https://github.com/apple/swift-syntax/pull/1225 type wrapperをdecl macroで実装している

[Macros] Implement a type wrapper transformation as a macro. by hbo...

The type wrapper macro struct conforms to MemberDeclarationMacro, MemberAttributeMacro, and AccessorDeclarationMacro. Each macro capability provides a different part of the type wrapper transform: ...