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, 7 insertions, 6 deletions

diff --git a/src/nix/main.cc b/src/nix/main.cc
index da920615f..c79d39459 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -54,12 +54,11 @@ static bool haveInternet()
 
 std::string programPath;
 
-struct HelpRequested { };
-
 struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
 {
     bool useNet = true;
     bool refresh = false;
+    bool helpRequested = false;
     bool showVersion = false;
 
     NixArgs() : MultiCommand(RegisterCommand::getCommandsFor({})), MixCommonArgs("nix")
@@ -74,7 +73,7 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
             .longName = "help",
             .description = "Show usage information.",
             .category = miscCategory,
-            .handler = {[&]() { throw HelpRequested(); }},
+            .handler = {[this]() { this->helpRequested = true; }},
         });
 
         addFlag({
@@ -337,7 +336,11 @@ void mainWrapped(int argc, char * * argv)
 
     try {
         args.parseCmdline(argvToStrings(argc, argv));
-    } catch (HelpRequested &) {
+    } catch (UsageError &) {
+        if (!args.helpRequested && !completions) throw;
+    }
+
+    if (args.helpRequested) {
         std::vector<std::string> subcommand;
         MultiCommand * command = &args;
         while (command) {
@@ -349,8 +352,6 @@ void mainWrapped(int argc, char * * argv)
         }
         showHelp(subcommand, args);
         return;
-    } catch (UsageError &) {
-        if (!completions) throw;
     }
 
     if (completions) {