Not the OP, but I'm thinking about like closures?

Say you wanted to make a Node.js framework with callbacks that react to an event. The callback might be a closure that captured some of its surrounding variables. At that point, any of the captured variables are not trivially stack-allocated.

You might be able to do something similar to what Rust does with moving though.

Say you wanted a map of some key to growable arrays (or maps), where the number of keys isn't known until runtime. It I understand correctly, you can't really do that because the number of growable stacks nees to be known at compile time.