Add test for output warning to ensure stability

tests/functional/flakes/meson.build

@@ -33,6 +33,7 @@ suites += {
     'debugger.sh',
     'source-paths.sh',
     'old-lockfiles.sh',
+    'trace-ifd.sh',
   ],
   'workdir': meson.current_source_dir(),
 }

tests/functional/flakes/trace-ifd.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+
+source ./common.sh
+
+requireGit
+
+flake1Dir="$TEST_ROOT/flake"
+
+createGitRepo "$flake1Dir"
+createSimpleGitFlake "$flake1Dir"
+
+cat > "$flake1Dir/flake.nix" <<'EOF'
+{
+  outputs = { self }: let inherit (import ./config.nix) mkDerivation; in {
+    drv = mkDerivation {
+      name = "drv";
+      buildCommand = ''
+        echo drv >$out
+      '';
+    };
+
+    ifd = mkDerivation {
+      name = "ifd";
+      buildCommand = ''
+        echo ${builtins.readFile self.drv} >$out
+      '';
+    };
+  };
+}
+EOF
+
+nix build "$flake1Dir#ifd" --option trace-import-from-derivation true 2>&1 \
+  | grepQuiet 'warning: built .* during evaluation due to an import from derivation'