Allow overring module options from the command line

E.g.

  $ nix build github:tweag/nix-ux/configs?dir=configs#hello --argstr who Everybody
  $ ./result/bin/hello
  Hello Everybody

This works by generating a new flake that imports the specified one
and sets the specified module options.

src/nix/bundle.cc

@@ -78,7 +78,7 @@ struct CmdBundle : InstallableCommand
         auto bundler = InstallableFlake(
             evalState, std::move(bundlerFlakeRef),
             Strings{bundlerName == "" ? "defaultBundler" : bundlerName},
-            Strings({"bundlers."}), lockFlags);
+            Strings({"bundlers."}), lockFlags, nullptr);
 
         Value * arg = evalState->allocValue();
         evalState->mkAttrs(*arg, 2);

src/nix/develop.cc

@@ -395,7 +395,8 @@ struct CmdDevelop : Common, MixEnvironment
                 installable->nixpkgsFlakeRef(),
                 Strings{"bashInteractive"},
                 Strings{"legacyPackages." + settings.thisSystem.get() + "."},
-                lockFlags);
+                lockFlags,
+                nullptr);
 
             shell = state->store->printStorePath(
                 toStorePath(state->store, Realise::Outputs, OperateOn::Output, bashInstallable)) + "/bin/bash";

src/nix/flake.cc

@@ -563,7 +563,7 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
         auto installable = InstallableFlake(
             evalState, std::move(templateFlakeRef),
             Strings{templateName == "" ? "defaultTemplate" : templateName},
-            Strings(attrsPathPrefixes), lockFlags);
+            Strings(attrsPathPrefixes), lockFlags, nullptr);
 
         auto [cursor, attrPath] = installable.getCursor(*evalState);
 

src/nix/installables.cc

@@ -13,6 +13,8 @@
 #include "url.hh"
 #include "registry.hh"
 
+#include "../cpptoml/cpptoml.h"
+
 #include <regex>
 #include <queue>
 
@@ -549,8 +551,61 @@ InstallableFlake::getCursors(EvalState & state)
 
 std::shared_ptr<flake::LockedFlake> InstallableFlake::getLockedFlake() const
 {
-    if (!_lockedFlake)
+    if (!_lockedFlake) {
         _lockedFlake = std::make_shared<flake::LockedFlake>(lockFlake(*state, flakeRef, lockFlags));
+
+        if (options && options->size()) {
+            /* Get the modules defined by this flake. */
+            auto cache = openEvalCache(*state, _lockedFlake);
+            auto root = cache->getRoot();
+            auto aModules = root->maybeGetAttr(state->symbols.create("modules"));
+            if (!aModules)
+                throw Error("flake '%s' has no modules, so --arg cannot override anything", flakeRef);
+
+            auto toml = cpptoml::make_table();
+
+            auto base = cpptoml::make_table();
+            base->insert("type", "path");
+            base->insert("path", _lockedFlake->flake.sourceInfo->actualPath);
+            if (_lockedFlake->flake.lockedRef.subdir != "")
+                base->insert("dir", _lockedFlake->flake.lockedRef.subdir);
+            // FIXME: copy rev etc.
+            auto inputs = cpptoml::make_table();
+            inputs->insert("base", base);
+
+            toml->insert("inputs", inputs);
+
+            for (auto & moduleName : aModules->getAttrs()) {
+                auto module = cpptoml::make_table();
+                auto extends = cpptoml::make_array();
+                extends->push_back("base#" + (std::string) moduleName);
+                module->insert("extends", extends);
+                for (auto & option : options->lexicographicOrder()) {
+                    state->forceValue(*option->value);
+                    if (option->value->type == tString)
+                        module->insert(option->name, state->forceString(*option->value));
+                    else if (option->value->type == tInt)
+                        module->insert(option->name, option->value->integer);
+                    else
+                        throw Error("option '%s' is %s which is not supported",
+                            option->name, showType(*option->value));
+                }
+                toml->insert(moduleName, module);
+            }
+
+            auto tempDir = createTempDir();
+            AutoDelete cleanup(tempDir);
+
+            std::ostringstream str;
+            str << *toml;
+            debug("writing temporary flake:\n%s", str.str());
+            writeFile(tempDir + "/nix.toml", str.str());
+
+            _lockedFlake = std::make_shared<flake::LockedFlake>(lockFlake(*state,
+                    parseFlakeRef("path://" + tempDir), lockFlags));
+        }
+    }
+
     return _lockedFlake;
 }
 
@@ -600,10 +655,14 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
 
             try {
                 auto [flakeRef, fragment] = parseFlakeRefWithFragment(s, absPath("."));
+                auto state = getEvalState();
                 result.push_back(std::make_shared<InstallableFlake>(
-                        getEvalState(), std::move(flakeRef),
+                        state,
+                        std::move(flakeRef),
                         fragment == "" ? getDefaultFlakeAttrPaths() : Strings{fragment},
-                        getDefaultFlakeAttrPathPrefixes(), lockFlags));
+                        getDefaultFlakeAttrPathPrefixes(),
+                        lockFlags,
+                        getAutoArgs(*state)));
                 continue;
             } catch (...) {
                 ex = std::current_exception();

src/nix/installables.hh

@@ -98,11 +98,17 @@ struct InstallableFlake : InstallableValue
     Strings prefixes;
     const flake::LockFlags & lockFlags;
     mutable std::shared_ptr<flake::LockedFlake> _lockedFlake;
-
-    InstallableFlake(ref<EvalState> state, FlakeRef && flakeRef,
-        Strings && attrPaths, Strings && prefixes, const flake::LockFlags & lockFlags)
+    Bindings * options;
+
+    InstallableFlake(
+        ref<EvalState> state,
+        FlakeRef && flakeRef,
+        Strings && attrPaths,
+        Strings && prefixes, const
+        flake::LockFlags & lockFlags,
+        Bindings * options)
         : InstallableValue(state), flakeRef(flakeRef), attrPaths(attrPaths),
-          prefixes(prefixes), lockFlags(lockFlags)
+          prefixes(prefixes), lockFlags(lockFlags), options(options)
     { }
 
     std::string what() override { return flakeRef.to_string() + "#" + *attrPaths.begin(); }

src/nix/profile.cc

@@ -342,7 +342,11 @@ struct CmdProfileUpgrade : virtual SourceExprCommand, MixDefaultProfile, MixProf
                 Activity act(*logger, lvlChatty, actUnknown,
                     fmt("checking '%s' for updates", element.source->attrPath));
 
-                InstallableFlake installable(getEvalState(), FlakeRef(element.source->originalRef), {element.source->attrPath}, {}, lockFlags);
+                InstallableFlake installable(
+                    getEvalState(),
+                    FlakeRef(element.source->originalRef),
+                    {element.source->attrPath},
+                    {}, lockFlags, nullptr);
 
                 auto [attrPath, resolvedRef, drv] = installable.toDerivation();