Suppose we have a vector of some type that can be cloned
let foo_vec = vec![clonable_item_1, clonable_item_2, ...];
How to determine whether to use .clone() and .cloned() when iterating?
foo_vec.iter().cloned()...
// vs
foo_vec.clone().iter()...
I couldn't find anything written about the difference between the two. What's the difference?
They're not at all equal. If anything, it should be v.iter().cloned() vs. v.clone().into_iter(), both produce an iterator over owned T while v.clone().iter() produces an iterator over &T.
v.clone().into_iter() clones the Vec, allocating a Vec with the same size and cloning all elements into it, then converts this newly created Vec into an iterator. v.iter().cloned(), OTOH, creates a borrowed iterator over the Vec that yields &T, then applies the cloned() iterator adapter to it, which on-the-fly clones the &T produced by Iterator::next() to produce an owned T. Thus it doesn't need to allocate a new vector.
Because of that, you should always prefer v.iter().cloned() when possible (usually it is, but Vec's IntoIter has additional capabilities, like getting the underlying slice that may be required).