Language: C++

// from https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

#[allow(unused_macros)]
macro_rules! debug { ($x: expr) => { println!("{}: {:?}", stringify!($x), $x) } }
// macro_rules! debug { ($x: expr) => () }

#[allow(dead_code)]
fn show<T>(iter: T) -> String
where
    T: Iterator,
    T::Item: std::fmt::Display
{
    let mut res = iter.fold(String::new(), |sum, e| sum + &format!("{} ", e));
    res.pop();
    res
}

#[allow(unused_imports)]
use std::cmp::{max, min};
#[allow(unused_imports)]
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
#[allow(unused_imports)]
use std::collections::btree_map::Entry::{Occupied, Vacant};
#[allow(unused_imports)]
use std::mem::swap;

#[allow(dead_code)]
fn getline() -> String {
    let mut res = String::new();
    std::io::stdin().read_line(&mut res).ok();
    res
}

#[allow(unused_macros)]
macro_rules! readl {
    ($t: ty) => {
        {
            let s = getline();
            s.trim().parse::<$t>().unwrap()
        }
    };
    ($( $t: ty),+ ) => {
        {
            let s = getline();
            let mut iter = s.trim().split(' ');
            ($(iter.next().unwrap().parse::<$t>().unwrap(),)*)
        }
    };
}

#[allow(unused_macros)]
macro_rules! readlvec {
    ($t: ty) => {
        {
            let s = getline();
            let iter = s.trim().split(' ');
            iter.map(|x| x.parse().unwrap()).collect::<Vec<$t>>()
        }
    }
}

fn main() {
    use std::io::Write;
    let out = std::io::stdout();
    let mut out = std::io::BufWriter::new(out.lock());
    macro_rules! printf { ($($arg:tt)*) => (write!(out, $($arg)*).unwrap()); }
    macro_rules! printfln { () => (writeln!(out).unwrap()); ($($arg:tt)*) => (writeln!(out, $($arg)*).unwrap()); }
    
    let t = readl!(usize);
    if t == 1 {
        printfln!("Hello world!");
    } else {
        let a = readl!(usize);
        let b = readl!(usize);
        printfln!("{}", a+b);
    }
}


Compiler output:
a.cc:35:10: error: too many decimal points in number
   35 |         (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
      |          ^~~~~~~
a.cc:51:2: error: invalid preprocessing directive #[
   51 | #[allow(unused_macros)]
      |  ^
a.cc:55:2: error: invalid preprocessing directive #[
   55 | #[allow(dead_code)]
      |  ^
a.cc:66:2: error: invalid preprocessing directive #[
   66 | #[allow(unused_imports)]
      |  ^
a.cc:68:2: error: invalid preprocessing directive #[
   68 | #[allow(unused_imports)]
      |  ^
a.cc:70:2: error: invalid preprocessing directive #[
   70 | #[allow(unused_imports)]
      |  ^
a.cc:72:2: error: invalid preprocessing directive #[
   72 | #[allow(unused_imports)]
      |  ^
a.cc:75:2: error: invalid preprocessing directive #[
   75 | #[allow(dead_code)]
      |  ^
a.cc:82:2: error: invalid preprocessing directive #[
   82 | #[allow(unused_macros)]
      |  ^
a.cc:99:2: error: invalid preprocessing directive #[
   99 | #[allow(unused_macros)]
      |  ^
a.cc:2:1: error: 'macro_rules' does not name a type
    2 | macro_rules! input {
      | ^~~~~~~~~~~
a.cc:19:1: error: 'macro_rules' does not name a type
   19 | macro_rules! input_inner {
      | ^~~~~~~~~~~
a.cc:29:1: error: 'macro_rules' does not name a type
   29 | macro_rules! read_value {
      | ^~~~~~~~~~~
a.cc:52:1: error: 'macro_rules' does not name a type
   52 | macro_rules! debug { ($x: expr) => { println!("{}: {:?}", stringify!($x), $x) } }
      | ^~~~~~~~~~~
a.cc:56:1: error: 'fn' does not name a type
   56 | fn show<T>(iter: T) -> String
      | ^~
a.cc:67:1: error: 'use' does not name a type
   67 | use std::cmp::{max, min};
      | ^~~
a.cc:69:1: error: 'use' does not name a type
   69 | use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
      | ^~~
a.cc:71:1: error: 'use' does not name a type
   71 | use std::collections::btree_map::Entry::{Occupied, Vacant};
      | ^~~
a.cc:73:1: error: 'use' does not name a type
   73 | use std::mem::swap;
      | ^~~
a.cc:76:1: error: 'fn' does not name a type
   76 | fn getline() -> String {
      | ^~
a.cc:83:1: error: 'macro_rules' does not name a type
   83 | macro_rules! readl {
      | ^~~~~~~~~~~
a.cc:100:1: error: 'macro_rules' does not name a type
  100 | macro_rules! readlvec {
      | ^~~~~~~~~~~
a.cc:110:1: error: 'fn' does not name a type
  110 | fn main() {
      | ^~