Understanding Rust Error E0080: Constant Evaluation Failed

Introduction

Rust Error E0080 occurs when the compiler encounters a constant expression that it cannot evaluate. This generally happens when an operation results in an integer overflow or involves dividing by zero. In this article, we'll examine the causes and solutions for this error in more depth.

Common Causes

1. Integer Overflow

Integer overflow occurs when an arithmetic operation on integer values goes beyond the range of the integer type. Here's an example that generates Rust Error E0080 due to integer overflow:

#![allow(unused)]
fn main() {
    enum Enum {
        X = (1 << 500),
    }
}

In this code, the constant expression (1 << 500) attempts to left shift the value 1 five hundred places. This results in an integer overflow for the default i32 type and throws error E0080.

2. Division by Zero

Another common cause of Rust Error E0080 is trying to divide a number by zero. Here's an example that demonstrates this:

#![allow(unused)]
fn main() {
    enum Enum {
        Y = (1 / 0),
    }
}

In this code, the constant expression (1 / 0) attempts to divide a number by zero. Division by zero is undefined behavior, and the compiler raises error E0080.

Solutions

To fix Rust Error E0080, you must examine the constant expressions in your code and ensure that they do not result in integer overflows or divisions by zero.

Solution for Integer Overflow

You can resolve integer overflow issues by using a larger integer type if the resulting value still fits within its range. For example, you can replace the i32 type with the i64 type in the erroneous code:

use std::num::NonZeroI64;

#![allow(unused)]
fn main() {
    enum Enum {
        X = ((1_i64 << 50) as isize),
    }
}

Here, we use i64 to accommodate the larger value without causing an integer overflow.

Solution for Division by Zero

To address divisions by zero, you must ensure that the divisor is a non-zero value. You can use conditional checks to verify that the divisor is not zero before performing the division. Alternatively, you can handle division-by-zero errors using the checked_div() method, which returns an Option type:

fn checked_divide(a: i32, b: i32) -> Option<i32> {
    a.checked_div(b)
}

fn main() {
    let a = 1;
    let b = 0;

    match checked_divide(a, b) {
        Some(result) => println!("Result: {}", result),
        None => println!("Error: Division by zero"),
    }
}

This code snippet uses the checked_divide function, which returns an Option type containing either the division result or None if the divisor is zero. This way, you can gracefully handle division-by-zero errors and prevent Rust Error E0080.