From 49a932fb18add471feefc469fb45fc44313bd5c6 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Mon, 13 Sep 2021 14:41:28 +0200
Subject: nix --help: Display help using lowdown instead of man

Fixes #4476.
Fixes #5231.
---
 src/nix/local.mk |  4 +++-
 src/nix/main.cc  | 46 ++++++++++++++++++++++++++++++++++++++++++----
 2 files changed, 45 insertions(+), 5 deletions(-)

(limited to 'src/nix')

diff --git a/src/nix/local.mk b/src/nix/local.mk
index 83b6dd08b..e4ec7634d 100644
--- a/src/nix/local.mk
+++ b/src/nix/local.mk
@@ -14,7 +14,7 @@ nix_SOURCES := \
   $(wildcard src/nix-instantiate/*.cc) \
   $(wildcard src/nix-store/*.cc) \
 
-nix_CXXFLAGS += -I src/libutil -I src/libstore -I src/libfetchers -I src/libexpr -I src/libmain -I src/libcmd
+nix_CXXFLAGS += -I src/libutil -I src/libstore -I src/libfetchers -I src/libexpr -I src/libmain -I src/libcmd -I doc/manual
 
 nix_LIBS = libexpr libmain libfetchers libstore libutil libcmd
 
@@ -30,3 +30,5 @@ src/nix-env/user-env.cc: src/nix-env/buildenv.nix.gen.hh
 src/nix/develop.cc: src/nix/get-env.sh.gen.hh
 
 src/nix-channel/nix-channel.cc: src/nix-channel/unpack-channel.nix.gen.hh
+
+src/nix/main.cc: doc/manual/generate-manpage.nix.gen.hh doc/manual/utils.nix.gen.hh
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;
-- 
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