bundler-env: add .env

pkgs/development/interpreters/ruby/bundler-env/default.nix

@@ -246,9 +246,7 @@ let
       (map mkBuildFlags
         (lib.filter needsBuildFlags (attrValues instantiated)));
 
-in
+  derivation = stdenv.mkDerivation {
-
-stdenv.mkDerivation {
     inherit name;
 
     buildInputs = [
@@ -306,7 +304,36 @@ stdenv.mkDerivation {
     passthru = {
       inherit ruby;
       inherit bundler;
+
+      env = let
+        irbrc = builtins.toFile "irbrc" ''
+          if not ENV["OLD_IRBRC"].empty?
+            require ENV["OLD_IRBRC"]
+          end
+          require 'rubygems'
+          require 'bundler/setup'
+        '';
+        in stdenv.mkDerivation {
+          name = "interactive-${name}-environment";
+          nativeBuildInputs = [ ruby derivation ];
+          shellHook = ''
+            export BUNDLE_GEMFILE=${derivation.bundle}/Gemfile
+            export GEM_HOME=${derivation}/${ruby.gemPath}
+            export NIX_BUNDLER_GEMPATH=${bundler}/${ruby.gemPath}
+            export GEM_PATH=$NIX_BUNDLER_GEMPATH:$GEM_HOME
+            export OLD_IRBRC="$IRBRC"
+            export IRBRC=${irbrc}
+          '';
+          buildCommand = ''
+            echo >&2 ""
+            echo >&2 "*** Ruby 'env' attributes are intended for interactive nix-shell sessions, not for building! ***"
+            echo >&2 ""
+            exit 1
+          '';
+        };
     };
 
     inherit meta;
-}
+  };
+
+in derivation