Borrowing a collection

We cannot modify a collection once we have a reference on it:

let mut v = vec![1, 2, 3];
let elem = &v[0]; // immutable borrow
v.push(10); // mutable borrow ==> COMPILER ERROR!
(elem, v)

Error: The variable `elem` contains a reference with a non-static lifetime so
can't be persisted. You can prevent this error by making sure that the
variable goes out of scope - i.e. wrapping the code in {}.

[E0502] Error: cannot borrow `v` as mutable because it is also borrowed as immutable
   ╭─[ command_2:1:1 ]
   │
 3 │ v.push(10); // mutable borrow ==> COMPILER ERROR!
   │ ─────┬────  
   │      ╰────── mutable borrow occurs here
   │
   ├─[ command_2:1:1 ]
   │
 2 │ let elem = &v[0]; // immutable borrow
   │             ┬  
   │             ╰── immutable borrow occurs here
   │ 
   │ Note: note: requirement that the value outlives `'static` introduced here
───╯

[E0505] Error: cannot move out of `v` because it is borrowed
   ╭─[ command_2:1:1 ]
   │
 4 │ (elem, v)
   │        ┬  
   │        ╰── move out of `v` occurs here
   │
   ├─[ command_2:1:1 ]
   │
 2 │ let elem = &v[0]; // immutable borrow
   │             ┬  
   │             ╰── borrow of `v` occurs here
   │
   ├─[ command_2:1:1 ]
   │
 1 │ let mut v = vec![1, 2, 3];
   │     ──┬──  
   │       ╰──── binding `v` declared here
 2 │ let elem = &v[0]; // immutable borrow
   │              │ 
   │              ╰─ help: consider cloning the value if the performance cost is acceptable: `.clone()`
───╯

30.2 Borrowing a collection