Add feature to disable URL literals

E.g.

  $ nix-build '<nixpkgs>' -A hello --experimental-features no-url-literals
  error: URL literals are disabled, at /nix/store/vsjamkzh15r3c779q2711az826hqgvzr-nixpkgs-20.03pre194957.bef773ed53f/nixpkgs/pkgs/top-level/all-packages.nix:1236:11

Helps with implementing https://github.com/NixOS/rfcs/pull/45.

1 files changed, 7 insertions, 2 deletions

diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index aa0823f41..6d3b95740 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -105,10 +105,15 @@ StringSet Settings::getDefaultSystemFeatures()
     return features;
 }
 
-void Settings::requireExperimentalFeature(const std::string & name)
+bool Settings::isExperimentalFeatureEnabled(const std::string & name)
 {
     auto & f = experimentalFeatures.get();
-    if (std::find(f.begin(), f.end(), name) == f.end())
+    return std::find(f.begin(), f.end(), name) != f.end();
+}
+
+void Settings::requireExperimentalFeature(const std::string & name)
+{
+    if (!isExperimentalFeatureEnabled(name))
         throw Error("experimental Nix feature '%s' is disabled", name);
 }