@
 #include <nlohmann/json.hpp>
 #include "config.hh"
 #include "json-utils.hh"
+// Required for instances of to_json and from_json for ExperimentalFeature
+#include "experimental-features-json.hh"
 
 namespace nix {
 template<typename T>
diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index 3e39b4d7c..520f50c30 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -2,6 +2,7 @@
 #include "args/root.hh"
 #include "hash.hh"
 #include "json-utils.hh"
+#include "experimental-features-json.hh"
 
 #include <glob.h>
 
diff --git a/src/libutil/experimental-features-json.hh b/src/libutil/experimental-features-json.hh
new file mode 100644
index 000000000..a4f999a93
--- /dev/null
+++ b/src/libutil/experimental-features-json.hh
@@ -0,0 +1,29 @@
+#pragma once
+///@file
+
+#include "experimental-features.hh"
+#include "json-utils.hh"
+
+namespace nix {
+
+/**
+ * Compute the documentation of all experimental features.
+ *
+ * See `doc/manual` for how this information is used.
+ */
+nlohmann::json documentExperimentalFeatures();
+
+/**
+ * Semi-magic conversion to and from json.
+ * See the nlohmann/json readme for more details.
+ */
+void to_json(nlohmann::json &, const ExperimentalFeature &);
+void from_json(const nlohmann::json &, ExperimentalFeature &);
+
+/**
+ * It is always rendered as a string
+ */
+template<>
+struct json_avoids_null<ExperimentalFeature> : std::true_type {};
+
+};
diff --git a/src/libutil/experimental-features.cc b/src/libutil/experimental-features.cc
index d1e1d7423..25beba467 100644
--- a/src/libutil/experimental-features.cc
+++ b/src/libutil/experimental-features.cc
@@ -1,4 +1,6 @@
 #include "experimental-features.hh"
+// Required for instances of to_json and from_json for ExperimentalFeature
+#include "experimental-features-json.hh"
 #include "util.hh"
 
 #include "nlohmann/json.hpp"
diff --git a/src/libutil/experimental-features.hh b/src/libutil/experimental-features.hh
index cc841c0a6..38889e7bc 100644
--- a/src/libutil/experimental-features.hh
+++ b/src/libutil/experimental-features.hh
@@ -1,9 +1,7 @@
 #pragma once
 ///@file
 
-#include "comparator.hh"
 #include "error.hh"
-#include "json-utils.hh"
 #include "types.hh"
 
 namespace nix {
@@ -53,13 +51,6 @@ const std::optional<ExperimentalFeature> parseExperimentalFeature(
 std::string_view showExperimentalFeature(const ExperimentalFeature);
 
 /**
- * Compute the documentation of all experimental features.
- *
- * See `doc/manual` for how this information is used.
- */
-nlohmann::json documentExperimentalFeatures();
-
-/**
  * Shorthand for `str << showExperimentalFeature(feature)`.
  */
 std::ostream & operator<<(
@@ -87,17 +78,4 @@ public:
     MissingExperimentalFeature(ExperimentalFeature missingFeature);
 };
 
-/**
- * Semi-magic conversion to and from json.
- * See the nlohmann/json readme for more details.
- */
-void to_json(nlohmann::json &, const ExperimentalFeature &);
-void from_json(const nlohmann::json &, ExperimentalFeature &);
-
-/**
- * It is always rendered as a string
- */
-template<>
-struct json_avoids_null<ExperimentalFeature> : std::true_type {};
-
 }
diff --git a/src/libutil/logging-json.hh b/src/libutil/logging-json.hh
new file mode 100644
index 000000000..8263ad707
--- /dev/null
+++ b/src/libutil/logging-json.hh
@@ -0,0 +1,23 @@
+#pragma once
+///@file logging-json.hh
+///
+///@brief Logging functions for json specifically, split due to the cost of
+///including nlohmann.
+
+#include "logging.hh"
+
+#include <nlohmann/json_fwd.hpp>
+
+namespace nix {
+
+std::optional<nlohmann::json> parseJSONMessage(const std::string & msg);
+
+bool handleJSONLogMessage(nlohmann::json & json,
+    const Activity & act, std::map<ActivityId, Activity> & activities,
+    bool trusted);
+
+bool handleJSONLogMessage(const std::string & msg,
+    const Activity & act, std::map<ActivityId, Activity> & activities,
+    bool trusted);
+
+};
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index 7a6341d70..dc6f53d77 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -5,8 +5,6 @@
 #include "error.hh"
 #include "config.hh"
 
-#include <nlohmann/json_fwd.hpp>
-
 namespace nix {
 
 typedef enum {
@@ -184,16 +182,6 @@ Logger * makeSimpleLogger(bool printBuildLogs = true);
 
 Logger * makeJSONLogger(Logger & prevLogger);
 
-std::optional<nlohmann::json> parseJSONMessage(const std::string & msg);
-
-bool handleJSONLogMessage(nlohmann::json & json,
-    const Activity & act, std::map<ActivityId, Activity> & activities,
-    bool trusted);
-
-bool handleJSONLogMessage(const std::string & msg,
-    const Activity & act, std::map<ActivityId, Activity> & activities,
-    bool trusted);
-
 /**
  * suppress msgs > this
  */
diff --git a/src/libutil/meson.build b/src/libutil/meson.build
index 084d7ed11..66eba9d85 100644
--- a/src/libutil/meson.build
+++ b/src/libutil/meson.build
@@ -50,6 +50,7 @@ libutil_headers = files(
   'error.hh',
   'exit.hh',
   'experimental-features.hh',
+  'experimental-features-json.hh',
   'finally.hh',
   'fmt.hh',
   'git.hh',
@@ -59,6 +60,7 @@ libutil_headers = files(
   'json-impls.hh',
   'json-utils.hh',
   'logging.hh',
+  'logging-json.hh',
   'lru-cache.hh',
   'monitor-fd.hh',
   'namespaces.hh',