What Is the Difference Between iter(), iter_mut(), and into_iter()?

The difference lies in ownership and mutability: iter() borrows immutably, iter_mut() borrows mutably, and into_iter() takes ownership.

let v = vec![1, 2, 3];

// Immutable borrow: reads values, keeps v usable
for x in v.iter() { println!("{}", x); }

// Mutable borrow: modifies values, keeps v usable
for x in v.iter_mut() { *x += 1; }

// Takes ownership: consumes v, returns owned values
for x in v.into_iter() { println!("{}", x); }

These methods control how you access items in a collection. iter() lets you read items without changing them, iter_mut() lets you change them, and into_iter() takes the collection apart to give you the items themselves. Think of iter() as looking at a book, iter_mut() as writing in the margins, and into_iter() as tearing the pages out.