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.

8 files changed, 55 insertions, 37 deletions

diff --git a/.gitignore b/.gitignore
index e326966d6..53442751f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -19,9 +19,11 @@ perl/Makefile.config
 /doc/manual/nix.json
 /doc/manual/conf-file.json
 /doc/manual/builtins.json
+/doc/manual/xp-features.json
 /doc/manual/src/SUMMARY.md
 /doc/manual/src/command-ref/new-cli
 /doc/manual/src/command-ref/conf-file.md
+/doc/manual/src/command-ref/experimental-features.md
 /doc/manual/src/language/builtins.md
 
 # /scripts/
diff --git a/doc/manual/generate-xp-features.nix b/doc/manual/generate-xp-features.nix
new file mode 100644
index 000000000..db1ba6092
--- /dev/null
+++ b/doc/manual/generate-xp-features.nix
@@ -0,0 +1,11 @@
+with builtins;
+with import ./utils.nix;
+
+let
+  showExperimentalFeature = name: doc:
+    squash ''
+      - <span id="xp-feature-${name}">[`${name}`](#xp-feature-${name})</span>
+
+      ${indent "  " doc}
+    '';
+in xps: indent "  " (concatStringsSep "\n" (attrValues (mapAttrs showExperimentalFeature xps)))
diff --git a/doc/manual/local.mk b/doc/manual/local.mk
index df941d460..0d4b9d640 100644
--- a/doc/manual/local.mk
+++ b/doc/manual/local.mk
@@ -88,12 +88,12 @@ $(d)/src/SUMMARY.md: $(d)/src/SUMMARY.md.in $(d)/src/command-ref/new-cli
 	@cp $< $@
 	@$(call process-includes,$@,$@)
 
-$(d)/src/command-ref/new-cli: $(d)/nix.json $(d)/generate-manpage.nix $(bindir)/nix
+$(d)/src/command-ref/new-cli: $(d)/nix.json $(d)/utils.nix $(d)/generate-manpage.nix $(bindir)/nix
 	@rm -rf $@ $@.tmp
 	$(trace-gen) $(nix-eval) --write-to $@.tmp --expr 'import doc/manual/generate-manpage.nix (builtins.readFile $<)'
 	@mv $@.tmp $@
 
-$(d)/src/command-ref/conf-file.md: $(d)/conf-file.json $(d)/utils.nix $(d)/src/command-ref/conf-file-prefix.md $(bindir)/nix
+$(d)/src/command-ref/conf-file.md: $(d)/conf-file.json $(d)/utils.nix $(d)/src/command-ref/conf-file-prefix.md $(d)/src/command-ref/experimental-features.md $(bindir)/nix
 	@cat doc/manual/src/command-ref/conf-file-prefix.md > $@.tmp
 	$(trace-gen) $(nix-eval) --expr '(import doc/manual/utils.nix).showSettings { useAnchors = true; } (builtins.fromJSON (builtins.readFile $<))' >> $@.tmp;
 	@mv $@.tmp $@
@@ -106,6 +106,15 @@ $(d)/conf-file.json: $(bindir)/nix
 	$(trace-gen) $(dummy-env) $(bindir)/nix show-config --json --experimental-features nix-command > $@.tmp
 	@mv $@.tmp $@
 
+$(d)/src/command-ref/experimental-features.md: $(d)/xp-features.json $(d)/utils.nix $(d)/generate-xp-features.nix $(bindir)/nix
+	@rm -rf $@ $@.tmp
+	$(trace-gen) $(nix-eval) --write-to $@.tmp --expr 'import doc/manual/generate-xp-features.nix (builtins.fromJSON (builtins.readFile $<))'
+	@mv $@.tmp $@
+
+$(d)/xp-features.json: $(bindir)/nix
+	$(trace-gen) $(dummy-env) NIX_PATH=nix/corepkgs=corepkgs $(bindir)/nix __dump-xp-features > $@.tmp
+	@mv $@.tmp $@
+
 $(d)/src/language/builtins.md: $(d)/builtins.json $(d)/generate-builtins.nix $(d)/src/language/builtins-prefix.md $(bindir)/nix
 	@cat doc/manual/src/language/builtins-prefix.md > $@.tmp
 	$(trace-gen) $(nix-eval) --expr 'import doc/manual/generate-builtins.nix (builtins.fromJSON (builtins.readFile $<))' >> $@.tmp;
diff --git a/doc/manual/utils.nix b/doc/manual/utils.nix
index 5eacce0dd..f78e6bb02 100644
--- a/doc/manual/utils.nix
+++ b/doc/manual/utils.nix
@@ -74,10 +74,10 @@ rec {
           if aliases == [] then "" else
             "**Deprecated alias:** ${(concatStringsSep ", " (map (s: "`${s}`") aliases))}";
 
-      indent = prefix: s:
-        concatStringsSep "\n" (map (x: if x == "" then x else "${prefix}${x}") (splitLines s));
-
     in result;
 
+  indent = prefix: s:
+    concatStringsSep "\n" (map (x: if x == "" then x else "${prefix}${x}") (splitLines s));
+
   showSettings = args: settingsInfo: concatStrings (attrValues (mapAttrs (showSetting args) settingsInfo));
 }
diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index a001056f7..8b0fe6555 100644
--- a/src/libutil/config.hh
+++ b/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.
diff --git a/src/libutil/experimental-features.cc b/src/libutil/experimental-features.cc
index a64e9715a..010ab1d68 100644
--- a/src/libutil/experimental-features.cc
+++ b/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)
diff --git a/src/libutil/experimental-features.hh b/src/libutil/experimental-features.hh
index 3c479dbd9..8ef66263a 100644
--- a/src/libutil/experimental-features.hh
+++ b/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)`.
diff --git a/src/nix/main.cc b/src/nix/main.cc
index 4d4164333..62b1f98d1 100644
--- a/src/nix/main.cc
+++ b/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) {