Update safety requirements of Vec::from_raw_parts?

[EFanZh]

The “Safety” section of the document of Vec::from_raw_parts says:

• length needs to be less than or equal to capacity.
• capacity needs to be the capacity that the pointer was allocated with.

I think the first requirement should also add “The first length elements starting from ptr are initialized”. And the second requirement could be relaxed into “capacity needs to be less than or equal to the capacity that the pointer was allocated with”, this is because Allocator::deallocate does not require specifying exact size that the pointer was allocated with.

[CryZe]

Two opinions:

1. The Allocator API is unstable, I don't think it would make sense to adjust documentation to guarantee unstable behavior.
2. It's actually even more tricky than your wording, because it also needs to be at least the size of the Layout that was used when allocating the memory block, which you may or may not even know about depending on whether the Vec itself was using the Allocator or you yourself are constructing a Vec from a memory block you manually allocated. So given that Vec probably doesn't want to guarantee any growing behavior, you can't know the Layout that the Vec used, and thus it wouldn't be safe (in general) to use any capacity that is less than what you got from the Vec.

[TheDestroyer19]

I just noticed that from_raw_parts and from_raw_parts_in doesn't list a requirement for the first length elements of ptrmust be initialized. A possible wording would be

• The first length must be initialized.