Make explicit metadata queries lightweight

internal/engine/metadata_test.go

@@ -285,6 +285,75 @@ func TestResolutionInputKeepsDetachedEntryMetadata(t *testing.T) {
 	}
 }
 
+func TestMetadataIsExplicitAgreesWithLookup(t *testing.T) {
+	t.Parallel()
+
+	type entry struct {
+		Kept    string `yaml:"kept"`
+		Set     string `yaml:"set"`
+		Enabled bool   `yaml:"enabled"`
+		Count   int    `yaml:"count"`
+	}
+	type config struct {
+		Nested   entry            `yaml:"nested"`
+		Nullable *string          `yaml:"nullable"`
+		Nodes    map[string]entry `yaml:"nodes"`
+	}
+
+	input := []byte(`nested: {set: configured, enabled: false, count: 0}
+nullable: null
+nodes:
+  base: {enabled: false, count: 0}
+  copy: {extends: base}
+`)
+	result, err := testLoad(Bytes("metadata-presence.yml", input), Options[config]{Definition: Definition[config]{
+		Defaults: func() config {
+			return config{
+				Nested: entry{Kept: "default", Set: "default", Enabled: true, Count: 7},
+				Nodes:  map[string]entry{"copy": {Kept: "node default"}},
+			}
+		},
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	metadata := result.Metadata()
+	tests := []struct {
+		name          string
+		path          Path
+		wantExplicit  bool
+		wantNull      bool
+		wantInherited bool
+	}{
+		{name: "default", path: Keys("nested", "kept")},
+		{name: "direct input", path: Keys("nested", "set"), wantExplicit: true},
+		{name: "explicit false", path: Keys("nested", "enabled"), wantExplicit: true},
+		{name: "explicit zero", path: Keys("nested", "count"), wantExplicit: true},
+		{name: "explicit null", path: Keys("nullable"), wantExplicit: true, wantNull: true},
+		{name: "inherited input", path: Keys("nodes", "copy", "enabled"), wantExplicit: true, wantInherited: true},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			info, found := metadata.Lookup(test.path)
+			if !found {
+				t.Fatalf("Lookup(%s) did not find an effective path", test.path)
+			}
+			if got := metadata.IsExplicit(test.path); got != info.Explicit || got != test.wantExplicit {
+				t.Fatalf("IsExplicit(%s) = %v, Lookup explicit = %v, want %v", test.path, got, info.Explicit, test.wantExplicit)
+			}
+			if info.Null != test.wantNull {
+				t.Fatalf("Lookup(%s).Null = %v, want %v", test.path, info.Null, test.wantNull)
+			}
+			if inherited := len(info.Origin.Inheritance) != 0; inherited != test.wantInherited {
+				t.Fatalf("Lookup(%s) inherited = %v, want %v", test.path, inherited, test.wantInherited)
+			}
+		})
+	}
+	if result.Config.Nested.Enabled || result.Config.Nested.Count != 0 || result.Config.Nullable != nil {
+		t.Fatalf("explicit values changed during loading: %#v", result.Config)
+	}
+}
+
 func TestQueriesRespectActualHistoricalAndEffectiveContainers(t *testing.T) {
 	result, err := testLoad[map[string]any](Bytes("base", []byte("items: [1]\nm: {'0': 2}\nx: 3\nretired: {child: 4}\n")), Options[map[string]any]{Overrides: []Override{{Path: Keys("retired"), Value: "removed", Flag: "--retired"}}})
 	if err != nil {