Understanding Rust Error E0010

Error E0010: Creating Boxed Values at Compile Time

In Rust, error E0010 occurs when you attempt to create a boxed value at compile time. Because statics and constants in Rust need to be known at compile time, creating boxed values, which allocates memory on the heap at runtime, is not allowed.

Erroneous Code Example

#![allow(unused)]
fn main() {
const CON : Vec<i32> = vec![1, 2, 3];
}

In this example, we attempt to create a constant named CON that contains a Vec<i32> with the values 1, 2, and 3. However, because Vec<i32> allocates memory on the heap at runtime, this results in error E0010.

Non-boxed Values for Constants

To solve this issue, consider using non-boxed values for constants. Commonly used alternatives include arrays, which can be initialized with a fixed size at compile time.

Correct Code Example

#![allow(unused)]
fn main() {
const CON : [i32; 3] = [1, 2, 3];
}

Here, we update the erroneous code by substituting the Vec<i32> with a fixed-size array [i32; 3]. Error E0010 no longer occurs, as array sizes are determined at compile time.

Lazy_static Library

If you need to use more complex data structures or require a heap-allocated value, consider using the lazy_static library. The library provides a macro that allows you to declare static values with heap-allocated data that are initialized at runtime when first accessed.

Lazy_static Example

#![allow(unused)]
#[macro_use]
extern crate lazy_static;
use std::collections::HashMap;

lazy_static! {
    static ref CON: HashMap<i32, &'static str> = {
        let mut map = HashMap::new();
        map.insert(1, "One");
        map.insert(2, "Two");
        map.insert(3, "Three");
        map
    };
}

fn main() {
    println!("{}", CON.get(&1).unwrap());
}

We use the lazy_static macro to declare a static HashMap, and it is initialized only when first accessed, which avoids error E0010.