Fix handling of experimental features mid-parse

If we conditionally "declare" the argument, as we did before, based upon
weather the feature is enabled, commands like

    nix --experimental-features=foo ... --thing-gated-on-foo

won't work, because the experimental feature isn't enabled until *after*
we start parsing.

Instead, allow arguments to also be associated with experimental
features (just as we did for builtins and settings), and then the
command line parser will filter out the experimental ones.

Since the effects of arguments (handler functions) are performed right
away, we get the required behavior: earlier arguments can enable later
arguments enabled!

There is just one catch: we want to keep non-positional
flags...non-positional. So if

    nix --experimental-features=foo ... --thing-gated-on-foo

works, then

    nix --thing-gated-on-foo --experimental-features=foo ...

should also work.

This is not my favorite long-term solution, but for now this is
implemented by delaying the requirement of needed experimental features
until *after* all the arguments have been parsed.

1 files changed, 22 insertions, 3 deletions

diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index 35686a8aa..fc009592c 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -52,7 +52,7 @@ std::shared_ptr<Completions> completions;
 
 std::string completionMarker = "___COMPLETE___";
 
-std::optional<std::string> needsCompletion(std::string_view s)
+static std::optional<std::string> needsCompletion(std::string_view s)
 {
     if (!completions) return {};
     auto i = s.find(completionMarker);
@@ -120,6 +120,12 @@ void Args::parseCmdline(const Strings & _cmdline)
 
     if (!argsSeen)
         initialFlagsProcessed();
+
+    /* Now that we are done parsing, make sure that any experimental
+     * feature required by the flags is enabled */
+    for (auto & f : flagExperimentalFeatures)
+        experimentalFeatureSettings.require(f);
+
 }
 
 bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
@@ -128,12 +134,18 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
 
     auto process = [&](const std::string & name, const Flag & flag) -> bool {
         ++pos;
+
+        if (auto & f = flag.experimentalFeature)
+            flagExperimentalFeatures.insert(*f);
+
         std::vector<std::string> args;
         bool anyCompleted = false;
         for (size_t n = 0 ; n < flag.handler.arity; ++n) {
             if (pos == end) {
                 if (flag.handler.arity == ArityAny || anyCompleted) break;
-                throw UsageError("flag '%s' requires %d argument(s)", name, flag.handler.arity);
+                throw UsageError(
+                    "flag '%s' requires %d argument(s), but only %d were given",
+                    name, flag.handler.arity, n);
             }
             if (auto prefix = needsCompletion(*pos)) {
                 anyCompleted = true;
@@ -152,7 +164,11 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
             for (auto & [name, flag] : longFlags) {
                 if (!hiddenCategories.count(flag->category)
                     && hasPrefix(name, std::string(*prefix, 2)))
+                {
+                    if (auto & f = flag->experimentalFeature)
+                        flagExperimentalFeatures.insert(*f);
                     completions->add("--" + name, flag->description);
+                }
             }
             return false;
         }
@@ -172,7 +188,8 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
         if (prefix == "-") {
             completions->add("--");
             for (auto & [flagName, flag] : shortFlags)
-                completions->add(std::string("-") + flagName, flag->description);
+                if (experimentalFeatureSettings.isEnabled(flag->experimentalFeature))
+                    completions->add(std::string("-") + flagName, flag->description);
         }
     }
 
@@ -219,6 +236,8 @@ nlohmann::json Args::toJSON()
     auto flags = nlohmann::json::object();
 
     for (auto & [name, flag] : longFlags) {
+        /* Skip experimental flags when listing flags. */
+        if (!experimentalFeatureSettings.isEnabled(flag->experimentalFeature)) continue;
         auto j = nlohmann::json::object();
         if (flag->aliases.count(name)) continue;
         if (flag->shortName)