Iterator adaptors take an iterator (which usually, but not always, comes from a collection, e.g. "my_vec.into_iter()") and give you another iterator back, without allocating an intermediate collection; the iterator itself stores only the current state of the iteration process. You can apply iterator adaptors to such iterators as the receiving end of a channel ( http://doc.rust-lang.org/std/sync/mpsc/struct.Receiver.html#... ), and you can pass around the iterator and add other computations to the end of it before you "run" the whole thing.
However, I'm not sure what you mean by the transformation processes being functions. In Rust these processes (iterators) are values, and have methods that produce a derived process by appending a step; for a concrete example:
let fb_chars = "foobar".chars(); //fb_chars is an iterator
let not_o = fb_chars.filter(|&x| x!='o'); //not_o is an iterator; we got it by applying the filter iterator adaptor
for i in not_o {println!("char: {}", i);} //run the iterator with a for loop; we could also have run it by calling .collect() and obtaining a collection