Ownership and moves
Because variables are in charge of freeing their own resources, resources can only have one owner. This prevents resources from being freed more than once. Note that not all variables own resources (e.g. references).
When doing assignments (let x = y) or passing function arguments by value
(foo(x)), the ownership of the resources is transferred. In Rust-speak,
this is known as a move.
After moving resources, the previous owner can no longer be used. This avoids creating dangling pointers.