Define main-relative input options

internal/engine/load_request.go

@@ -16,6 +16,7 @@ type loadRequest[T any] struct {
 	effectiveVersion  *VersionRule
 	overlay           string
 	overlayFilesystem *FilesystemContext
+	overlayFallback   *FilesystemContext
 	overrides         []Override
 	ordinary          bool
 }
@@ -31,6 +32,7 @@ type loadInput struct {
 	source        Source
 	imports       bool
 	filesystem    *FilesystemContext
+	mainRelative  *MainRelativeContext
 	destination   Path
 	placementMode PlacementMode
 }
@@ -57,6 +59,9 @@ func loadGroupedWithSecretLookupLimit[T any](request loadRequest[T], secrets Sec
 	request.importDefaults = defaults
 	request.sequencePolicy = policy
 	request.definition.sequenceIdentityRegistrations = nil
+	if request.main.mainRelative != nil && request.main.filesystem == nil {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "main input cannot use main-relative context"))
+	}
 	main, mainDiagnostics := normalizeInput(request.main, "")
 	request.main = main
 	diagnostics = append(diagnostics, mainDiagnostics...)
@@ -64,6 +69,13 @@ func loadGroupedWithSecretLookupLimit[T any](request loadRequest[T], secrets Sec
 	for groupIndex := range request.underlays {
 		for inputIndex := range request.underlays[groupIndex].inputs {
 			input := &request.underlays[groupIndex].inputs[inputIndex]
+			if input.mainRelative != nil && input.filesystem == nil {
+				if input.source.kind == sourceFile || input.source.kind == sourceFilesystem || input.source.kind == sourceSearch {
+					diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "file and search underlays cannot use main-relative context"))
+				} else if input.mainRelative.Fallback != nil {
+					diagnostics = append(diagnostics, filesystemOptionDiagnostics(Source{kind: sourceBytes}, input.mainRelative.Fallback)...)
+				}
+			}
 			normalized, inputDiagnostics := normalizeInput(*input, "")
 			*input = normalized
 			diagnostics = append(diagnostics, inputDiagnostics...)
@@ -71,10 +83,10 @@ func loadGroupedWithSecretLookupLimit[T any](request loadRequest[T], secrets Sec
 	}
 	diagnostics = append(diagnostics, overlayOptionDiagnostics(request.overlay)...)
 	if activeOverlay(request.overlay) {
-		if request.overlayFilesystem == nil {
-			diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "selected overlay requires explicit filesystem context"))
-		} else {
+		if request.overlayFilesystem != nil {
 			diagnostics = append(diagnostics, filesystemOptionDiagnostics(Source{kind: sourceBytes}, request.overlayFilesystem)...)
+		} else if request.overlayFallback != nil {
+			diagnostics = append(diagnostics, filesystemOptionDiagnostics(Source{kind: sourceBytes}, request.overlayFallback)...)
 		}
 	}
 	if len(diagnostics) != 0 {