Swift Macro如何移除原变量并替换为闭包实现？

我正在开发一个Swift宏（PeerMacro），基础功能接近完成，但现在需要移除原变量（如static let loans: String），用闭包完全替换它。目前宏展开后原变量仍会保留，以下是相关代码和展开结果，求实现方法（刚接触Swift Macros）。

宏扩展代码 #

public static func expansion(
    of node: SwiftSyntax.AttributeSyntax,
    providingPeersOf declaration: some SwiftSyntax.DeclSyntaxProtocol,
    in context: some SwiftSyntaxMacros.MacroExpansionContext
) throws -> [SwiftSyntax.DeclSyntax] {
    guard let variableSyntax = declaration.as(VariableDeclSyntax.self),
          let variableSyntaxAttributes = variableSyntax.attributes.first?.as(AttributeSyntax.self),
          let labelListSyntax = variableSyntaxAttributes.arguments?.as(LabeledExprListSyntax.self)
    else {
        throw MacroExpansionError.message(
            "Could not read the variable syntax"
        )
    }

    // 校验变量格式
    guard let staticModifierListSyntax = variableSyntax.modifiers.first?.as(DeclModifierSyntax.self),
          let bindingsListSyntax = variableSyntax.bindings.first?.as(PatternBindingSyntax.self),
          let identifierSyntax = bindingsListSyntax.pattern.as(IdentifierPatternSyntax.self)
    else {
        throw MacroExpansionError.message(
            "Variable is not properly formatted"
        )
    }
    let staticKeyWord = staticModifierListSyntax.name.text
    let letKeyword = variableSyntax.bindingSpecifier.text
    let variableName = identifierSyntax.identifier.text

    guard staticKeyWord == "static",
          letKeyword == "let",
          !variableName.isEmpty
    else {
        throw MacroExpansionError.message(
            "Variable is not properly formatted"
        )
    }

    // 校验URL参数
    guard let urlStringLabelSyntax = labelListSyntax.first?.as(LabeledExprSyntax.self),
          let urlStringLiteralSyntax = urlStringLabelSyntax.expression.trimmed.as(StringLiteralExprSyntax.self),
          let urlStringSegmentSyntax = urlStringLiteralSyntax.segments.first?.as(StringSegmentSyntax.self)
    else  {
        throw MacroExpansionError.message(
            "First argument for URL is missing or not the right type"
        )
    }
    let urlString = urlStringSegmentSyntax.content.text

    return [
        DeclSyntax(stringLiteral:
        """
            \(staticKeyWord) \(letKeyword) \(variableName): String = {
                return String(format: "\(urlString)")
            }()
        """
        )
    ]
}

测试代码 #

func testMacro() throws {
    #if canImport(AFEndpointMacros)
    assertMacroExpansion(
        """
        struct Endpoints {
            @AFEndpoint("/v2/test/")
            static let test: String
        }
        """,
        expandedSource: 
        """
        struct Endpoints {
            static let test: String = {
                return String(format: "/v2/test/")
            }()
        }
        """,
        macros: testMacros
    )
    #else
    throw XCTSkip("macros are only supported when running tests for the host platform")
    #endif
}

实际与预期展开结果 #

预期输出：

struct Endpoints {
    static let test: String = {
        return String(format: "/v2/test/")
    }()
}

实际输出：

struct Endpoints {
    static let test: String

    static let test: String = {
                    return String(format: "/v2/test/")
        }()
}

问题出在你使用的是Peer Macro（同伴宏），这类宏的作用是在原声明的旁边添加新代码，不会替换原声明，所以原变量会被保留。要实现替换原变量的需求，你需要改用Declaration Macro（声明宏），它会直接替换掉被标记的原声明。

具体修改步骤：

1. 修改宏的协议实现：把宏结构体的继承协议从PeerMacro改成DeclarationMacro
2. 调整扩展方法签名：将方法参数从providingPeersOf改为for，对应DeclarationMacro的标准方法签名
3. 保留内部逻辑：原有的变量解析、闭包生成逻辑不需要修改

修改后的宏扩展方法签名示例：

public static func expansion(
    of node: SwiftSyntax.AttributeSyntax,
    for declaration: some SwiftSyntax.DeclSyntaxProtocol,
    in context: some SwiftSyntaxMacros.MacroExpansionContext
) throws -> [SwiftSyntax.DeclSyntax] {
    // 原有的内部逻辑完全保留
}

修改后，宏展开时会用你生成的带闭包的变量声明直接替换原变量声明，不会保留原代码，就能得到预期的展开结果。

内容的提问来源于stack exchange，提问作者Kwoner