Allows selectively adding environment variables to pure shells.

Includes documentation and test.

src/nix-build/nix-build.cc

@@ -99,6 +99,9 @@ void mainWrapped(int argc, char * * argv)
 
     std::string outLink = "./result";
 
+    // List of environment variables kept for --pure
+    std::set<string> keepVars{"HOME", "USER", "LOGNAME", "DISPLAY", "PATH", "TERM", "IN_NIX_SHELL", "TZ", "PAGER", "NIX_BUILD_SHELL", "SHLVL"};
+
     Strings args;
     for (int i = 1; i < argc; ++i)
         args.push_back(argv[i]);
@@ -218,6 +221,9 @@ void mainWrapped(int argc, char * * argv)
             }
         }
 
+        else if (*arg == "--keep")
+            keepVars.insert(getArg(*arg, arg, end));
+
         else if (*arg == "-")
             readStdin = true;
 
@@ -368,7 +374,6 @@ void mainWrapped(int argc, char * * argv)
         auto tmp = getEnv("TMPDIR", getEnv("XDG_RUNTIME_DIR", "/tmp"));
 
         if (pure) {
-            std::set<string> keepVars{"HOME", "USER", "LOGNAME", "DISPLAY", "PATH", "TERM", "IN_NIX_SHELL", "TZ", "PAGER", "NIX_BUILD_SHELL", "SHLVL"};
             decltype(env) newEnv;
             for (auto & i : env)
                 if (keepVars.count(i.first))