Allow suppression of objc prefix checks.

- Accept "-" as the expected prefixes file and turn off all validations of
  prefixes (even the Apple conventions).
- Reflow some of the prefix checks to hopefully make things a little easier
  to follow.
- Don't print warnings about updates to the expected prefix file until there is
  a file.

src/google/protobuf/compiler/objectivec/objectivec_helpers.cc

@@ -1234,6 +1234,8 @@ bool ValidateObjCClassPrefix(
   // without any prefix at all (for legacy reasons).
 
   bool has_prefix = file->options().has_objc_class_prefix();
+  bool have_expected_prefix_file = !expected_prefixes_path.empty();
+
   const std::string prefix = file->options().objc_class_prefix();
   const std::string package = file->package();
 
@@ -1267,30 +1269,10 @@ bool ValidateObjCClassPrefix(
     return true;
   }
 
-  // Check: Warning - Make sure the prefix is is a reasonable value according
+  // When the prefix is non empty, check it against the expected entries.
-  // to Apple's rules (the checks above implicitly whitelist anything that
+  if (!prefix.empty() && have_expected_prefix_file) {
-  // doesn't meet these rules).
+    // For a non empty prefix, look for any other package that uses the prefix.
-  if (!prefix.empty() && !ascii_isupper(prefix[0])) {
-    std::cerr
-         << "protoc:0: warning: Invalid 'option objc_class_prefix = \""
-         << prefix << "\";' in '" << file->name() << "';"
-         << " it should start with a capital letter." << std::endl;
-    std::cerr.flush();
-  }
-  if (!prefix.empty() && prefix.length() < 3) {
-    // Apple reserves 2 character prefixes for themselves. They do use some
-    // 3 character prefixes, but they haven't updated the rules/docs.
-    std::cerr
-         << "protoc:0: warning: Invalid 'option objc_class_prefix = \""
-         << prefix << "\";' in '" << file->name() << "';"
-         << " Apple recommends they should be at least 3 characters long."
-         << std::endl;
-    std::cerr.flush();
-  }
-
-  // Look for any other package that uses the same (non empty) prefix.
     std::string other_package_for_prefix;
-  if (!prefix.empty()) {
     for (std::map<std::string, std::string>::const_iterator i =
              expected_package_prefixes.begin();
          i != expected_package_prefixes.end(); ++i) {
@@ -1299,11 +1281,11 @@ bool ValidateObjCClassPrefix(
         break;
       }
     }
-  }
 
-  // Check: Warning - If the file does not have a package, check whether the non
+    // Check: Warning - If the file does not have a package, check whether the
-  // empty prefix declared is being used by another package or not.
+    // prefix was declared is being used by another package or not. This is
-  if (package.empty() && !prefix.empty()) {
+    // a special case for empty packages.
+    if (package.empty()) {
       // The file does not have a package and ...
       if (other_package_for_prefix.empty()) {
         // ... no other package has declared that prefix.
@@ -1340,10 +1322,32 @@ bool ValidateObjCClassPrefix(
           expected_prefixes_path + ").";
       return false;  // Only report first usage of the prefix.
     }
+  } // !prefix.empty()
+
+  // Check: Warning - Make sure the prefix is is a reasonable value according
+  // to Apple's rules (the checks above implicitly whitelist anything that
+  // doesn't meet these rules).
+  if (!prefix.empty() && !ascii_isupper(prefix[0])) {
+    std::cerr
+         << "protoc:0: warning: Invalid 'option objc_class_prefix = \""
+         << prefix << "\";' in '" << file->name() << "';"
+         << " it should start with a capital letter." << std::endl;
+    std::cerr.flush();
+  }
+  if (!prefix.empty() && prefix.length() < 3) {
+    // Apple reserves 2 character prefixes for themselves. They do use some
+    // 3 character prefixes, but they haven't updated the rules/docs.
+    std::cerr
+         << "protoc:0: warning: Invalid 'option objc_class_prefix = \""
+         << prefix << "\";' in '" << file->name() << "';"
+         << " Apple recommends they should be at least 3 characters long."
+         << std::endl;
+    std::cerr.flush();
+  }
 
   // Check: Warning - If the given package/prefix pair wasn't expected, issue a
   // warning suggesting it gets added to the file.
-  if (!expected_package_prefixes.empty()) {
+  if (have_expected_prefix_file) {
     std::cerr
          << "protoc:0: warning: Found unexpected 'option objc_class_prefix = \""
          << prefix << "\";' in '" << file->name() << "';"
@@ -1360,6 +1364,12 @@ bool ValidateObjCClassPrefix(
 bool ValidateObjCClassPrefixes(const std::vector<const FileDescriptor*>& files,
                                const Options& generation_options,
                                std::string* out_error) {
+  // Allow a '-' as the path for the expected prefixes to completely disable
+  // even the most basic of checks.
+  if (generation_options.expected_prefixes_path == "-") {
+    return true;
+  }
+
   // Load the expected package prefixes, if available, to validate against.
   std::map<std::string, std::string> expected_package_prefixes;
   if (!LoadExpectedPackagePrefixes(generation_options,