Don't inspect hidden classes when printing captured calls

ext/fiber/profiler/capture.c

@@ -627,6 +627,15 @@ static size_t Fiber_Profiler_Capture_absolute_nesting(struct Fiber_Profiler_Capt
 // If a call is within this threshold of the parent call, it will be skipped when printing the call stack - it's considered inconsequential to the performance of the parent call.
 static const double Fiber_Profiler_Capture_SKIP_THRESHOLD = 0.98;
 
+// Hidden classes (e.g. the singleton class of RubyVM::FrozenCore, which implements lambda literals) raise NotImplementedError from #inspect, so we use their class path instead:
+static VALUE Fiber_Profiler_Capture_Call_class_inspect(struct Fiber_Profiler_Capture_Call *call) {
+	if (!RB_SPECIAL_CONST_P(call->klass) && RBASIC_CLASS(call->klass) == 0) {
+		return rb_class_path(call->klass);
+	}
+
+	return rb_inspect(call->klass);
+}
+
 void Fiber_Profiler_Capture_print_tty(struct Fiber_Profiler_Capture *capture, FILE *restrict stream, double duration) {
 	double start_time = Fiber_Profiler_Time_delta(&capture->start_time, &capture->switch_time);
 
@@ -677,7 +686,7 @@ void Fiber_Profiler_Capture_print_tty(struct Fiber_Profiler_Capture *capture, FI
 			fprintf(stream, "\e[31m");
 		}
 
-		VALUE class_inspect = rb_inspect(call->klass);
+		VALUE class_inspect = Fiber_Profiler_Capture_Call_class_inspect(call);
 		const char *name = rb_id2name(call->id);
 
 		struct timespec offset;
@@ -731,7 +740,7 @@ void Fiber_Profiler_Capture_print_json(struct Fiber_Profiler_Capture *capture, F
 			call->nesting = call->parent->nesting + 1;
 		}
 
-		VALUE class_inspect = rb_inspect(call->klass);
+		VALUE class_inspect = Fiber_Profiler_Capture_Call_class_inspect(call);
 		const char *name = rb_id2name(call->id);
 
 		size_t nesting = Fiber_Profiler_Capture_absolute_nesting(capture, call);

test/fiber/profiler/capture.rb

@@ -140,6 +140,32 @@ def nested(n = 100, &block)
 		end
 	end
 
+	with "hidden classes" do
+		let(:capture) {subject.new(stall_threshold: 0, filter_threshold: 0, output: output)}
+
+		it "can report garbage collection inside methods of hidden classes" do
+			capture.start
+
+			Fiber.new do
+				GC.stress = true
+
+				# Lambda literals call a method on the hidden singleton class of RubyVM::FrozenCore:
+				->{}
+			ensure
+				GC.stress = false
+			end.resume
+
+			capture.stop
+
+			stall = JSON.parse(output.string)
+			calls = stall["calls"]
+			expect(calls).to have_value(have_keys(
+				"class" => be =~ /\A#<Class:0x\h+>\z/,
+				"method" => be == "lambda",
+			))
+		end
+	end
+
 	with "Process.fork" do
 		it "should disable the profiler in the child process after fork" do
 			capture.start