如何从Rust的BigInt中减去1？

Question

I'd like this program to compile and print 314158 when executed:

extern crate num;

use num::{BigInt, FromPrimitive, One};

fn main() {
    let p: BigInt = FromPrimitive::from_usize(314159).unwrap();
    let q: BigInt = p - One::one();
    println!("q = {}", q);
} // end main

The compiler error is:

error[E0284]: type annotations required: cannot resolve `<num::BigInt as std::ops::Sub<_>>::Output == num::BigInt`
 --> src/main.rs:7:23
  |
7 |     let q: BigInt = p - One::one();
  |                       ^

Answer 1

Rust follows an open world hypothesis when it comes to traits. It knows, based on your annotations, that p is BigInt . It also knows that One::one() has a type which implements One . So Rust is looking for a subtraction operator on BigInt which takes a One -like thing as an argument.

num::BigInt as std::ops::Sub<Foo>>

where Foo implements One . Trouble is, BigInt implements Sub in several different ways , so Rust doesn't know whether you're trying to subtract a i32 , a u64 , or another BigInt from p .

One answer is to be more explicit with your types.

let p: BigInt = FromPrimitive::from_usize(314159).unwrap();
let one: BigInt = One::one();
let q: BigInt = p - one;

However, more succinctly, you may take advantage of the fact that BigInt implements One and help the compiler with type inference that way.

let p: BigInt = FromPrimitive::from_usize(314159).unwrap();
let q: BigInt = p - BigInt::one();

(Thanks, @loganfsmyth, for this latter solution!)