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.

src/libexpr/parser.y

@@ -20,6 +20,7 @@
 
 #include "nixexpr.hh"
 #include "eval.hh"
+#include "globals.hh"
 
 namespace nix {
 
@@ -401,7 +402,12 @@ expr_simple
               new ExprVar(data->symbols.create("__nixPath"))),
           new ExprString(data->symbols.create(path)));
   }
-  | URI { $$ = new ExprString(data->symbols.create($1)); }
+  | URI {
+      static bool noURLLiterals = settings.isExperimentalFeatureEnabled("no-url-literals");
+      if (noURLLiterals)
+          throw ParseError("URL literals are disabled, at %s", CUR_POS);
+      $$ = new ExprString(data->symbols.create($1));
+  }
   | '(' expr ')' { $$ = $2; }
   /* Let expressions `let {..., body = ...}' are just desugared
      into `(rec {..., body = ...}).body'. */

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);
 }
 

src/libstore/globals.hh

@@ -357,6 +357,8 @@ public:
     Setting<Strings> experimentalFeatures{this, {}, "experimental-features",
         "Experimental Nix features to enable."};
 
+    bool isExperimentalFeatureEnabled(const std::string & name);
+
     void requireExperimentalFeature(const std::string & name);
 };