libmain/shared: factor out nix features

This way, other parts of the codebase can use it.

Change-Id: I58673a6a56362692980426a6c8f70f5c6aff24d4
Signed-off-by: Raito Bezarius <raito@lix.systems>

lix/libmain/shared.cc

@@ -21,6 +21,7 @@
 #include "lix/libutil/terminal.hh"
 #include "lix/libutil/strings.hh"
 #include "lix/libutil/exit.hh"
+#include "lix/libutil/compile-time-features.hh"
 
 #include <algorithm>
 #include <exception>
@@ -302,14 +303,9 @@ bool LegacyArgs::processArgs(const Strings & args, bool finish)
 void printVersion(const std::string & programName)
 {
     std::cout << fmt("%1% (Lix, like Nix) %2%", programName, nixVersion) << std::endl;
-    Strings cfg;
-#if HAVE_BOEHMGC
-    cfg.push_back("gc");
-#endif
-    cfg.push_back("signed-caches");
     std::cout << "System type: " << settings.thisSystem << "\n";
     std::cout << "Additional system types: " << concatStringsSep(", ", settings.extraPlatforms.get()) << "\n";
-    std::cout << "Features: " << concatStringsSep(", ", cfg) << "\n";
+    std::cout << "Features: " << concatStringsSep(", ", getNixFeatures()) << "\n";
     std::cout << "System configuration file: " << settings.nixConfDir + "/nix.conf" << "\n";
     std::cout << "User configuration files: " <<
         concatStringsSep(":", settings.nixUserConfFiles)

lix/libutil/compile-time-features.hh

@@ -0,0 +1,26 @@
+#pragma once
+///@file
+
+#include "lix/libutil/types.hh"
+
+namespace nix {
+/**
+ * This returns a list of compile-time features.
+ *
+ * Historically, `signed-caches` seemed to have been one.
+ * So we include it always.
+ *
+ * NOTE: There's no guarantee that the output will be stable
+ * across Lix versions.
+ */
+constexpr Strings getNixFeatures()
+{
+    Strings features = {"signed-caches"};
+
+#if HAVE_BOEHMGC
+    features.push_back("gc");
+#endif
+
+    return features;
+}
+}

lix/libutil/meson.build

@@ -81,6 +81,7 @@ libutil_headers = files(
   'chunked-vector.hh',
   'closure.hh',
   'comparator.hh',
+  'compile-time-features.hh',
   'compression.hh',
   'compute-levels.hh',
   'concepts.hh',