Sure, here is the playground link, crippled a bit to give you some output (ie, doesn't use b.N).
The sources for the func (r BenchmarkResult) String() stringhere basically don't give a call to r.MemString(), or have access to B.showAllocResult, because the *B or showAllocResult isn't copied into the struct (ie, there isn't a property for it, yet).
I don't think we should always print the memory results, and I don't think we should add a new field to BenchmarkResult, a struct that currently has only exported fields. It's already possible to call MemString; adding the results to String would be a minor convenience in a few cases. So all in all I don't think this is worth doing. Sorry.