I don't know Zig, but aren't we missing a return here?

  fn count_nonzero(a: []const i32) i32 {
    var count: i32 = 0;
    for (items) |value| { // "for" works only on arrays and slices, use >"while" for generic loops.
       if (value == 0) {
        continue;
     }
    count += 1; // there is no increment operator, but there are shortcuts for +=, \*=, >>= etc.
   }
  }