Inspired by a discussion here: https://github.com/JuliaLang/julia/issues/14112#issuecomment-159715454, it might be nice to be able to reduce the amount of "padded" (allocated-but-not-yet-used) memory for dynamic structures in certain cases where we're not planning to expand the structures significantly (and can tolerate allocation delays if we do).
Does a function that does something like this exist already?
