Is reusing an allocation while changing its size a thing you expect to be able to do? I would believe that some languages/systems/etc can do that, but it certainly feels like an exception rather than a rule. Reuse generally means the whole block of memory is retained, from what I've seen, because you'd have to track that half-freed memory for reuse somehow and that has some associated cost. (A compacting-GC language would be a reasonable exception here, for example)
> Is reusing an allocation while changing its size a thing you expect to be able to do?
It's something you should expect to be able to try to do. The underlying allocator may reject the request depending on context (maybe it is works for large sizes only, for example). This is provided by Rust's Allocator trait realloc_in_place() API, which returns CannotReallocInPlace if it isn't possible.
For Vec::collect, in the event that the storage cannot be reused smaller in place, I think it would be reasonable to free it and allocate an appropriately sized buffer instead.
