A Comprehensive Guide to Swift Macros and SwiftSyntax Implementation

Welcome to the world of Swift Macros. This article explores the revolutionary feature introduced in Swift 5.9 - Macros, which provide the ability to process source code at compile time. Macros help automate code generation, reduce boilerplate code, and make code more concise and understandable.

The guide covers the fundamental concepts of Swift Macros, distinguishing between attached macros and freestanding macros. Attached macros must be associated with an existing type or declaration, starting with "@", while freestanding macros can be used independently, starting with "#".

Key topics include:

Macro expansion process: how the compiler reads code, creates an in-memory syntax representation, passes it to the macro implementation, and replaces the macro call with its expanded form

Creating macros: implementing the macro (using SwiftSyntax to analyze code and follow specific protocols), declaring the macro, and creating the plugin

SwiftSyntax library: provides high-level APIs for inspecting, processing, and manipulating Swift source code through Abstract Syntax Trees (AST)

Syntax nodes and token types that form the building blocks of Swift code analysis

A practical example demonstrates implementing a PeerMacro (@attached(peer)) that adds async versions to existing completion handler-based functions, showing the complete workflow from AST analysis to code generation.