Cartesian Product in Go
I think you should replace mixedBaseInc with a generator that returns the combinations of indexes. That would simplify XProd by taking out numElms and the construction of digits.

That gives you the option to parallelise XProd by instantiating more instances of the goroutine that outputs the product vectors (because the closure no longer binds digits). If that is the bottleneck then that improves throughput.

However it depends on the program where this is used; if most of the work is done by the consumer of the output vectors then the best speed-up is for the consumer to consume in a way that can be parallelised.

An alternative approach is to build the output vectors one element at a time — https://github.com/schwarmco/go-cartesian-product/blob/master/cartesian.go for example . That solution has pros and cons and it's a bit more complicated to increase its parallelism, but it might be much better on some inputs (perhaps if there are a large number of small input sets to the product).