Address code review comments

Author: DougGregor

Sources/SwiftCompilerPluginMessageHandling/Macros.swift

@@ -14,8 +14,8 @@ import SwiftBasicFormat
 import SwiftDiagnostics
 import SwiftOperators
 import SwiftSyntax
-import SwiftSyntaxMacroExpansion
-import SwiftSyntaxMacros
+@_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacroExpansion
+@_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacros
 
 extension CompilerPluginMessageHandler {
   /// Get concrete macro type from a pair of module name and type name.

Sources/SwiftCompilerPluginMessageHandling/PluginMessages.swift

@@ -124,8 +124,8 @@ public enum PluginMessage {
     case conformance
     case codeItem
     case `extension`
-    case preamble
-    case body
+    @_spi(ExperimentalLanguageFeature) case preamble
+    @_spi(ExperimentalLanguageFeature) case body
   }
 
   public struct SourceLocation: Codable {

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -12,7 +12,7 @@
 
 import SwiftBasicFormat
 import SwiftSyntax
-@_spi(MacroExpansion) import SwiftSyntaxMacros
+@_spi(MacroExpansion) @_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacros
 
 public enum MacroRole {
   case expression
@@ -24,8 +24,8 @@ public enum MacroRole {
   case conformance
   case codeItem
   case `extension`
-  case preamble
-  case body
+  @_spi(ExperimentalLanguageFeature) case preamble
+  @_spi(ExperimentalLanguageFeature) case body
 }
 
 extension MacroRole {
@@ -55,7 +55,7 @@ enum MacroExpansionError: Error, CustomStringConvertible {
   case declarationHasNoBody
   case noExtendedTypeSyntax
   case noFreestandingMacroRoles(Macro.Type)
-  case tooManyBodyMacros
+  case moreThanOneBodyMacro
   case preambleWithoutBody
 
   var description: String {
@@ -81,7 +81,7 @@ enum MacroExpansionError: Error, CustomStringConvertible {
     case .noFreestandingMacroRoles(let type):
       return "macro implementation type '\(type)' does not conform to any freestanding macro protocol"
 
-    case .tooManyBodyMacros:
+    case .moreThanOneBodyMacro:
       return "function can not have more than one body macro applied to it"
 
     case .preambleWithoutBody:

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -15,7 +15,7 @@ import SwiftOperators
 @_spi(MacroExpansion) import SwiftParser
 import SwiftSyntax
 import SwiftSyntaxBuilder
-@_spi(MacroExpansion) import SwiftSyntaxMacros
+@_spi(MacroExpansion) @_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacros
 
 // MARK: - Public entry function
 
@@ -379,7 +379,7 @@ private func expandPreambleMacro(
   // Match the indentation of the statements if we can, and put newlines around
   // the preamble to separate it from the rest of the body.
   let indentation = decl.body?.statements.indentationOfFirstLine ?? (decl.indentationOfFirstLine + indentationWidth)
-  let indentedSource = "\n" + expanded.indented(by: indentation) + "\n\n\n"
+  let indentedSource = "\n" + expanded.indented(by: indentation) + "\n\n"
   return "\(raw: indentedSource)"
 }
 
@@ -410,7 +410,8 @@ private func expandBodyMacro(
   }
 
   // Wrap the body in braces.
-  let indentedSource = " {\n" + expanded.indented(by: decl.indentationOfFirstLine + indentationWidth) + "\n}\n"
+  let beforeBody = decl.body == nil ? " " : "";
+  let indentedSource = beforeBody + "{\n" + expanded.indented(by: decl.indentationOfFirstLine + indentationWidth) + "\n}\n"
   return "\(raw: indentedSource)" as CodeBlockSyntax
 }
 
@@ -704,7 +705,7 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
       body = expandedBodies[0]
 
     default:
-      context.addDiagnostics(from: MacroExpansionError.tooManyBodyMacros, node: node)
+      context.addDiagnostics(from: MacroExpansionError.moreThanOneBodyMacro, node: node)
       body = expandedBodies[0]
     }
 
@@ -713,9 +714,7 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
       return node.with(\.body, body)
     }
 
-    var statements = body.statements
-    statements.insert(contentsOf: preamble, at: statements.startIndex)
-    return node.with(\.body, body.with(\.statements, statements))
+    return node.with(\.body, body.with(\.statements, preamble + body.statements))
   }
 
   override func visit(_ node: CodeBlockItemListSyntax) -> CodeBlockItemListSyntax {

Sources/SwiftSyntaxMacros/MacroProtocols/BodyMacro.swift

@@ -11,6 +11,7 @@ import SwiftSyntax
 
 /// Describes a macro that can create the body for a function that does not
 /// have one.
+@_spi(ExperimentalLanguageFeature)
 public protocol BodyMacro: AttachedMacro {
   /// Expand a macro described by the given custom attribute and
   /// attached to the given declaration and evaluated within a

Sources/SwiftSyntaxMacros/MacroProtocols/PreambleMacro.swift

@@ -11,6 +11,7 @@ import SwiftSyntax
 
 /// Describes a macro that can introduce "preamble" code into an existing
 /// function body.
+@_spi(ExperimentalLanguageFeature)
 public protocol PreambleMacro: AttachedMacro {
   /// Expand a macro described by the given custom attribute and
   /// attached to the given declaration and evaluated within a

Tests/SwiftSyntaxMacroExpansionTest/BodyMacroTests.swift

@@ -21,7 +21,7 @@
 import SwiftDiagnostics
 import SwiftSyntax
 @_spi(Testing) import SwiftSyntaxMacroExpansion
-import SwiftSyntaxMacros
+@_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacros
 import SwiftSyntaxMacrosTestSupport
 import XCTest
 
@@ -39,11 +39,7 @@ struct RemoteBodyMacro: BodyMacro {
 
     let funcBaseName = funcDecl.name.text
     let paramNames = funcDecl.signature.parameterClause.parameters.map { param in
-      if let name = param.parameterName {
-        return name
-      } else {
-        return TokenSyntax(.wildcard, presence: .present)
-      }
+      param.parameterName ?? TokenSyntax(.wildcard, presence: .present)
     }
 
     let passedArgs = DictionaryExprSyntax(
@@ -109,6 +105,24 @@ final class BodyMacroTests: XCTestCase {
       macros: ["Remote": RemoteBodyMacro.self],
       indentationWidth: indentationWidth
     )
+  }
+
+  func testBodyExpansionReplacement() {
+    assertMacroExpansion(
+      """
+      @Remote
+      func f(a: Int, b: String) async throws -> String {
+        this; code; is; unused
+      }
+      """,
+      expandedSource: """
 
+        func f(a: Int, b: String) async throws -> String {
+          return try await remoteCall(function: "f", arguments: ["a": a, "b": b])
+        }
+        """,
+      macros: ["Remote": RemoteBodyMacro.self],
+      indentationWidth: indentationWidth
+    )
   }
 }

Tests/SwiftSyntaxMacroExpansionTest/PreambleMacroTests.swift

@@ -21,7 +21,7 @@
 import SwiftDiagnostics
 import SwiftSyntax
 @_spi(Testing) import SwiftSyntaxMacroExpansion
-import SwiftSyntaxMacros
+@_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacros
 import SwiftSyntaxMacrosTestSupport
 import XCTest
 
@@ -39,17 +39,13 @@ struct TracedPreambleMacro: PreambleMacro {
 
     let funcBaseName = funcDecl.name
     let paramNames = funcDecl.signature.parameterClause.parameters.map { param in
-      if let name = param.parameterName {
-        return name.text
-      } else {
-        return "_"
-      }
+      param.parameterName?.text ?? "_"
     }
 
     let passedArgs = paramNames.map { "\($0): \\(\($0))" }.joined(separator: ", ")
 
     let entry: CodeBlockItemSyntax = """
-      log("Entering \(raw: funcBaseName)(\(raw: passedArgs))")
+      log("Entering \(funcBaseName)(\(raw: passedArgs))")
       """
 
     let argLabels = paramNames.map { "\($0):" }.joined()