Clarify confusing wording on Vec

[steveklabnik]

Originally reported here: https://news.ycombinator.com/item?id=15969317

---

I don't understand the following sentence:

... it is strongly recommended that you only free memory allocated by a Vec by creating a new Vec and dropping it.

Could someone parse this for me? To me, it reads like "to free memory allocated by a Vec v, create a new Vec w and drop it (i.e., w)".

This is obviously not what it is intended. What's the role of the second Vec? Is this only in a context where v is empty but its memory not yet freed? If so, why wouldn't shrink_to_fit work in that case?

[hanna-kruppe]

This refers to unsafe code that e.g. uses Vec::with_capacity to allocate memory, and then mem::forgets the Vec to use the buffer in different ways (e.g., passing it to C, or implement a custom data structure over the allocated memory).

[Manishearth]

#46884