Allow absent inputs to use import contexts

internal/engine/filesystem.go

@@ -9,6 +9,9 @@ import (
 )
 
 func validateFilesystemContext(source Source, context *FilesystemContext, main parsedSource) *Diagnostic {
+	if source.kind == sourceNone && context.FS == nil {
+		return validateAbsentHostFilesystemContext(context)
+	}
 	if nativeContext(source, context) {
 		access, basePath, mainPath, diagnostic := nativeFilesystemAccess(source, context)
 		if diagnostic != nil {
@@ -33,10 +36,54 @@ func validateFilesystemContext(source Source, context *FilesystemContext, main p
 	return nil
 }
 
+func validateAbsentHostFilesystemContext(context *FilesystemContext) *Diagnostic {
+	rootPath, err := canonicalExistingPath(context.Root)
+	if err != nil {
+		value := importDiagnostic(CodeImportInvalid, "configuration root could not be canonicalized", nil)
+		return &value
+	}
+	basePath := rootPath
+	if context.BaseDir != "" {
+		basePath, err = canonicalExistingPath(context.BaseDir)
+		if err != nil {
+			value := importDiagnostic(CodeImportInvalid, "import base directory could not be canonicalized", nil)
+			return &value
+		}
+	}
+	if !containedPath(rootPath, basePath) {
+		value := importDiagnostic(CodeImportOutsideRoot, "import base must stay inside the configuration root", nil)
+		return &value
+	}
+	for _, candidate := range []string{rootPath, basePath} {
+		info, err := os.Stat(candidate)
+		if err != nil || !info.IsDir() {
+			value := importDiagnostic(CodeImportInvalid, "configuration root and import base must be directories", nil)
+			return &value
+		}
+	}
+	root, err := os.OpenRoot(rootPath)
+	if err != nil {
+		value := globFilesystemDiagnostic(err, "configuration root could not be opened", Location{})
+		value.Primary = nil
+		return &value
+	}
+	if err := root.Close(); err != nil {
+		value := importDiagnostic(CodeImportRead, "configuration root could not be closed", nil)
+		return &value
+	}
+	return nil
+}
+
 func filesystemOptionDiagnostics(source Source, context *FilesystemContext) []Diagnostic {
 	switch source.kind {
 	case sourceNone:
-		return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "NoSource cannot use filesystem context")}
+		if context.FS != nil {
+			if context.FS.namespace == nil || nilFilesystem(context.FS.namespace.filesystem) || context.BaseDir == "" || context.Root == "" || !fs.ValidPath(context.BaseDir) || !fs.ValidPath(context.Root) {
+				return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "native filesystem context requires a namespace, base, and root directory")}
+			}
+		} else if context.Root == "" || !filepath.IsAbs(context.Root) || context.BaseDir != "" && !filepath.IsAbs(context.BaseDir) {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "absent-input filesystem root and base directories must be absolute host paths")}
+		}
 	case sourceFile:
 		if context.FS != nil {
 			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "host file sources cannot use a native filesystem context")}