Add an NB to @_implementationOnly import.

Author: mbrandonw

Package.resolved

@@ -10,6 +10,8 @@ import SwiftSyntaxMacros
 import XCTest
 
 #if canImport(Testing)
+  // NB: We are importing only the implementation of Testing because that framework is not available
+  //     in Xcode UI test targets.
   @_implementationOnly import Testing
 #endif
 

Sources/MacroTesting/AssertMacro.swift

@@ -1,6 +1,8 @@
 import XCTest
 
 #if canImport(Testing)
+  // NB: We are importing only the implementation of Testing because that framework is not available
+  //     in Xcode UI test targets.
   @_implementationOnly import Testing
 #endif
 

Sources/MacroTesting/Internal/RecordIssue.swift

@@ -2,6 +2,8 @@
   import SnapshotTesting
   import SwiftSyntax
   import SwiftSyntaxMacros
+  // NB: We are importing only the implementation of Testing because that framework is not available
+  //     in Xcode UI test targets.
   @_implementationOnly import Testing
 
   @_spi(Experimental)