Language: C

macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let 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")
    };
}

pub trait LexicalPermutation {
    /// Return `true` if the slice was permuted, `false` if it is already
    /// at the last ordered permutation.
    fn next_permutation(&mut self) -> bool;
    /// Return `true` if the slice was permuted, `false` if it is already
    /// at the first ordered permutation.
    fn prev_permutation(&mut self) -> bool;
}

impl<T> LexicalPermutation for [T]
where
    T: Ord,
{
    /// Original author in Rust: Thomas Backman <serenity@exscape.org>
    fn next_permutation(&mut self) -> bool {
        // These cases only have 1 permutation each, so we can't do anything.
        if self.len() < 2 {
            return false;
        }

        // Step 1: Identify the longest, rightmost weakly decreasing part of the vector
        let mut i = self.len() - 1;
        while i > 0 && self[i - 1] >= self[i] {
            i -= 1;
        }

        // If that is the entire vector, this is the last-ordered permutation.
        if i == 0 {
            return false;
        }

        // Step 2: Find the rightmost element larger than the pivot (i-1)
        let mut j = self.len() - 1;
        while j >= i && self[j] <= self[i - 1] {
            j -= 1;
        }

        // Step 3: Swap that element with the pivot
        self.swap(j, i - 1);

        // Step 4: Reverse the (previously) weakly decreasing part
        self[i..].reverse();

        true
    }

    fn prev_permutation(&mut self) -> bool {
        // These cases only have 1 permutation each, so we can't do anything.
        if self.len() < 2 {
            return false;
        }

        // Step 1: Identify the longest, rightmost weakly increasing part of the vector
        let mut i = self.len() - 1;
        while i > 0 && self[i - 1] <= self[i] {
            i -= 1;
        }

        // If that is the entire vector, this is the first-ordered permutation.
        if i == 0 {
            return false;
        }

        // Step 2: Reverse the weakly increasing part
        self[i..].reverse();

        // Step 3: Find the rightmost element equal to or bigger than the pivot (i-1)
        let mut j = self.len() - 1;
        while j >= i && self[j - 1] < self[i - 1] {
            j -= 1;
        }

        // Step 4: Swap that element with the pivot
        self.swap(i - 1, j);

        true
    }
}

#[allow(dead_code)]
fn sieve_of_eratosthenes(n: usize) -> Vec<usize> {
    // spf:
    // store smallest prime factor of number
    // 最小の素因数を格納
    // is_prime:
    // 配列番号が素数であるかどうかを示す
    let mut spf = vec![None; n + 1];
    let mut is_prime = vec![true; n + 1];
    let mut primes = Vec::new();

    // 0 と 1 は素数ではない
    is_prime[0] = false;
    is_prime[1] = false;

    for i in 2..n + 1 {
        // is_prime[i]:
        // それ以下の素数で割り切られていなければ True となる
        if is_prime[i] {
            // この if 文より i は素数なので
            primes.push(i);

            // 素数はそれ自身の最も小さい素因数であるから
            spf[i] = Some(i);
        }

        // 全ての prime の倍数(prime * i)を取り除く
        for prime in &primes {
            // prime が i の最も小さい因数よりも大きくなってしまったら探索終了
            if i * prime >= n + 1 || prime > &spf[i].unwrap() {
                break;
            }

            is_prime[i * prime] = false;

            // i * primes[j] の最も小さい素数は primes[j] 自身
            spf[i * prime] = Some(*prime);
        }
    }
    primes
}
use std::cmp::Ordering;
#[derive(Eq, PartialEq, Clone, Debug)]
pub struct Rev<T>(pub T);

impl<T: PartialOrd> PartialOrd for Rev<T> {
    fn partial_cmp(&self, other: &Rev<T>) -> Option<Ordering> {
        other.0.partial_cmp(&self.0)
    }
}

impl<T: Ord> Ord for Rev<T> {
    fn cmp(&self, other: &Rev<T>) -> Ordering {
        other.0.cmp(&self.0)
    }
}
#[allow(dead_code)]
fn gcd(a: u64, b: u64) -> u64 {
    if b == 0 {
        a
    } else {
        gcd(b, a % b)
    }
}
#[allow(dead_code)]
fn lcm(a: u64, b: u64) -> u64 {
    a / gcd(a, b) * b
}

#[allow(dead_code)]
fn expected(p: u64) -> f64 {
    let p = p as f64;
    ((p * (p + 1.0)) / 2.0) / p
}

#[allow(unused_imports)]
use std::collections::BTreeSet;
#[allow(unused_imports)]
use std::collections::VecDeque;

#[allow(unused_mut)]
#[allow(unused_variables)]
#[allow(non_snake_case)]
fn main() {
    input! {R: f64}

    println!("{}", std::f64::consts::PI * 2. * R);
}


Compiler output:
main.c:1:12: error: expected '=', ',', ';', 'asm' or '__attribute__' before '!' token
    1 | macro_rules! input {
      |            ^
main.c:18:12: error: expected '=', ',', ';', 'asm' or '__attribute__' before '!' token
   18 | macro_rules! input_inner {
      |            ^
main.c:22:12: error: expected '=', ',', ';', 'asm' or '__attribute__' before '!' token
   22 | macro_rules! read_value {
      |            ^
main.c:28:10: error: too many decimal points in number
   28 |         (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
      |          ^~~~~~~
main.c:44:1: error: unknown type name 'pub'
   44 | pub trait LexicalPermutation {
      | ^~~
main.c:44:11: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'LexicalPermutation'
   44 | pub trait LexicalPermutation {
      |           ^~~~~~~~~~~~~~~~~~
main.c:53:5: error: expected '=', ',', ';', 'asm' or '__attribute__' before '<' token
   53 | impl<T> LexicalPermutation for [T]
      |     ^
main.c:123:2: error: invalid preprocessing directive #[
  123 | #[allow(dead_code)]
      |  ^
main.c:124:1: error: unknown type name 'fn'
  124 | fn sieve_of_eratosthenes(n: usize) -> Vec<usize> {
      | ^~
main.c:124:27: error: expected ')' before ':' token
  124 | fn sieve_of_eratosthenes(n: usize) -> Vec<usize> {
      |                           ^
      |                           )
main.c:138:14: error: too many decimal points in number
  138 |     for i in 2..n + 1 {
      |              ^~~~
main.c:164:1: error: unknown type name 'use'
  164 | use std::cmp::Ordering;
      | ^~~
main.c:164:8: error: expected '=', ',', ';', 'asm' or '__attribute__' before '::' token
  164 | use std::cmp::Ordering;
      |        ^~
main.c:165:2: error: invalid preprocessing directive #[
  165 | #[derive(Eq, PartialEq, Clone, Debug)]
      |  ^
main.c:166:4: error: expected ';' before 'struct'
  166 | pub struct Rev<T>(pub T);
      |    ^~~~~~~
      |    ;
main.c:166:15: error: expected identifier or '(' before '<' token
  166 | pub struct Rev<T>(pub T);
      |               ^
main.c:168:5: error: expected '=', ',', ';', 'asm' or '__attribute__' before '<' token
  168 | impl<T: PartialOrd> PartialOrd for Rev<T> {
      |     ^
main.c:170:14: error: too many decimal points in number
  170 |         other.0.partial_cmp(&self.0)
      |              ^~~~~~~~~~~~~~
main.c:174:5: error: expected '=', ',', ';', 'asm' or '__attribute__' before '<' token
  174 | impl<T: Ord> Ord for Rev<T> {
      |     ^
main.c:176:14: error: too many decimal points in number
  176 |         other.0.cmp(&self.0)
      |              ^~~~~~
main.c:179:2: error: invalid preprocessing directive #[
  179 | #[allow(dead_code)]
      |  ^
main.c:180:1: error: unknown type name 'fn'
  180 | fn gcd(a: u64, b: u64) -> u64 {
      | ^~
main.c:180:9: error: expected ')' before ':' token
  180 | fn gcd(a: u64, b: u64) -> u64 {
      |         ^
      |         )
main.c:187:2: error: invalid preprocessing directive #[
  187 | #[allow(dead_code)]
      |  ^
main.c:188:1: error: unknown type name 'fn'
  188 | fn lcm(a: u64, b: u64) -> u64 {
      | ^~
main.c:188:9: error: expected ')' before ':' token
  188 | fn lcm(a: u64, b: u64) -> u64 {
      |         ^
      |         )
main.c:192:2: error: invalid preprocessing directive #[
  192 | #[allow(dead_code)]
      |  ^
main.c:193:1: error: unknown type name 'fn'
  193 | fn expected(p: u64) -> f64 {
      | ^~
main.c:193:14: error: expected ')' before ':' token
  193 | fn expected(p: u64) -> f64 {
      |              ^
      |              )
main.c:198:2: error: invalid preprocessing directive #[
  198 | #[allow(unused_imports)]
      |  ^
main.c:199:1: error: unknown type name 'use'
  199 | use std::collections::BTreeSet;
      | ^~~
main.c:199:8: error: expected '=', ',', ';', 'asm' or '__attribute__' before '::' token
  199 | use std::collections::BTreeSet;
      |        ^~
main.c:200:2: error: invalid preprocessing directive #[
  200 | #[allow(unused_imports)]
      |  ^
main.c:201:1: error: unknown type name 'use'
  201 | use std::collections::VecDeque;
      | ^~~
main.c:201:8: error: expected '=', ',', ';', 'asm' or '__attribute__' before '::' token
  201 | use std::collections::VecDeque;
      |        ^~
main.c:203:2: error: invalid preprocessing directive #[
  203 | #[allow(unused_mut)]
      |  ^
main.c:204:2: error: invalid preprocessing directive #[
  204 | #[allow(unused_variables)]
      |  ^
main.c:205:2: error: invalid preprocessing directive #[
  205 | #[allow(non_snake_case)]
      |  ^
main.c:206:1: error: unknown type name 'fn'
  206 | fn main() {
      | ^~
main.c: In function 'main':
main.c:207:5: error: 'input' undeclared (first use in this function); did you mean 'int'?
  207 |     input! {R: f64}
      |     ^~~~~
      |     int
main.c:207:5: note: each undeclared identifier is reported only once for each function it appears in
main.c:207:10: error: expected ';' before '!' token
  207 |     input! {R: f64}
      |          ^
      |          ;