> Or there is a way to heap allocate value of inline type by wrapping it into some utility type, similar to Rust's Box?

There isn't. For this to work, functions for inline types have to be compiled such that one of these approaches is used:

1. They always take inline values by pointer, and we have to guarantee those pointers are never invalidated. This again means you need some sort of borrow checking scheme.

2. We compile two versions for each function: one that takes the data by value, and one by pointer, resulting in significant code bloat and compiler complexity.

I think the split also better captures the intent: heap types are for cyclic and heavily mutated values, inline types are more for short-lived mostly immutable values.