feat: support fixit

Author: RayZhao1998

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -728,15 +728,25 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
         }) {
           self.expandedAttributes.remove(at: index)
         } else {
-            if let macroRole = try? inferAttachedMacroRole(definition: spec.type), isInvalidAttachedMacro(macroRole: macroRole, attachedTo: declSyntax) {
-              contextGenerator(node).addDiagnostics(
-                from: MacroApplicationError.macroAttachedToInvalidDecl(
-                  macroRole.rawValue,
-                  declSyntax.nodeTypeNameForDiagnostics(allowBlockNames: true) ?? ""
-                ),
-                node: declSyntax
-              )
-            }
+          if let macroRole = try? inferAttachedMacroRole(definition: spec.type),
+            isInvalidAttachedMacro(macroRole: macroRole, attachedTo: declSyntax)
+          {
+            contextGenerator(node).addDiagnostics(
+              from: MacroApplicationError.macroAttachedToInvalidDecl(
+                macroRole.rawValue,
+                declSyntax.nodeTypeNameForDiagnostics(allowBlockNames: true) ?? ""
+              ),
+              node: declSyntax,
+              fixIts: [
+                FixIt(
+                  message: SwiftSyntaxMacros.MacroExpansionFixItMessage(
+                    "Remove '\(attribute.trimmedDescription)'"
+                  ),
+                  changes: [.replace(oldNode: Syntax(attribute), newNode: Syntax(AttributeListSyntax()))]
+                )
+              ]
+            )
+          }
         }
       }
       return AttributeRemover(removingWhere: { attributesToRemove.map(\.attributeNode).contains($0) }).rewrite(

Sources/SwiftSyntaxMacros/MacroExpansionContext.swift

@@ -108,21 +108,28 @@ private struct ThrownErrorDiagnostic: DiagnosticMessage {
 
 extension MacroExpansionContext {
   /// Adds diagnostics from the error thrown during a macro expansion.
-  public func addDiagnostics(from error: Error, node: some SyntaxProtocol) {
+  public func addDiagnostics(from error: Error, node: some SyntaxProtocol, fixIts: [FixIt] = []) {
     // Inspect the error to form an appropriate set of diagnostics.
     var diagnostics: [Diagnostic]
     if let diagnosticsError = error as? DiagnosticsError {
       diagnostics = diagnosticsError.diagnostics
     } else if let message = error as? DiagnosticMessage {
-      diagnostics = [Diagnostic(node: Syntax(node), message: message)]
+      diagnostics = [Diagnostic(node: Syntax(node), message: message, fixIts: fixIts)]
     } else if let error = error as? SyntaxStringInterpolationInvalidNodeTypeError {
       let diagnostic = Diagnostic(
         node: Syntax(node),
-        message: ThrownErrorDiagnostic(message: "Internal macro error: \(error.description)")
+        message: ThrownErrorDiagnostic(message: "Internal macro error: \(error.description)"),
+        fixIts: fixIts
       )
       diagnostics = [diagnostic]
     } else {
-      diagnostics = [Diagnostic(node: Syntax(node), message: ThrownErrorDiagnostic(message: String(describing: error)))]
+      diagnostics = [
+        Diagnostic(
+          node: Syntax(node),
+          message: ThrownErrorDiagnostic(message: String(describing: error)),
+          fixIts: fixIts
+        )
+      ]
     }
 
     // Emit the diagnostics.

Tests/SwiftSyntaxMacroExpansionTest/AccessorMacroTests.swift

@@ -427,7 +427,12 @@ final class AccessorMacroTests: XCTestCase {
       "@Test struct Foo {}",
       expandedSource: "struct Foo {}",
       diagnostics: [
-        DiagnosticSpec(message: "'accessor' macro cannot be attached to struct", line: 1, column: 1)
+        DiagnosticSpec(
+          message: "'accessor' macro cannot be attached to struct",
+          line: 1,
+          column: 1,
+          fixIts: [FixItSpec(message: "Remove '@Test'")]
+        )
       ],
       macros: ["Test": TestMacro.self]
     )
@@ -437,7 +442,12 @@ final class AccessorMacroTests: XCTestCase {
       expandedSource: "func Foo() {}",
       diagnostics: [
         // The compiler will reject this with "'accessor' macro cannot be attached to global function"
-        DiagnosticSpec(message: "'accessor' macro cannot be attached to function", line: 1, column: 1)
+        DiagnosticSpec(
+          message: "'accessor' macro cannot be attached to function",
+          line: 1,
+          column: 1,
+          fixIts: [FixItSpec(message: "Remove '@Test'")]
+        )
       ],
       macros: ["Test": TestMacro.self]
     )
@@ -456,7 +466,12 @@ final class AccessorMacroTests: XCTestCase {
         """,
       diagnostics: [
         // The compiler will reject this with "'accessor' macro cannot be attached to instance method"
-        DiagnosticSpec(message: "'accessor' macro cannot be attached to function", line: 2, column: 3)
+        DiagnosticSpec(
+          message: "'accessor' macro cannot be attached to function",
+          line: 2,
+          column: 3,
+          fixIts: [FixItSpec(message: "Remove '@Test'")]
+        )
       ],
       macros: ["Test": TestMacro.self],
       indentationWidth: indentationWidth