Ownership and borrowing with references

References let you use a value without taking ownership of it: the & syntax creates a reference, and the action of creating one is called borrowing. Because a reference does not own the value, the value is not dropped when the reference goes out of scope, so a function taking a reference parameter never has to return the value to hand ownership back. References are immutable by default — you cannot modify something you only have a & reference to; use &mut for a mutable reference, which makes the mutation intent explicit in the signature. A mutable reference is exclusive: while one exists you can have no other references to that value, mutable or immutable, and this restriction lets Rust prevent data races at compile time. A reference's scope runs from its introduction to its last use, so non-overlapping scopes let a mutable reference follow immutable ones. The compiler also guarantees references are never dangling. The rules: at any given time, either one mutable reference or any number of immutable references, and references must always be valid. Scope: Rust. Reviewed October 4, 2026.