To expand (hopefully correctly, I'm still learning Rust): You can have a single "mutable borrow" in a single lexical scope where you can mutate the vector _and

To expand (hopefully correctly, I'm still learning Rust): You can have a single "mutable borrow" in a single lexical scope where you can mutate the vector _and its contents_ to your liking, but you can't pass a vector mid-mutation to another scope to read the vector while that borrow is active. It is safe to mutate the vector and its contents, as the vector owns the items inside it and there is only one active user of the collection. If you don't have a mutable borrow active, you can immutably borrow it as many times as you like and iterate over it to your heart's content in as many scopes as you like, but you can't mutate it. These borrows are lexically scoped. This is a start to explaining ownership: https://doc.rust-lang.org/book/second-edition/ch04-01-what-is-ownership.html https://doc.rust-lang.org/book/second-edition/ch04-01-what-i...