If we have a varible, we can borrow the variable with a reference
so that we can read it later without owning the variable ourselves.
let a = String::from("xyz");
let aref = &a;
println!("{aref}"); // the same as println!("{a}");We can also take a mutable reference and use it to change the contents of the variable by remote control.
let a = String::from("xyz");
let amut = &mut a;
*amut = String::from("abc"); // changes the contents of a.
println!("{amut}"); // prints "abc"But because of potential multithreaded badness, taking a reference effectively locks that variable in a read-only state. This prevents one thread writing to a variable as another thread is reading it. This prevents race conditions between the two threads which will result in you being called out of bed at 3am on new years day to fix a bug.
let a = String::from("xyz");
let aref = &a; // 'locks' a.
let bref = &a; // this is fine as two immutable refs are ok.
let amut = &mut a; // Error: a is in a locked state.Only a single mutable reference is allowed to a variable when no immutable references are still alive.