100-exercises-to-learn-rust/book/src/06_ticket_management/11_mutable_slices.md

Mutable slices

Every time we’ve talked about slice types (like str and [T]), we’ve used their immutable borrow form (&str and &[T]).
But slices can also be mutable!

Here’s how you create a mutable slice:

let mut numbers = vec![1, 2, 3];
let slice: &mut [i32] = &mut numbers;

You can then modify the elements in the slice:

slice[0] = 42;

This will change the first element of the Vec to 42.

Limitations

When working with immutable borrows, the recommendation was clear: prefer slice references over references to the owned type (e.g. &[T] over &Vec<T>).
That’s not the case with mutable borrows.

Consider this scenario:

let mut numbers = Vec::with_capacity(2);
let mut slice: &mut [i32] = &mut numbers;
slice.push(1);

It won’t compile!
push is a method on Vec, not on slices. This is the manifestation of a more general principle: Rust won’t allow you to add or remove elements from a slice. You will only be able to modify/replace the elements that are already there.

In this regard, a &mut Vec or a &mut String are strictly more powerful than a &mut [T] or a &mut str.
Choose the type that best fits based on the operations you need to perform.