Language: C++

#[allow(unused_imports)]
use std::cmp::{max, min, Ordering};
#[allow(unused_imports)]
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
#[allow(unused_imports)]
use std::iter::FromIterator;
// input macro
// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String {
            bytes
                .by_ref()
                .map(|r| r.unwrap() as char)
                .skip_while(|c| c.is_whitespace())
                .take_while(|c| !c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};
 
    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
 
    ($next:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}
 
macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };
 
    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };
 
    ($next:expr, [ $t:tt ]) => {
        {
            let len = read_value!($next, usize);
            (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
        }
    };
 
    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };
 
    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };
 
    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}

fn main() {
    input!{
        _n: usize,
        k: usize,
        mut s: chars,
    }
    s[k] = s[k].clone().to_lowercase().collect::<Vec<_>>()[0];
    println!("{:?}", s.into_iter().collect::<String>());
}


Compiler output:
a.cc:1:2: error: invalid preprocessing directive #[
    1 | #[allow(unused_imports)]
      |  ^
a.cc:3:2: error: invalid preprocessing directive #[
    3 | #[allow(unused_imports)]
      |  ^
a.cc:5:2: error: invalid preprocessing directive #[
    5 | #[allow(unused_imports)]
      |  ^
a.cc:51:10: error: too many decimal points in number
   51 |         (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
      |          ^~~~~~~
a.cc:57:14: error: too many decimal points in number
   57 |             (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
      |              ^~~~~~
a.cc:2:1: error: 'use' does not name a type
    2 | use std::cmp::{max, min, Ordering};
      | ^~~
a.cc:4:1: error: 'use' does not name a type
    4 | use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
      | ^~~
a.cc:6:1: error: 'use' does not name a type
    6 | use std::iter::FromIterator;
      | ^~~
a.cc:9:1: error: 'macro_rules' does not name a type
    9 | macro_rules! input {
      | ^~~~~~~~~~~
a.cc:30:1: error: 'macro_rules' does not name a type
   30 | macro_rules! input_inner {
      | ^~~~~~~~~~~
a.cc:45:1: error: 'macro_rules' does not name a type
   45 | macro_rules! read_value {
      | ^~~~~~~~~~~
a.cc:74:1: error: 'fn' does not name a type
   74 | fn main() {
      | ^~