[Macros] Freestanding macros expand to CodeBlockItemList; parse top-level macro expansion decls

- Change the return type of `FreestandingDeclarationMacro.expanion(of:in:)` to ``CodeBlockItemListSyntax` to allow expressions, statements, and declarations.
- Parse freestanding macro expansions as declarations rather than expressions when it's not part of a statement or expression (i.e. "top-level").

Author: rxwei

Sources/_SwiftSyntaxMacros/CodeItemMacro.swift

@@ -16,5 +16,5 @@ public protocol CodeItemMacro: FreestandingMacro {
   static func expansion(
     of node: MacroExpansionDeclSyntax,
     in context: inout MacroExpansionContext
-  ) throws -> [CodeBlockItemSyntax]
+  ) throws -> CodeBlockItemListSyntax
 }

Sources/_SwiftSyntaxMacros/DeclarationMacro.swift

@@ -16,7 +16,7 @@ public protocol DeclarationMacro: FreestandingMacro {
   static func expansion(
     of node: MacroExpansionDeclSyntax,
     in context: inout MacroExpansionContext
-  ) throws -> [DeclSyntax]
+  ) throws -> CodeBlockItemListSyntax
 }
 
 @available(*, deprecated, renamed: "DeclarationMacro")

Sources/_SwiftSyntaxMacros/MacroSystem.swift

@@ -134,18 +134,12 @@ class MacroApplication: SyntaxRewriter {
         do {
           if let macro = macro as? DeclarationMacro.Type {
             let expandedItemList = try macro.expansion(
-              of: declExpansion,
-              in: &context
-            )
-            newItems.append(
-              contentsOf: expandedItemList.map {
-                CodeBlockItemSyntax(item: .decl($0))
-              }
+              of: declExpansion, in: &context
             )
+            newItems.append(contentsOf: expandedItemList)
           } else if let macro = macro as? ExpressionMacro.Type {
             let expandedExpr = try macro.expansion(
-              of: declExpansion.asMacroExpansionExpr(),
-              in: &context
+              of: declExpansion.asMacroExpansionExpr(), in: &context
             )
             newItems.append(CodeBlockItemSyntax(item: .init(expandedExpr)))
           }
@@ -194,11 +188,15 @@ class MacroApplication: SyntaxRewriter {
             in: &context
           )
 
-          newItems.append(
-            contentsOf: expandedList.map { decl in
-              return MemberDeclListItemSyntax(decl: decl)
-            }
-          )
+          newItems.append(contentsOf: expandedList.compactMap { item -> MemberDeclListItemSyntax? in
+            guard let decl = item.item.as(DeclSyntax.self) else { return nil }
+            return MemberDeclListItemSyntax(
+              leadingTrivia: item.leadingTrivia,
+              item.unexpectedBeforeItem,
+              decl: decl,
+              semicolon: item.semicolon,
+              trailingTrivia: item.trailingTrivia)
+          })
         } catch {
           // Record the error
           context.diagnose(

Tests/SwiftSyntaxMacrosTest/MacroSystemTests.swift

@@ -158,7 +158,7 @@ public struct ErrorMacro: DeclarationMacro {
   public static func expansion(
     of node: MacroExpansionDeclSyntax,
     in context: inout MacroExpansionContext
-  ) throws -> [DeclSyntax] {
+  ) throws -> CodeBlockItemListSyntax {
     guard let firstElement = node.argumentList.first,
       let stringLiteral = firstElement.expression
         .as(StringLiteralExprSyntax.self),
@@ -187,7 +187,7 @@ struct DefineBitwidthNumberedStructsMacro: DeclarationMacro {
   static func expansion(
     of node: MacroExpansionDeclSyntax,
     in context: inout MacroExpansionContext
-  ) throws -> [DeclSyntax] {
+  ) throws -> CodeBlockItemListSyntax {
     guard let firstElement = node.argumentList.first,
       let stringLiteral = firstElement.expression
         .as(StringLiteralExprSyntax.self),
@@ -199,12 +199,17 @@ struct DefineBitwidthNumberedStructsMacro: DeclarationMacro {
       )
     }
 
-    return [8, 16, 32, 64].map { bitwidth in
+    let decls: [DeclSyntax] = [8, 16, 32, 64].map { bitwidth in
       """
 
       struct \(raw: prefix)\(raw: String(bitwidth)) { }
       """
     }
+    // TODO: Interpolate to `CodeBlockItemList` directly when it supports
+    // string interpolation.
+    return CodeBlockItemListSyntax(decls.map {
+      CodeBlockItemSyntax(item: .init($0))
+    })
   }
 }