Stack + Heap allocation store

[Per Nordlöw]

Walter's talk on DConf 2019 talks about a favorite design pattern 
of his - a hybrid approach to memory allocation.

A store first allocates on the stack and then when it grows too 
large it moves the data to a heap allocation (array).

I know such a struct is not hard to write but I ask anyway to 
collect advice on writing it.

Does Phobos have anything like that?

What about code.dlang.org?