Report the attempted depth in generator nesting errors

ext/json/ext/generator/generator.c

@@ -1045,11 +1045,10 @@ json_object_i(VALUE key, VALUE val, VALUE _arg)
 static inline long increase_depth(struct generate_json_data *data)
 {
     JSON_Generator_State *state = data->state;
-    long depth = ++data->depth;
-    if (RB_UNLIKELY(depth > state->max_nesting && state->max_nesting)) {
-        rb_raise(eNestingError, "nesting of %ld is too deep. Did you try to serialize objects with circular references?", --data->depth);
+    if (RB_UNLIKELY(data->depth >= state->max_nesting && state->max_nesting)) {
+        rb_raise(eNestingError, "nesting of %ld is too deep. Did you try to serialize objects with circular references?", data->depth + 1);
     }
-    return depth;
+    return ++data->depth;
 }
 
 static void generate_json_object(FBuffer *buffer, struct generate_json_data *data, VALUE obj)

java/src/json/ext/GeneratorState.java

@@ -649,23 +649,21 @@ public RubyHash to_h(ThreadContext context) {
     }
 
     public int increaseDepth(ThreadContext context) {
-        depth++;
         checkMaxNesting(context);
-        return depth;
+        return ++depth;
     }
 
     public int decreaseDepth() {
         return --depth;
     }
 
     /**
-     * Checks if the current depth is allowed as per this state's options.
+     * Checks if increasing the depth is allowed as per this state's options.
      * @param context The current context
      */
     private void checkMaxNesting(ThreadContext context) {
-        if (maxNesting != 0 && depth > maxNesting) {
-            depth--;
-            throw Utils.newException(context, Utils.M_NESTING_ERROR, "nesting of " + depth + " is too deep. Did you try to serialize objects with circular references?");
+        if (maxNesting != 0 && depth >= maxNesting) {
+            throw Utils.newException(context, Utils.M_NESTING_ERROR, "nesting of " + (depth + 1) + " is too deep. Did you try to serialize objects with circular references?");
         }
     }
 }

test/json/json_generator_test.rb

@@ -477,6 +477,18 @@ def test_depth_nesting_error
     assert_raise(JSON::NestingError) { JSON.pretty_generate(ary) }
   end
 
+  def test_nesting_error_reports_attempted_depth
+    [[[]], { a: {} }].each do |object|
+      [0, 1].each do |depth|
+        state = JSON.state.new(depth: depth, max_nesting: depth + 1)
+        error = assert_raise(JSON::NestingError) { state.generate(object) }
+        assert_match(/\Anesting of #{depth + 2} is too deep\./, error.message)
+        assert_equal depth, state.depth
+        assert_equal '[]', state.generate([])
+      end
+    end
+  end
+
   def test_depth_nesting_error_to_json
     ary = []; ary << ary
     s = JSON.state.new(depth: 1)