Check import opt-in before processing source headers

facade_source_version_mode_test.go

@@ -78,6 +78,41 @@ func TestFacadeSourceVersionEvidenceRedactsCollidingShapes(t *testing.T) {
 	}
 }
 
+func TestFacadeSourceVersionModesPreserveImportOptIn(t *testing.T) {
+	t.Parallel()
+	for _, mode := range []configkit.SourceVersionMode{configkit.SourceVersionModeIfPresent, configkit.SourceVersionModeRequired} {
+		for _, underlay := range []bool{false, true} {
+			bad := configkit.Bytes("disabled-imports", []byte("version: accepted\nconfigkit: {imports: [{path: missing.yml}]}\nvalue: visible\n"))
+			definition := configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+				Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { return nil },
+			}}
+			var err error
+			var resultPresent bool
+			wantSource := "source-1"
+			if underlay {
+				loaded, loadErr := configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte("version: main\n"))}, configkit.LayeredOptions[map[string]any]{
+					Definition: definition, SourceVersionMode: mode,
+					Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: bad}}}},
+				})
+				err, resultPresent, wantSource = loadErr, loaded != nil, "source-2"
+			} else {
+				loaded, loadErr := configkit.Load(bad, configkit.Options[map[string]any]{Definition: definition, SourceVersionMode: mode})
+				err, resultPresent = loadErr, loaded != nil
+			}
+			var failure *configkit.LoadError
+			if resultPresent || !errors.As(err, &failure) {
+				t.Errorf("mode=%v underlay=%v: import opt-in did not fail safely", mode, underlay)
+				continue
+			}
+			diagnostics := failure.Diagnostics()
+			if len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeImportDisabled || diagnostics[0].Path.String() != "/configkit" ||
+				diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != wantSource {
+				t.Errorf("mode=%v underlay=%v: diagnostics=%#v", mode, underlay, diagnostics)
+			}
+		}
+	}
+}
+
 func (reader *sourceVersionModeReader) Read(data []byte) (int, error) {
 	reader.reads++
 	return reader.Reader.Read(data)