Borrowing: References

Instead of transferring ownership every time you want to inspect a value, you can borrow it. Borrowing creates references (pointers to the original data).

The Borrowing Rules

Rust strictly enforces these two compile-time constraints:

1. You may have any number of shared references (&T) to a resource.
2. OR you may have exactly one exclusive mutable reference (&mut T) to a resource.
3. You cannot have both at the same time.

let mut active_profile = String::from("Default");

let ref1 = &active_profile; // OK: Shared borrow
let ref2 = &active_profile; // OK: Multiple shared borrows are allowed

// let mut_ref = &mut active_profile; // ERROR: Cannot borrow as mutable while shared borrows exist!

Why?

Enforcing these rules prevents data races. A data race occurs when two references access the exact same memory address concurrently, at least one is writing, and there is no thread synchronization. By blocking concurrent read-write access, Rust eliminates crashes in multi-threaded input listeners before they can compile.