printValueAsJSON(): Don't devirtualize

This is already done by consumers of builtins.toJSON (like
builtins.toFile or builtins.derivation), so we can delay this until
it's actually needed.

src/libexpr/print.cc

@@ -249,7 +249,11 @@ private:
 
     void printString(Value & v)
     {
-        printLiteralString(output, v.string_view(), options.maxStringLength, options.ansiColors);
+        NixStringContext context;
+        copyContext(v, context);
+        std::ostringstream s;
+        printLiteralString(s, v.string_view(), options.maxStringLength, options.ansiColors);
+        output << state.devirtualize(s.str(), context);
     }
 
     void printPath(Value & v)

src/libexpr/value-to-json.cc

@@ -32,11 +32,7 @@ json printValueAsJSON(EvalState & state, bool strict,
 
         case nString:
             copyContext(v, context);
-            // FIXME: only use the context from `v`.
+            out = v.c_str();
-            // FIXME: make devirtualization configurable?
-            // FIXME: don't devirtualize here? It's redundant if
-            // 'toFile' or 'derivation' also do it.
-            out = state.devirtualize(v.c_str(), context);
             break;
 
         case nPath:

src/nix/eval.cc

@@ -122,7 +122,10 @@ struct CmdEval : MixJSON, InstallableValueCommand, MixReadOnlyOption
         }
 
         else if (json) {
-            logger->cout("%s", printValueAsJSON(*state, true, *v, pos, context, false));
+            logger->cout("%s",
+                state->devirtualize(
+                    printValueAsJSON(*state, true, *v, pos, context, false).dump(),
+                    context));
         }
 
         else {