It's not a concern about C, it's a concern about the underlying possibility expressed in C terms: sizeclass arenas are useful to an allocator, that means slack, which means the opportunity for in-place allocation resizing.
You advocated the use of a very specific behaviour of `append` as a DID and possibly a correctness requirement of programs.
My worry is about whether this behaviour is a hard specification of the Go language, or just an implementation detail of the primary Go implementation. And how programs applying your recommendation would handle such behaviour changing.
