Define main-relative input options

layered.go

@@ -19,8 +19,19 @@ type Input struct {
 	PlacementMode PlacementMode
 	Imports       bool
 	Filesystem    *FilesystemContext
+	MainRelative  *MainRelativeContext
 }
 
+// MainRelativeContext selects the main input's resolved context for an underlay.
+// Fallback is used when the main has no usable context; RequireBase requires one.
+type MainRelativeContext struct {
+	Fallback    *FilesystemContext
+	RequireBase bool
+}
+
+// MainRelative returns a fresh default main-relative binding description.
+func MainRelative() *MainRelativeContext { return &MainRelativeContext{} }
+
 // Underlay contains inputs that must compose strictly within one tier. Tiers
 // appear in lowest-to-highest precedence order; an empty tier is a no-op.
 type Underlay struct{ Inputs []Input }
@@ -42,6 +53,7 @@ type LayeredOptions[T any] struct {
 	Underlays         []Underlay
 	Overlay           string
 	OverlayFilesystem *FilesystemContext
+	OverlayFallback   *FilesystemContext
 	Overrides         []Override
 }
 
@@ -68,6 +80,9 @@ func toEngineInput(input Input) engine.Input {
 		Filesystem: toEngineFilesystemContext(input.Filesystem), Destination: toEnginePath(input.Destination),
 		PlacementMode: engine.PlacementMode(input.PlacementMode),
 	}
+	if input.MainRelative != nil {
+		result.MainRelative = &engine.MainRelativeContext{Fallback: toEngineFilesystemContext(input.MainRelative.Fallback), RequireBase: input.MainRelative.RequireBase}
+	}
 	return result
 }
 
@@ -77,6 +92,7 @@ func toEngineLayeredOptions[T any](options LayeredOptions[T]) engine.LayeredOpti
 		SourceVersionMode: engine.SourceVersionModeFromFacade(uint8(options.SourceVersionMode)),
 		Overlay:           options.Overlay,
 		OverlayFilesystem: toEngineFilesystemContext(options.OverlayFilesystem),
+		OverlayFallback:   toEngineFilesystemContext(options.OverlayFallback),
 		ImportDefaults:    toEngineImportDefaults(options.ImportDefaults),
 	}
 	if options.Underlays != nil {