Assemble experimental feature docs outside of Nix itself

Instead of constructing a markdown list in C++ (which involved all sorts
of nasty string literals), export some JSON and assemble it with the
manual build system.

Besides following the precedent set with other dumped data, this is a
better separate of content and presentation; if we decide for example we
want to display this information in a different way, or in a different
section of the manual, it will become much easier to do so.

src/libutil/config.hh

@@ -371,8 +371,21 @@ extern GlobalConfig globalConfig;
 
 struct ExperimentalFeatureSettings : Config {
 
-    Setting<std::set<ExperimentalFeature>> experimentalFeatures{this, {}, "experimental-features",
-        getExperimentalFeaturesList()};
+    Setting<std::set<ExperimentalFeature>> experimentalFeatures{
+        this, {}, "experimental-features",
+        R"(
+          Experimental features that are enabled.
+
+          Example:
+
+          ```
+          experimental-features = nix-command flakes
+          ```
+
+          Experimental features available:
+
+          {{#include experimental-features.md}}
+        )"};
 
     /**
      * Check whether the given experimental feature is enabled.

src/libutil/experimental-features.cc

@@ -140,32 +140,12 @@ std::string_view showExperimentalFeature(const ExperimentalFeature tag)
     return xpFeatureDetails[(size_t)tag].name;
 }
 
-std::string getExperimentalFeaturesList() {
-    std::string experimentalFeaturesList = R"(
-        Experimental features that can be enabled.
-        
-        Example:
-        
-        ```
-        experimental-features = nix-command flakes
-        
-        Experimental features available:
-
-)";
-
-    for (auto & xpFeature : xpFeatureDetails) {
-        experimentalFeaturesList += std::string {}
-            /* length of this first string must be 12, matching the indent of
-               the descriptions in the xpFeatureDetails literal. FIXME compute
-               markdown in a less hacky way. */
-            + "          - "
-            + "`" + xpFeature.name + "`"
-            + "\n"
-            + xpFeature.description
-            + "\n\n";
-    }
-
-    return experimentalFeaturesList;
+nlohmann::json documentExperimentalFeatures() {
+    StringMap res;
+    for (auto & xpFeature : xpFeatureDetails)
+        res[std::string { xpFeature.name }] =
+            trim(stripIndentation(xpFeature.description));
+    return (nlohmann::json) res;
 }
 
 std::set<ExperimentalFeature> parseFeatures(const std::set<std::string> & rawFeatures)

src/libutil/experimental-features.hh

@@ -51,11 +51,9 @@ std::string_view showExperimentalFeature(const ExperimentalFeature);
 /**
  * Compute the documentation of all experimental features.
  *
- * This a markdown bulleted list where each item is first (a) the
- * experimental feature flag name in backticks, and then (b) the
- * description of the experimental feature.
+ * See `doc/manual` for how this information is used.
  */
-std::string getExperimentalFeaturesList();
+nlohmann::json documentExperimentalFeatures();
 
 /**
  * Shorthand for `str << showExperimentalFeature(feature)`.

src/nix/main.cc

@@ -374,6 +374,11 @@ void mainWrapped(int argc, char * * argv)
         return;
     }
 
+    if (argc == 2 && std::string(argv[1]) == "__dump-xp-features") {
+        logger->cout(documentExperimentalFeatures().dump());
+        return;
+    }
+
     Finally printCompletions([&]()
     {
         if (completions) {