I.e. a `HashMap` struct, or `Vec` struct don't directly contain the data.

For example the `Vec` is defined internally as something similar to:

`struct Vec<T> { data: *mut [T], capacity: usize, len: usize, marker: PhantomData<T> }`

(Slightly simplified, not actual Vec type).

So a move of a Vec copies at most 3 usize (24 bytes on 64bit systems), similar thinks apply for a HashMap.

Additionally the copy can often be elided through compiler optimizations.

As a interesting side note a new empty Vec/HashMap will not actually allocate any memory, only once elements get added it will start doing so. This is why it crates vec's of vecs of length 1. Or else it wouldn't need to do "number of element" free calls.