nix --help: Display help using lowdown instead of man

Fixes #4476.
Fixes #5231.

1 files changed, 42 insertions, 4 deletions

diff --git a/src/nix/main.cc b/src/nix/main.cc
index 008482be3..8aaf08813 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -10,6 +10,7 @@
 #include "filetransfer.hh"
 #include "finally.hh"
 #include "loggers.hh"
+#include "markdown.hh"
 
 #include <sys/types.h>
 #include <sys/socket.h>
@@ -163,9 +164,43 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
     }
 };
 
-static void showHelp(std::vector<std::string> subcommand)
+/* Render the help for the specified subcommand to stdout using
+   lowdown. */
+static void showHelp(std::vector<std::string> subcommand, MultiCommand & toplevel)
 {
-    showManPage(subcommand.empty() ? "nix" : fmt("nix3-%s", concatStringsSep("-", subcommand)));
+    auto mdName = subcommand.empty() ? "nix" : fmt("nix3-%s", concatStringsSep("-", subcommand));
+
+    evalSettings.restrictEval = false;
+    evalSettings.pureEval = false;
+    EvalState state({}, openStore("dummy://"));
+
+    auto vGenerateManpage = state.allocValue();
+    state.eval(state.parseExprFromString(
+        #include "generate-manpage.nix.gen.hh"
+        , "/"), *vGenerateManpage);
+
+    auto vUtils = state.allocValue();
+    state.cacheFile(
+        "/utils.nix", "/utils.nix",
+        state.parseExprFromString(
+            #include "utils.nix.gen.hh"
+            , "/"),
+        *vUtils);
+
+    auto vJson = state.allocValue();
+    mkString(*vJson, toplevel.toJSON().dump());
+
+    auto vRes = state.allocValue();
+    state.callFunction(*vGenerateManpage, *vJson, *vRes, noPos);
+
+    auto attr = vRes->attrs->get(state.symbols.create(mdName + ".md"));
+    if (!attr)
+        throw UsageError("Nix has no subcommand '%s'", concatStringsSep("", subcommand));
+
+    auto markdown = state.forceString(*attr->value);
+
+    RunPager pager;
+    std::cout << renderMarkdownToTerminal(markdown) << "\n";
 }
 
 struct CmdHelp : Command
@@ -194,7 +229,10 @@ struct CmdHelp : Command
 
     void run() override
     {
-        showHelp(subcommand);
+        assert(parent);
+        MultiCommand * toplevel = parent;
+        while (toplevel->parent) toplevel = toplevel->parent;
+        showHelp(subcommand, *toplevel);
     }
 };
 
@@ -277,7 +315,7 @@ void mainWrapped(int argc, char * * argv)
             } else
                 break;
         }
-        showHelp(subcommand);
+        showHelp(subcommand, args);
         return;
     } catch (UsageError &) {
         if (!completions) throw;