Run the Nix test suite with lazy trees enabled

flake.nix

@@ -220,6 +220,12 @@
             '';
           repl-completion = nixpkgsFor.${system}.native.callPackage ./tests/repl-completion.nix { };
 
+          lazyTrees =
+            nixpkgsFor.${system}.native.nixComponents2.nix-functional-tests.override {
+              pname = "nix-lazy-trees-tests";
+              lazyTrees = true;
+            };
+
           /**
             Checks for our packaging expressions.
             This shouldn't build anything significant; just check that things

tests/functional/common/init.sh

@@ -54,6 +54,7 @@ flake-registry = $TEST_ROOT/registry.json
 show-trace = true
 include nix.conf.extra
 trusted-users = $(whoami)
+${_NIX_TEST_EXTRA_CONFIG:-}
 EOF
 
 cat > "$NIX_CONF_DIR"/nix.conf.extra <<EOF

tests/functional/package.nix

@@ -26,6 +26,9 @@
 
   # For running the functional tests against a different pre-built Nix.
   test-daemon ? null,
+
+  # Whether to run tests with lazy trees enabled.
+  lazyTrees ? false
 }:
 
 let
@@ -95,6 +98,8 @@ mkMesonDerivation (
       mkdir $out
     '';
 
+    _NIX_TEST_EXTRA_CONFIG = lib.optionalString lazyTrees "lazy-trees = true";
+
     meta = {
       platforms = lib.platforms.unix;
     };