Make members of a private extension fileprivate.

Extensions are declared at file scope, so "private" on an extension means
that the members are actually "fileprivate". In this case we need to add
the correct keyword to the individual members, or we'll actually change
their visibility incorrectly.

Fixes SR-11110.

Author: allevato

Sources/SwiftFormatRules/NoAccessLevelOnExtensionDeclaration.swift

@@ -36,9 +36,20 @@ public final class NoAccessLevelOnExtensionDeclaration: SyntaxFormatRule {
     // Public, private, or fileprivate keywords need to be moved to members
     case .publicKeyword, .privateKeyword, .fileprivateKeyword:
       diagnose(.moveAccessKeyword(keyword: accessKeyword.name.text), on: accessKeyword)
+
+      // The effective access level of the members of a `private` extension is `fileprivate`, so
+      // we have to update the keyword to ensure that the result is correct.
+      let accessKeywordToAdd: DeclModifierSyntax
+      if keywordKind == .privateKeyword {
+        accessKeywordToAdd
+          = accessKeyword.withName(accessKeyword.name.withKind(.fileprivateKeyword))
+      } else {
+        accessKeywordToAdd = accessKeyword
+      }
+
       let newMembers = SyntaxFactory.makeMemberDeclBlock(
         leftBrace: node.members.leftBrace,
-        members: addMemberAccessKeywords(memDeclBlock: node.members, keyword: accessKeyword),
+        members: addMemberAccessKeywords(memDeclBlock: node.members, keyword: accessKeywordToAdd),
         rightBrace: node.members.rightBrace)
       let newKeyword = replaceTrivia(
         on: node.extensionKeyword,
@@ -47,6 +58,7 @@ public final class NoAccessLevelOnExtensionDeclaration: SyntaxFormatRule {
       return node.withMembers(newMembers)
         .withModifiers(modifiers.remove(name: accessKeyword.name.text))
         .withExtensionKeyword(newKeyword)
+
     // Internal keyword redundant, delete
     case .internalKeyword:
       diagnose(
@@ -58,6 +70,7 @@ public final class NoAccessLevelOnExtensionDeclaration: SyntaxFormatRule {
         leadingTrivia: accessKeyword.leadingTrivia) as! TokenSyntax
       return node.withModifiers(modifiers.remove(name: accessKeyword.name.text))
         .withExtensionKeyword(newKeyword)
+
     default:
       break
     }

Tests/SwiftFormatRulesTests/NoAccessLevelOnExtensionDeclarationTests.swift

@@ -84,4 +84,20 @@ public class NoAccessLevelOnExtensionDeclarationTests: DiagnosingTestCase {
         """
     )
   }
+
+  public func testPrivateIsEffectivelyFileprivate() {
+    XCTAssertFormatting(
+      NoAccessLevelOnExtensionDeclaration.self,
+      input: """
+        private extension Foo {
+          func f() {}
+        }
+        """,
+      expected: """
+        extension Foo {
+          fileprivate func f() {}
+        }
+        """
+    )
+  }
 }

Tests/SwiftFormatRulesTests/XCTestManifests.swift

@@ -157,6 +157,7 @@ extension NoAccessLevelOnExtensionDeclarationTests {
     static let __allTests__NoAccessLevelOnExtensionDeclarationTests = [
         ("testExtensionDeclarationAccessLevel", testExtensionDeclarationAccessLevel),
         ("testPreservesCommentOnRemovedModifier", testPreservesCommentOnRemovedModifier),
+        ("testPrivateIsEffectivelyFileprivate", testPrivateIsEffectivelyFileprivate),
     ]
 }