Patch MSVC system includes to use -isystem

Monkey-patch ffi-clang MswinArgs to use -isystem instead of -I for
auto-discovered system includes. This ensures clang treats MSVC STL
headers as system headers, preventing the generator from processing
internals like xstring.

Remove once ffi-clang >= 0.15.1 is released.

Author: cfis

lib/ruby-bindgen/parser.rb

@@ -36,6 +36,7 @@ def initialize(inputter, clang_args, libclang: nil)
       require 'ffi/clang'
       require 'ruby-bindgen/refinements/cursor'
       require 'ruby-bindgen/refinements/type'
+      require 'ruby-bindgen/refinements/mswin_args'
 
       @index = FFI::Clang::Index.new(exclude_declarations_from_pch: false, display_diagnostics: true)
     end

lib/ruby-bindgen/refinements/mswin_args.rb

@@ -0,0 +1,28 @@
+# Compat patch for ffi-clang < 0.15.1 on MSVC.
+# Uses -isystem instead of -I for auto-discovered system includes so
+# that clang treats them as system headers (in_system_header? returns true).
+# Without this, the generator processes STL internals like xstring.
+#
+# Remove once ffi-clang >= 0.15.1 is released.
+
+if defined?(FFI::Clang::MswinArgs)
+  module FFI
+    module Clang
+      class MswinArgs < Args
+        private
+
+        def extra_args(command_line_args)
+          args = []
+
+          system_includes.each do |path|
+            unless command_line_args.include?(path)
+              args.push("-isystem", path)
+            end
+          end
+
+          args
+        end
+      end
+    end
+  end
+end