Code review comment for lp:~albaguirre/mir/back-to-stack-alloc-mir-protobuf

I know I asked for this. Although now I realize it's actually less efficient than using Arenas. Because on the stack your object is indeed allocated super fast (adjusting the stack pointer has always taken one clock tick or less). But stack allocation doesn't solve the problem for the subobjects (variable length lists, strings) that are still allocated on the heap. Arenas however solve both problems with the speed of a stack:

   https://developers.google.com/protocol-buffers/docs/reference/arenas?hl=en

So I've got nothing against this proposal right now. But if it makes the syntax less like the Arena syntax then it might be harder for us to get that optimal performance with Arenas in the long run.