improve clone impl as per https://github.com/Lokathor/tinyvec/issues/143

[Lokathor]

• Closes TinyVec/ArrayVec's Clone uses the naive (default) version of Clone::clone_from. #143

[Shnatsel]

Wait, isn't clone_from supposed to discard the data that was previously contained by the collection? Why is there zip then? Nevermind I'm dumb

[Shnatsel]

I wonder if a fast path for Copy types is necessary, for cases like a long-ish vec of u64? I'm not sure the current implementation would lower to a memcpy()

Edit: something like Rgb struct would be more egregious that has several Copy types in it

[Lokathor]

Best as I understand, this is improving the situation in some but not all situations. If this code is bad for a particular copy type, then it was previously also bad for that type. EDIT: but i'm not generally familiar with when clone_from vs clone is called.

[Nemo157]

Might want to take-and-drop all the elements from o.len..self.len.

[Soveu]

Just remember to skip this part if o.len >= self.len

[Lokathor]

If I follow you right, we'd want to have something like:

if o.len < self.len {
self[o.len..].iter_mut().for_each(|r| take(r));
}

immediately after the for loop?

[Soveu]

if let Some(to_drop) = self.data.get_mut(o.len() .. self.len()) {
  to_drop.for_each(|x| take(x));
}