Fixed up two issues that were discovered

Sources/MacroTesting/SwiftDiagnostics/DiagnosticsFormatter.swift

@@ -324,12 +324,12 @@ struct DiagnosticsFormatter {
         for diag in diags.dropLast(1) {
           annotatedSource.append("\(preMessage)├─ \(colorizeIfRequested(diag.diagMessage))\n")
           for fixIt in diag.fixIts {
-            annotatedSource.append("\(preMessage)│  ✏️ \(fixIt.message.message)")
+            annotatedSource.append("\(preMessage)│  ✏️ \(fixIt.message.message)\n")
           }
         }
         annotatedSource.append("\(preMessage)╰─ \(colorizeIfRequested(diags.last!.diagMessage))\n")
         for fixIt in diags.last!.fixIts {
-          annotatedSource.append("\(preMessage)   ✏️ \(fixIt.message.message)")
+          annotatedSource.append("\(preMessage)   ✏️ \(fixIt.message.message)\n")
         }
       }
 

Tests/MacroTestingTests/DiagnosticsAndFixitsEmitterMacroTests.swift

@@ -0,0 +1,42 @@
+import MacroTesting
+import XCTest
+
+final class DiagnosticsAndFixitsEmitterMacroTests: BaseTestCase {
+  override func invokeTest() {
+    withMacroTesting(macros: [DiagnosticsAndFixitsEmitterMacro.self]) {
+      super.invokeTest()
+    }
+  }
+
+  func testExpansionEmitsDiagnosticsAndFixits() {
+    assertMacro {
+      """
+      @DiagnosticsAndFixitsEmitter
+      struct FooBar {
+        let foo: Foo
+        let bar: Bar
+      }
+      """
+    } diagnostics: {
+      """
+      @DiagnosticsAndFixitsEmitter
+       ┬──────────────────────────
+       ├─ ⚠️ This is the first diagnostic.
+       │  ✏️ This is the first fix-it.
+       │  ✏️ This is the second fix-it.
+       ╰─ ℹ️ This is the second diagnostic, it's a note.
+      struct FooBar {
+        let foo: Foo
+        let bar: Bar
+      }
+      """
+    } expansion: {
+      """
+      struct FooBar {
+        let foo: Foo
+        let bar: Bar
+      }
+      """
+    }
+  }
+}

Tests/MacroTestingTests/MacroExamples/DiagnosticsAndFixitsEmitterMacro.swift

@@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftSyntax
+import SwiftSyntaxBuilder
+import SwiftSyntaxMacros
+import SwiftDiagnostics
+
+/// Emits two diagnostics, the first of which is a warning and has two fix-its, and
+/// the second is a note and has no fix-its.
+public enum DiagnosticsAndFixitsEmitterMacro: MemberMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    let firstFixIt = FixIt(message: SimpleDiagnosticMessage(message: "This is the first fix-it.",
+                                                            diagnosticID: MessageID(domain: "domain", id: "fixit1"), 
+                                                            severity: .error),
+                           changes: [
+                             .replace(oldNode: Syntax(node), newNode: Syntax(node)) // no-op
+                           ])
+    let secondFixIt = FixIt(message: SimpleDiagnosticMessage(message: "This is the second fix-it.",
+                                                             diagnosticID: MessageID(domain: "domain", id: "fixit2"),
+                                                             severity: .error),
+                            changes: [
+                              .replace(oldNode: Syntax(node), newNode: Syntax(node)) // no-op
+                            ])
+
+    context.diagnose(Diagnostic(node: node.attributeName,
+                                message: SimpleDiagnosticMessage(message: "This is the first diagnostic.",
+                                                                 diagnosticID: MessageID(domain: "domain", id: "diagnostic2"),
+                                                                 severity: .warning),
+                                fixIts: [firstFixIt, secondFixIt]))
+    context.diagnose(Diagnostic(node: node.attributeName,
+                                message: SimpleDiagnosticMessage(message: "This is the second diagnostic, it's a note.",
+                                                                 diagnosticID: MessageID(domain: "domain", id: "diagnostic2"),
+                                                                 severity: .note)))
+
+    return []
+  }
+}