[Macros] Ability to opt-out from auto-propagation of attrs/modifiers

`DeclarationMacro` now have two optional requirements that is defaulted
to 'true':

  static var propagateFreestandingMacroAttributes: Bool { get }
  static var propagateFreestandingMacroModifiers: Bool { get }

My implementing these and returning 'false', macro expansion systam
won't copy attributes and/or modifiers to the expanded declarations.

rdar://110364154

Author: rintaro

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -46,8 +46,10 @@ public func expandFreestandingMacro(
         var rewritten = try declMacroDef.expansion(of: node, in: context)
         // Copy attributes and modifiers to the generated decls.
         if let expansionDecl = node.as(MacroExpansionDeclSyntax.self) {
+          let attributes = declMacroDef.propagateFreestandingMacroAttributes ? expansionDecl.attributes : nil
+          let modifiers = declMacroDef.propagateFreestandingMacroModifiers ? expansionDecl.modifiers : nil
           rewritten = rewritten.map {
-            $0.applying(attributes: expansionDecl.attributes, modifiers: expansionDecl.modifiers)
+            $0.applying(attributes: attributes, modifiers: modifiers)
           }
         }
         expandedSyntax = Syntax(

Sources/SwiftSyntaxMacros/MacroProtocols/DeclarationMacro.swift

@@ -20,4 +20,16 @@ public protocol DeclarationMacro: FreestandingMacro {
     of node: Node,
     in context: Context
   ) throws -> [DeclSyntax]
+
+  /// Whether to copy attributes on the expansion syntax to expanded declarations,
+  /// 'true' by default.
+  static var propagateFreestandingMacroAttributes: Bool { get }
+  /// Whether to copy modifiers on the expansion syntax to expanded declarations,
+  /// 'true' by default.
+  static var propagateFreestandingMacroModifiers: Bool { get }
+}
+
+public extension DeclarationMacro {
+  static var propagateFreestandingMacroAttributes: Bool { true }
+  static var propagateFreestandingMacroModifiers: Bool { true }
 }

Sources/SwiftSyntaxMacros/MacroSystem.swift

@@ -146,8 +146,10 @@ class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
               in: context
             )
             if let declExpansion = expansion.as(MacroExpansionDeclSyntax.self) {
+              let attributes = macro.propagateFreestandingMacroAttributes ? declExpansion.attributes : nil
+              let modifiers = macro.propagateFreestandingMacroModifiers ? declExpansion.modifiers : nil
               expandedItemList = expandedItemList.map {
-                $0.applying(attributes: declExpansion.attributes, modifiers: declExpansion.modifiers)
+                $0.applying(attributes: attributes, modifiers: modifiers)
               }
             }
             newItems.append(
@@ -203,8 +205,10 @@ class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
             of: declExpansion,
             in: context
           )
+          let attributes = freestandingMacro.propagateFreestandingMacroAttributes ? declExpansion.attributes : nil
+          let modifiers = freestandingMacro.propagateFreestandingMacroModifiers ? declExpansion.modifiers : nil
           expandedList = expandedList.map {
-            $0.applying(attributes: declExpansion.attributes, modifiers: declExpansion.modifiers)
+            $0.applying(attributes: attributes, modifiers: modifiers)
           }
 
           newItems.append(

Tests/SwiftSyntaxMacrosTest/MacroSystemTests.swift

@@ -672,6 +672,30 @@ public struct DeclsFromStringsMacro: DeclarationMacro {
   }
 }
 
+public struct DeclsFromStringsMacroNoAttrs: DeclarationMacro {
+  public static var propagateFreestandingMacroAttributes: Bool { false }
+  public static var propagateFreestandingMacroModifiers: Bool { false }
+
+  public static func expansion(
+    of node: some FreestandingMacroExpansionSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    var strings: [String] = []
+    for arg in node.argumentList {
+      guard
+        let value = arg.expression.as(StringLiteralExprSyntax.self)?.representedLiteralValue
+      else {
+        continue
+      }
+      strings.append(value)
+    }
+
+    return strings.map {
+      "\(raw: $0)"
+    }
+  }
+}
+
 // MARK: Tests
 
 /// The set of test macros we use here.
@@ -1114,5 +1138,22 @@ final class MacroSystemTests: XCTestCase {
       macros: ["decls": DeclsFromStringsMacro.self],
       indentationWidth: indentationWidth
     )
+
+    assertMacroExpansion(
+      #"""
+      @attribute
+      @otherAttribute(x: 1)
+      public #decls("@moreAttibute var global = 42",
+                    "private func foo() {}")
+      """#,
+      expandedSource: #"""
+        @moreAttibute var global = 42
+        private func foo() {
+        }
+        """#,
+      macros: ["decls": DeclsFromStringsMacroNoAttrs.self],
+      indentationWidth: indentationWidth
+    )
+
   }
 }