Language: C++

#![allow(unused_imports)]
#![allow(unused_macros)]
#![allow(unknown_lints)]
#![allow(unused_variables)]
#![allow(unused_must_use)]
#![allow(unused_mut)]
#![allow(dead_code)]
#![allow(unused_assignments)]

use std::cmp;
use std::process::exit;
use std::collections::{
    HashMap,
    VecDeque
};
use std::io::{
    stdin,
    stdout,
    stderr,
    Read,
    Write,
    BufReader,
    BufWriter
};

macro_rules! d {
    ($t:expr, $($e:expr),*) => {
        #[cfg(debug_assertions)]
        $({
            let (e, mut err) = (stringify!($e), stderr());
            writeln!(err, "\x1B[33m{}\x1B[0m = {:?}", $t, $e).unwrap()
        })*
    };
    ($($e:expr),*) => {
        #[cfg(debug_assertions)]
        $({
            let (e, mut err) = (stringify!($e), stderr());
            writeln!(err, "\x1B[33m{}\x1B[0m = {:?}", e, $e).unwrap()
        })*
    };
}

macro_rules! e {
    ($($arg:tt)*) => {
        #[cfg(debug_assertions)]
        {
            let mut err = stderr();
            let e = format!($($arg)*);
            write!(err, "\x1B[32m{}\x1B[0m", e).unwrap()
        }
    };
}

macro_rules! eln {
    ($($arg:tt)*) => {
        #[cfg(debug_assertions)]
        {
            let mut err = stderr();
            let e = format!($($arg)*);
            writeln!(err, "\x1B[32m{}\x1B[0m", e).unwrap()
        }
    };
}

macro_rules! stdin {
    () => ({
        use std::io::Read;
        let mut s = String::new();
        std::io::stdin().read_to_string(&mut s).unwrap();
        s
    })
}

macro_rules! test {
    (name = $name:ident, $($input:expr => $output:expr),* $(,)*) => (
        #[test]
        fn $name() {
            $(
                assert_eq!(solve($input.to_string()), $output);
            )*
        }
    );
    ($($input:expr => $output:expr),* $(,)*) => (
        #[test]
        fn solve_test() {
            $(
                assert_eq!(solve($input.to_string()), $output);
            )*
        }
    );
}

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));
        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)*}
    };
}

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<_>>()
    };

    ($next:expr, chars) => {
        read_value!($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() {
    print!("{}", solve(stdin!()));
}

/* TEMPLATE END */

use std::ascii::AsciiExt;

fn solve(src: String) -> String {
    let mut res = String::from("");
    input!{
        source = src,
        n: usize,
        k: usize,
        s: String
    }
    eln!("###########################################");
    d!(n);
    d!(k);
    d!(s);
    eln!("-------------------------------------------");
    let v: Vec<char> = s.chars().collect();
    let mut after: Vec<char> = Vec::new();
    for i in 0..v.len() {
        if i == k-1 {
            after.push(v[i].to_ascii_lowercase());
        } else {
            after.push(v[i]);
        }
    }
    let after: String = after.into_iter().collect();
    res.push_str(&after.to_string());
    d!(res);
    res.push_str("\n");
    res
}

Compiler output:
a.cc:1:2: error: invalid preprocessing directive #!
    1 | #![allow(unused_imports)]
      |  ^
a.cc:2:2: error: invalid preprocessing directive #!
    2 | #![allow(unused_macros)]
      |  ^
a.cc:3:2: error: invalid preprocessing directive #!
    3 | #![allow(unknown_lints)]
      |  ^
a.cc:4:2: error: invalid preprocessing directive #!
    4 | #![allow(unused_variables)]
      |  ^
a.cc:5:2: error: invalid preprocessing directive #!
    5 | #![allow(unused_must_use)]
      |  ^
a.cc:6:2: error: invalid preprocessing directive #!
    6 | #![allow(unused_mut)]
      |  ^
a.cc:7:2: error: invalid preprocessing directive #!
    7 | #![allow(dead_code)]
      |  ^
a.cc:8:2: error: invalid preprocessing directive #!
    8 | #![allow(unused_assignments)]
      |  ^
a.cc:28:10: error: invalid preprocessing directive #[
   28 |         #[cfg(debug_assertions)]
      |          ^
a.cc:35:10: error: invalid preprocessing directive #[
   35 |         #[cfg(debug_assertions)]
      |          ^
a.cc:45:10: error: invalid preprocessing directive #[
   45 |         #[cfg(debug_assertions)]
      |          ^
a.cc:56:10: error: invalid preprocessing directive #[
   56 |         #[cfg(debug_assertions)]
      |          ^
a.cc:76:10: error: invalid preprocessing directive #[
   76 |         #[test]
      |          ^
a.cc:84:10: error: invalid preprocessing directive #[
   84 |         #[test]
      |          ^
a.cc:130:10: error: too many decimal points in number
  130 |         (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
      |          ^~~~~~~
a.cc:169:14: error: too many decimal points in number
  169 |     for i in 0..v.len() {
      |              ^~~~~~~~
a.cc:10:1: error: 'use' does not name a type
   10 | use std::cmp;
      | ^~~
a.cc:11:1: error: 'use' does not name a type
   11 | use std::process::exit;
      | ^~~
a.cc:12:1: error: 'use' does not name a type
   12 | use std::collections::{
      | ^~~
a.cc:16:1: error: 'use' does not name a type
   16 | use std::io::{
      | ^~~
a.cc:26:1: error: 'macro_rules' does not name a type
   26 | macro_rules! d {
      | ^~~~~~~~~~~
a.cc:43:1: error: 'macro_rules' does not name a type
   43 | macro_rules! e {
      | ^~~~~~~~~~~
a.cc:54:1: error: 'macro_rules' does not name a type
   54 | macro_rules! eln {
      | ^~~~~~~~~~~
a.cc:65:1: error: 'macro_rules' does not name a type
   65 | macro_rules! stdin {
      | ^~~~~~~~~~~
a.cc:74:1: error: 'macro_rules' does not name a type
   74 | macro_rules! test {
      | ^~~~~~~~~~~
a.cc:93:1: error: 'macro_rules' does not name a type
   93 | macro_rules! input {
      | ^~~~~~~~~~~
a.cc:114:1: error: 'macro_rules' does not name a type
  114 | macro_rules! input_inner {
      | ^~~~~~~~~~~
a.cc:124:1: error: 'macro_rules' does not name a type
  124 | macro_rules! read_value {
      | ^~~~~~~~~~~
a.cc:146:1: error: 'fn' does not name a type
  146 | fn main() {
      | ^~
a.cc:152:1: error: 'use' does not name a type
  152 | use std::ascii::AsciiExt;
      | ^~~
a.cc:154:1: error: 'fn' does not name a type
  154 | fn solve(src: String) -> String {
      | ^~