From 7bafc99f200064b4efec56cc6112c23feff59f40 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Wed, 11 Jun 2025 16:34:36 +0200
Subject: [PATCH 1/3] Run nixpkgsLibTests in lazy trees mode

---
 packaging/hydra.nix | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/packaging/hydra.nix b/packaging/hydra.nix
index 664ee18ca..6df878239 100644
--- a/packaging/hydra.nix
+++ b/packaging/hydra.nix
@@ -187,6 +187,19 @@ in
           pkgs = nixpkgsFor.${system}.native;
         }
       );
+
+      nixpkgsLibTestsLazy = forAllSystems (
+        system:
+        lib.overrideDerivation
+          (import (nixpkgs + "/lib/tests/test-with-nix.nix") {
+            lib = nixpkgsFor.${system}.native.lib;
+            nix = self.packages.${system}.nix-cli;
+            pkgs = nixpkgsFor.${system}.native;
+          })
+          (_: {
+            "NIX_CONFIG" = "lazy-trees = true";
+          })
+      );
     };
 
   metrics.nixpkgs = import "${nixpkgs-regression}/pkgs/top-level/metrics.nix" {

From df7a2a1bed5af129f6f192bf30cdfdd022c62f1c Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Wed, 11 Jun 2025 17:14:33 +0200
Subject: [PATCH 2/3] nix-instantiate --eval --json: Devirtualize the output

---
 src/nix-instantiate/nix-instantiate.cc | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index 89a8505bb..6f386a5d8 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -17,6 +17,7 @@
 #include <map>
 #include <iostream>
 
+#include <nlohmann/json.hpp>
 
 using namespace nix;
 
@@ -61,8 +62,8 @@ void processExpr(EvalState & state, const Strings & attrPaths,
             else if (output == okXML)
                 printValueAsXML(state, strict, location, vRes, std::cout, context, noPos);
             else if (output == okJSON) {
-                printValueAsJSON(state, strict, vRes, v.determinePos(noPos), std::cout, context);
-                std::cout << std::endl;
+                auto j = printValueAsJSON(state, strict, vRes, v.determinePos(noPos), context);
+                std::cout << state.devirtualize(j.dump(), context) << std::endl;
             } else {
                 if (strict) state.forceValueDeep(vRes);
                 std::set<const void *> seen;

From 31e58dcb92b944a54a834d9b85a573a78b19acc5 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Wed, 11 Jun 2025 17:18:42 +0200
Subject: [PATCH 3/3] nix-instantiate --eval --xml: Devirtualize the output

---
 src/nix-instantiate/nix-instantiate.cc | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index 6f386a5d8..f327454ec 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -59,12 +59,16 @@ void processExpr(EvalState & state, const Strings & attrPaths,
                         context);
                 // We intentionally don't output a newline here. The default PS1 for Bash in NixOS starts with a newline
                 // and other interactive shells like Zsh are smart enough to print a missing newline before the prompt.
-            else if (output == okXML)
-                printValueAsXML(state, strict, location, vRes, std::cout, context, noPos);
+            else if (output == okXML) {
+                std::ostringstream s;
+                printValueAsXML(state, strict, location, vRes, s, context, noPos);
+                std::cout << state.devirtualize(s.str(), context);
+            }
             else if (output == okJSON) {
                 auto j = printValueAsJSON(state, strict, vRes, v.determinePos(noPos), context);
                 std::cout << state.devirtualize(j.dump(), context) << std::endl;
-            } else {
+            }
+            else {
                 if (strict) state.forceValueDeep(vRes);
                 std::set<const void *> seen;
                 printAmbiguous(state, vRes, std::cout, &seen, std::numeric_limits<int>::max());