[clang-tidy/google] Improve the Objective-C global variable declarati…

…on check 🔧

Summary:
The current Objective-C global variable declaration check restricts naming that is permitted by the Google Objective-C style guide.

The Objective-C style guide states the following:
"Global and file scope constants should have an appropriate prefix. [...] Constants may use a lowercase k prefix when appropriate"
http://google.github.io/styleguide/objcguide#constants

This change fixes the check to allow two or more capital letters as an appropriate prefix. This change intentionally avoids making a decision regarding whether to flag constants that use a two letter prefix (two letter prefixes are reserved by Apple¹ but many projects seem to violate this guideline).

This change eliminates an important category of false positives (constants prefixed with '[A-Z]{2,}') at the cost of introducing a less important category of false negatives (constants prefixed with only '[A-Z]'). The false positives are observed in standard recommended code while the false negatives occur in non-standard unrecommended code. The number of eliminated false positives is expected to be significantly larger than the number of exposed false negatives.

❧

(1)
"Two-letter prefixes like these are reserved by Apple for use in framework classes."
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Conventions/Conventions.html

Reviewers: aaron.ballman, Wizard, hokein, benhamilton

Reviewed By: aaron.ballman, Wizard

Subscribers: aaron.ballman, cfe-commits

Differential Revision: https://reviews.llvm.org/D43581

llvm-svn: 326046

clang-tools-extra/clang-tidy/google/GlobalVariableDeclarationCheck.cpp

@@ -72,7 +72,7 @@ void GlobalVariableDeclarationCheck::registerMatchers(MatchFinder *Finder) {
       this);
   Finder->addMatcher(varDecl(hasGlobalStorage(), hasType(isConstQualified()),
                              unless(isLocalVariable()),
-                             unless(matchesName("::k[A-Z]")))
+                             unless(matchesName("::(k[A-Z]|[A-Z]{2,})")))
                          .bind("global_const"),
                      this);
 }
@@ -88,7 +88,7 @@ void GlobalVariableDeclarationCheck::check(
   if (const auto *Decl = Result.Nodes.getNodeAs<VarDecl>("global_const")) {
     diag(Decl->getLocation(),
          "const global variable '%0' must have a name which starts with "
-         "'k[A-Z]'")
+         "an appropriate prefix")
         << Decl->getName() << generateFixItHint(Decl, true);
   }
 }

clang-tools-extra/test/clang-tidy/google-objc-global-variable-declaration.m

@@ -2,7 +2,7 @@
 
 @class NSString;
 static NSString* const myConstString = @"hello";
-// CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable 'myConstString' must have a name which starts with 'k[A-Z]' [google-objc-global-variable-declaration]
+// CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable 'myConstString' must have a name which starts with an appropriate prefix [google-objc-global-variable-declaration]
 // CHECK-FIXES: static NSString* const kMyConstString = @"hello";
 
 static NSString* MyString = @"hi";
@@ -22,16 +22,24 @@
 // CHECK-FIXES: static NSString* gNoDef;
 
 static NSString* const _notAlpha = @"NotBeginWithAlpha";
-// CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable '_notAlpha' must have a name which starts with 'k[A-Z]' [google-objc-global-variable-declaration]
+// CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable '_notAlpha' must have a name which starts with an appropriate prefix [google-objc-global-variable-declaration]
 // CHECK-FIXES: static NSString* const _notAlpha = @"NotBeginWithAlpha";
 
 static NSString* const k_Alpha = @"SecondNotAlpha";
-// CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable 'k_Alpha' must have a name which starts with 'k[A-Z]' [google-objc-global-variable-declaration]
+// CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable 'k_Alpha' must have a name which starts with an appropriate prefix [google-objc-global-variable-declaration]
 // CHECK-FIXES: static NSString* const k_Alpha = @"SecondNotAlpha";
 
 static NSString* const kGood = @"hello";
+static NSString* const XYGood = @"hello";
 static NSString* gMyIntGood = 0;
 
+extern NSString* const GTLServiceErrorDomain;
+
+enum GTLServiceError {
+  GTLServiceErrorQueryResultMissing = -3000,
+  GTLServiceErrorWaitTimedOut       = -3001,
+};
+
 @implementation Foo
 - (void)f {
     int x = 0;