Restore ambiguous value printer for nix-instantiate

The Nix team has requested that this output format remain unchanged.
I've added a warning to the man page explaining that `nix-instantiate
--eval` output will not parse correctly in many situations.

src/libexpr/print-ambiguous.cc

@@ -0,0 +1,100 @@
+#include "print-ambiguous.hh"
+#include "print.hh"
+#include "signals.hh"
+
+namespace nix {
+
+// See: https://github.com/NixOS/nix/issues/9730
+void printAmbiguous(
+    Value &v,
+    const SymbolTable &symbols,
+    std::ostream &str,
+    std::set<const void *> *seen,
+    int depth)
+{
+    checkInterrupt();
+
+    if (depth <= 0) {
+        str << "«too deep»";
+        return;
+    }
+    switch (v.type()) {
+    case nInt:
+        str << v.integer;
+        break;
+    case nBool:
+        printLiteralBool(str, v.boolean);
+        break;
+    case nString:
+        printLiteralString(str, v.string_view());
+        break;
+    case nPath:
+        str << v.path().to_string(); // !!! escaping?
+        break;
+    case nNull:
+        str << "null";
+        break;
+    case nAttrs: {
+        if (seen && !v.attrs->empty() && !seen->insert(v.attrs).second)
+            str << "«repeated»";
+        else {
+            str << "{ ";
+            for (auto & i : v.attrs->lexicographicOrder(symbols)) {
+                str << symbols[i->name] << " = ";
+                printAmbiguous(*i->value, symbols, str, seen, depth - 1);
+                str << "; ";
+            }
+            str << "}";
+        }
+        break;
+    }
+    case nList:
+        if (seen && v.listSize() && !seen->insert(v.listElems()).second)
+            str << "«repeated»";
+        else {
+            str << "[ ";
+            for (auto v2 : v.listItems()) {
+                if (v2)
+                    printAmbiguous(*v2, symbols, str, seen, depth - 1);
+                else
+                    str << "(nullptr)";
+                str << " ";
+            }
+            str << "]";
+        }
+        break;
+    case nThunk:
+        if (!v.isBlackhole()) {
+            str << "<CODE>";
+        } else {
+            // Although we know for sure that it's going to be an infinite recursion
+            // when this value is accessed _in the current context_, it's likely
+            // that the user will misinterpret a simpler «infinite recursion» output
+            // as a definitive statement about the value, while in fact it may be
+            // a valid value after `builtins.trace` and perhaps some other steps
+            // have completed.
+            str << "«potential infinite recursion»";
+        }
+        break;
+    case nFunction:
+        if (v.isLambda()) {
+            str << "<LAMBDA>";
+        } else if (v.isPrimOp()) {
+            str << "<PRIMOP>";
+        } else if (v.isPrimOpApp()) {
+            str << "<PRIMOP-APP>";
+        }
+        break;
+    case nExternal:
+        str << *v.external;
+        break;
+    case nFloat:
+        str << v.fpoint;
+        break;
+    default:
+        printError("Nix evaluator internal error: printAmbiguous: invalid value type");
+        abort();
+    }
+}
+
+}