Shared and Unique Borrows

Rust puts constraints on the ways you can borrow values:

You can have one or more &T values at any given time, or
You can have exactly one &mut T value.

fn main() {
    let mut a: i32 = 10;
    let b: &i32 = &a;

    {
        let c: &mut i32 = &mut a;
        *c = 20;
    }

    println!("a: {a}");
    println!("b: {b}");
}

The above code does not compile because a is borrowed as mutable (through c) and as immutable (through b) at the same time.
Move the println! statement for b before the scope that introduces c to make the code compile.
After that change, the compiler realizes that b is only ever used before the new mutable borrow of a through c. This is a feature of the borrow checker called “non-lexical lifetimes”.