[ObjC] Issue stderr warnings for deprecated generation options.

This should get use feedback so we know if usage, the intent is to
be able to remove the different generation code in the future if
folks don't need these.

PiperOrigin-RevId: 666318474

Author: thomasvl

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

@@ -10,6 +10,7 @@
 #include <cstddef>
 #include <cstdlib>
 #include <fstream>
+#include <iostream>
 #include <memory>
 #include <string>
 #include <utility>
@@ -18,6 +19,7 @@
 #include "absl/container/flat_hash_set.h"
 #include "absl/memory/memory.h"
 #include "absl/strings/ascii.h"
+#include "absl/strings/match.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
 #include "absl/strings/string_view.h"
@@ -314,6 +316,57 @@ bool ObjectiveCGenerator::GenerateAll(
 
   // -----------------------------------------------------------------
 
+  // NOTE: src/google/protobuf/compiler/plugin.cc makes use of cerr for some
+  // error cases, so it seems to be ok to use as a back door for warnings.
+
+  // This is a way to turn off these warnings, the intent is that if you find
+  // this then you also did as asked and filed an issue so the need for the
+  // generation option is known. But it allows you to keep your builds quiet
+  // after opening the issue. The value of the environment variable should be
+  // a comma separated list of the names of the options to suppress their usage
+  // warning.
+  char* options_warnings_suppressions_cstr =
+      getenv("GPB_OBJC_SUPPRESS_DEPRECATED_OPTIONS_WARNINGS");
+  const absl::string_view options_warnings_suppressions =
+      options_warnings_suppressions_cstr != nullptr
+          ? options_warnings_suppressions_cstr
+          : "";
+  if (generation_options.headers_use_forward_declarations &&
+      !absl::StrContains(options_warnings_suppressions,
+                         "headers_use_forward_declarations")) {
+    std::cerr << "WARNING: headers_use_forward_declarations is enabled, this "
+                 "is deprecated and will be removed in the future. If you have "
+                 "a need for enabling it please file an issue at "
+                 "https://github.com/protocolbuffers/protobuf/issues with "
+                 "your use case."
+              << std::endl;
+    std::cerr.flush();
+  }
+  if (!generation_options.generate_minimal_imports &&
+      !absl::StrContains(options_warnings_suppressions,
+                         "generate_minimal_imports")) {
+    std::cerr << "WARNING: generate_minimal_imports is disabled, this is "
+                 "deprecated and will be removed in the future. If you have a "
+                 "need for disabling it please file an issue at "
+                 "https://github.com/protocolbuffers/protobuf/issues with "
+                 "your use case."
+              << std::endl;
+    std::cerr.flush();
+  }
+  if (!generation_options.strip_custom_options &&
+      !absl::StrContains(options_warnings_suppressions,
+                         "strip_custom_options")) {
+    std::cerr << "WARNING: strip_custom_options is disabled, this is deprecated"
+                 "and will be removed in the future. If you have a need for "
+                 "disabling it please file an issue at "
+                 "https://github.com/protocolbuffers/protobuf/issues with "
+                 "your use case."
+              << std::endl;
+    std::cerr.flush();
+  }
+
+  // -----------------------------------------------------------------
+
   // These are not official generation options and could be removed/changed in
   // the future and doing that won't count as a breaking change.
   bool headers_only = getenv("GPB_OBJC_HEADERS_ONLY") != nullptr;
@@ -417,7 +470,7 @@ bool ObjectiveCGenerator::GenerateAll(
         }
       }
     }  // if (!headers_only && skip_impls.count(file->name()) == 0)
-  }    // for(file : files)
+  }  // for(file : files)
 
   return true;
 }