If you start worrying about "what if <C concern> underlying the Go runtime happens?" you'll find a lot worse than realloc. Luckily, in the absence of runtime bugs, you don't have to think about it.
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.
Sorry, this is nonsensically mixed up. Even if it reallocs in place, which it may be free to do, the language semantics still guarantee only one observer gets that extended space. Otherwise you would need to worry about this even when dealing with immutable structures like concatenating strings.
