Handle nil backtrace_locations in generate_snippet

lib/error_highlight/core_ext.rb

@@ -5,7 +5,7 @@ module CoreExt
     private def generate_snippet
       if ArgumentError === self && message =~ /\A(?:wrong number of arguments|missing keyword[s]?|unknown keyword[s]?|no keywords accepted)\b/
         locs = self.backtrace_locations
-        return "" if locs.size < 2
+        return "" if locs.nil? || locs.size < 2
         callee_loc, caller_loc = locs
         callee_spot = ErrorHighlight.spot(self, backtrace_location: callee_loc, point_type: :name)
         caller_spot = ErrorHighlight.spot(self, backtrace_location: caller_loc, point_type: :name)

test/test_error_highlight.rb

@@ -1815,6 +1815,30 @@ def test_detailed_message_does_not_raise_when_argument_error_is_rewrapped
     end
   end
 
+  def test_detailed_message_does_not_raise_when_backtrace_locations_is_nil
+    # This reproduces a real-world crash: when an exception crosses a process
+    # boundary via Marshal, `backtrace` survives as strings but
+    # `backtrace_locations` becomes nil. A keyword ArgumentError then crashed
+    # CoreExt#generate_snippet with `undefined method 'size' for nil`.
+    begin
+      def_with_required_keyword
+    rescue ArgumentError => original
+      exc = Marshal.load(Marshal.dump(original))
+    end
+
+    assert_nil exc.backtrace_locations
+
+    msg = nil
+    assert_nothing_raised do
+      msg = exc.detailed_message(highlight: false)
+    end
+    assert_match("missing keyword", msg)
+
+    assert_nothing_raised do
+      exc.full_message(highlight: false)
+    end
+  end
+
   private
 
   def find_node_by_id(node, node_id)