With line numbers:
function add(a: number, b: number): number {
  return a + b;
}
With title:
fib.rs
fn fib(n: u64) -> u64 {
  if n <= 1 {
    return n;
  }
  fib(n - 1) + fib(n - 2)
}
With character highlighting:
#!/bin/bash
set -euo pipefail
 
function fib() {
  local number=$1; local a=0; local b=1
  for ((index = 0; index < number; index++)); do
    local temp=$a
    a=$b
    b=$((temp + b))
  done
  echo $a
}
 
fib $1