Restore mkBlackhole() for testing

Author: edolstra

src/libexpr-tests/value/print.cc

@@ -185,14 +185,12 @@ TEST_F(ValuePrintingTests, vFloat)
     test(vFloat, "2");
 }
 
-#if 0
 TEST_F(ValuePrintingTests, vBlackhole)
 {
     Value vBlackhole;
     vBlackhole.mkBlackhole();
     test(vBlackhole, "«potential infinite recursion»");
 }
-#endif
 
 TEST_F(ValuePrintingTests, depthAttrs)
 {
@@ -632,7 +630,6 @@ TEST_F(ValuePrintingTests, ansiColorsThunk)
          });
 }
 
-#if 0
 TEST_F(ValuePrintingTests, ansiColorsBlackhole)
 {
     Value v;
@@ -644,7 +641,6 @@ TEST_F(ValuePrintingTests, ansiColorsBlackhole)
              .ansiColors = true
          });
 }
-#endif
 
 TEST_F(ValuePrintingTests, ansiColorsAttrsRepeated)
 {

src/libexpr/include/nix/expr/value.hh

@@ -478,6 +478,12 @@ public:
         finishValue(tLambda, { .lambda = { .env = e, .fun = f } });
     }
 
+    /// Only used for testing.
+    inline void mkBlackhole()
+    {
+        internalType = tPending;
+    }
+
     void mkPrimOp(PrimOp * p);
 
     inline void mkPrimOpApp(Value * l, Value * r)