Trying the 'Discover Ray Tracing' Sample Code in Swift

I have enjoyed looking through the 'Discover Ray Tracing with Metal' video and Sample Code. While trying to recreate the Project in Swift, I have run across an issue with setting up an MTLPrimativeAccelerationStructureDescriptor.

In Renderer.mm ( Objective C ), within the createAccelerationStructures function, a loop is created:

Code Block 
// Create a primitive acceleration structure for each piece of geometry in the scene
	for (NSUInteger i = 0; i < _scene.geometries.count; i++) {
	Geometry *mesh = _scene.geometries[i];
	MTLAccelerationStructureGeometryDescriptor *geometryDescriptor = [mesh geometryDescriptor];
	// Assign each piece of geometry a consecutive slot in the intersection function table
        geometryDescriptor.intersectionFunctionTableOffset = i;
	// Create a primitive acceleration structure descriptor to contain the single piece of acceleration structure geometry
	MTLPrimitiveAccelerationStructureDescriptor *accelDescriptor = [MTLPrimitiveAccelerationStructureDescriptor descriptor];
	accelDescriptor.geometryDescriptors = @[ geometryDescriptor ];
	// Build the acceleration structure.
	id <MTLAccelerationStructure> accelerationStructure = [self newAccelerationStructureWithDescriptor:accelDescriptor];
	// Add the acceleration structure to the array of primitive acceleration structures.
	[_primitiveAccelerationStructures addObject:accelerationStructure];
	}

In Swift, I am trying:

Code Block 
for i in 0..<( scene.nodes.count )
	{
	let aMesh = scene.nodes[i]
	let geometryDescriptor: MTLAccelerationStructureGeometryDescriptor = aMesh.getGeometryDescriptor()
	// Assign each piece of geometry a consecutive slot in the intersection function table
            geometryDescriptor.intersectionFunctionTableOffset = i
	// Create a primitive acceleration structure descriptor to contain the single piece of acceleration structure geometry
	var accelDescriptor: MTLPrimitiveAccelerationStructureDescriptor = MTLPrimitiveAccelerationStructureDescriptor()
	accelDescriptor.geometryDescriptors.append( geometryDescriptor )
	// Build the acceleration structure.
	// To follow...
	// Add the acceleration structure to the array of primitive acceleration structures.
	 // To follow...
	}

The compiler flags appending the geometryDescriptor with "Cannot use mutating member on immutable value: 'accelDescriptor is immutable'". I have tried a couple of compiler suggested ideas to unwrap the accelDescriptor.geometryDescriptors, but end up with a nil value in the geometryDescriptors array.

It may be naive to simply append the geometryDescriptor, but I would appreciate any suggestions to fill in the accelDescriptor.

In the larger picture, it would be great to have a Swift Project sample to use as a starting point. The concepts spelled out in the video and Objective C project are very compelling!

Thanks so much - John Lobe