The byte buffers that Go's Reader reads from and that Go's Writer writes into cannot in general be allocated on the stack because they are considered to escape. Because Reader and Writer are interfaces, calls are dispatched virtually, so escape analysis cannot always see through them. This is now fixed for simple cases in Go, but only very recently: https://github.com/golang/go/issues/19361

Ironically, Java HotSpot handles the use case of Reader and Writer better than Go does, since when it's not able to allocate on the stack it has fast allocation due to the use of a generational GC with bump allocation in the nursery. By virtue of the fact that it's a JIT, HotSpot can also do neat things like see that there's only one implementation of an interface and thereby devirtualize calls to that interface (allowing for escape analysis to kick in better), something Go cannot do in general as it's an AOT compiler.