Is it possible to have a recursive function computed at compile time in Rust?

Question

I want to calculate factorial a const. I want to get something like this:

const N: usize = 4;
const N_PERMUTATIONS = factorial(N);

Obvious solutions (which do not work):

• const fn- conditional statements are not allowed (or at least not implemented) in const fn, none of the following will compile:

  const fn factorial(n: usize) -> usize {
      match n {
          0 => 1,
          _ => n * factorial(n-1)
      }
  }
  

  const fn factorial(n: usize) -> usize {
      if n == 0 {
          1
      } else {
          n * factorial(n-1)
      }
  }
  

• macros - expression evaluations are performed after all macro extensions. This macro will never reach the base case, because after four iterations an argument 4-1-1-1-1that does not match 0:

  macro_rules!factorial {
      (0) => (1);
      ($n:expr) => ($n * factorial($n-1));
  }
  

I also tried the following, which will work if it *evaluates a short circuit, but as-is has unconditional recursion that gives a stack overflow:

const fn factorial(n: usize) -> usize {
    ((n == 0) as usize) + ((n != 0) as usize) * n * factorial(n-1)
}

( ., factorial(n - ((n != 0) as usize)).)

.

Answer 1

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