これですね https://github.com/apple/swift-syntax/blob/1313c36fbf86215d4d6894b8d5e85f1230f013d1/Sources/SwiftSyntax/SyntaxData.swift#L217-L223

 private enum ParentOrArena { // For non-root nodes. case parent(SyntaxBox) // For root node. case arena(SyntaxArena) } private let parentOrArena: ParentOrArena

(edited)