Define main-relative input options

internal/engine/layered.go

@@ -7,10 +7,16 @@ type Input struct {
 	Source        Source
 	Imports       bool
 	Filesystem    *FilesystemContext
+	MainRelative  *MainRelativeContext
 	Destination   Path
 	PlacementMode PlacementMode
 }
 
+type MainRelativeContext struct {
+	Fallback    *FilesystemContext
+	RequireBase bool
+}
+
 // Underlay contains strictly composed contributions at one precedence tier.
 type Underlay struct{ Inputs []Input }
 
@@ -22,6 +28,7 @@ type LayeredOptions[T any] struct {
 	Underlays         []Underlay
 	Overlay           string
 	OverlayFilesystem *FilesystemContext
+	OverlayFallback   *FilesystemContext
 	Overrides         []Override
 }
 
@@ -37,13 +44,14 @@ func LoadLayered[T any](main Input, secrets SecretBinding, supplied ...LayeredOp
 	}
 	request := loadRequest[T]{
 		main: loadInput{
-			source: main.Source, imports: main.Imports, filesystem: main.Filesystem,
+			source: main.Source, imports: main.Imports, filesystem: main.Filesystem, mainRelative: main.MainRelative,
 			destination: main.Destination.clone(), placementMode: main.PlacementMode,
 		},
 		definition: options.Definition, sourceVersionMode: options.SourceVersionMode,
 		importDefaults:    options.ImportDefaults,
 		overlay:           options.Overlay,
 		overlayFilesystem: options.OverlayFilesystem,
+		overlayFallback:   options.OverlayFallback,
 		overrides:         append([]Override(nil), options.Overrides...),
 		underlays:         make([]loadGroup, len(options.Underlays)),
 	}
@@ -51,7 +59,7 @@ func LoadLayered[T any](main Input, secrets SecretBinding, supplied ...LayeredOp
 		request.underlays[index].inputs = make([]loadInput, len(underlay.Inputs))
 		for inputIndex, input := range underlay.Inputs {
 			request.underlays[index].inputs[inputIndex] = loadInput{
-				source: input.Source, imports: input.Imports, filesystem: input.Filesystem,
+				source: input.Source, imports: input.Imports, filesystem: input.Filesystem, mainRelative: input.MainRelative,
 				destination: input.Destination.clone(), placementMode: input.PlacementMode,
 			}
 		}