Add Setting<std::filesystem::path> specialization

Like PathSetting, this normalizes the path (without resolving
symlinks).

Author: edolstra

src/libutil/configuration.cc

@@ -433,6 +433,26 @@ std::string BaseSetting<StringMap>::to_string() const
         [](const auto & kvpair) { return kvpair.first + "=" + kvpair.second; });
 }
 
+static Path parsePath(const AbstractSetting & s, const std::string & str)
+{
+    if (str == "")
+        throw UsageError("setting '%s' is a path and paths cannot be empty", s.name);
+    else
+        return canonPath(str);
+}
+
+template<>
+std::filesystem::path BaseSetting<std::filesystem::path>::parse(const std::string & str) const
+{
+    return parsePath(*this, str);
+}
+
+template<>
+std::string BaseSetting<std::filesystem::path>::to_string() const
+{
+    return value.string();
+}
+
 template class BaseSetting<int>;
 template class BaseSetting<unsigned int>;
 template class BaseSetting<long>;
@@ -445,14 +465,7 @@ template class BaseSetting<Strings>;
 template class BaseSetting<StringSet>;
 template class BaseSetting<StringMap>;
 template class BaseSetting<std::set<ExperimentalFeature>>;
-
-static Path parsePath(const AbstractSetting & s, const std::string & str)
-{
-    if (str == "")
-        throw UsageError("setting '%s' is a path and paths cannot be empty", s.name);
-    else
-        return canonPath(str);
-}
+template class BaseSetting<std::filesystem::path>;
 
 PathSetting::PathSetting(
     Config * options,

src/libutil/include/nix/util/config-impl.hh

@@ -134,6 +134,7 @@ DECLARE_CONFIG_SERIALISER(Strings)
 DECLARE_CONFIG_SERIALISER(StringSet)
 DECLARE_CONFIG_SERIALISER(StringMap)
 DECLARE_CONFIG_SERIALISER(std::set<ExperimentalFeature>)
+DECLARE_CONFIG_SERIALISER(std::filesystem::path)
 
 template<typename T>
 T BaseSetting<T>::parse(const std::string & str) const

src/libutil/include/nix/util/logging.hh

@@ -54,9 +54,9 @@ struct LoggerSettings : Config
           expression evaluation errors.
         )"};
 
-    Setting<Path> jsonLogPath{
+    Setting<std::filesystem::path> jsonLogPath{
         this,
-        "",
+        {},
         "json-log-path",
         R"(
           A file or unix socket to which JSON records of Nix's log output are

src/libutil/logging.cc

@@ -370,7 +370,7 @@ void applyJSONLogger()
     if (!loggerSettings.jsonLogPath.get().empty()) {
         try {
             std::vector<std::unique_ptr<Logger>> loggers;
-            loggers.push_back(makeJSONLogger(std::filesystem::path(loggerSettings.jsonLogPath.get()), false));
+            loggers.push_back(makeJSONLogger(loggerSettings.jsonLogPath.get(), false));
             try {
                 logger = makeTeeLogger(std::move(logger), std::move(loggers));
             } catch (...) {