Understanding Rust Error E0015: Calling Non-Const Functions in Const Context

Overview of Rust Error E0015

Rust Error E0015 occurs when attempting to call a non-const function within a const context, such as a constant or static expression. To resolve the error, any functions used in a const context must be declared as const.

Erroneous Code Example

#![allow(unused)]
fn main() {
fn create_some() -> Option<u8> {
    Some(1)
}

// error: cannot call non-const fn `create_some` in constants
const FOO: Option<u8> = create_some();
}

In the above code, the `create_some` function is not const, and it is being called in a constant expression. This results in the E0015 error.

Correcting the Error

To fix the error, declare the function as const. Here is the corrected code from the example:

#![allow(unused)]
fn main() {
// declared as a `const` function:
const fn create_some() -> Option<u8> {
    Some(1)
}

const FOO: Option<u8> = create_some(); // no error!
}

In this updated code, the `create_some` function is declared as const, resolving the E0015 error.

Rules of Const Functions

There are certain restrictions and guidelines for using const functions in Rust. When declaring a const function, it is important to note that it cannot:

1. Call other non-const functions or methods.
2. Access mutable data.
3. Use unsafe blocks.
4. Contain non-integer mutables.
5. Modify any non-integer mutable variables.

Conclusion

In summary, Rust Error E0015 emerges when a non-const function is called within a const context. To rectify this error, ensure that all functions used in constant or static expressions are declared as const. Remember to follow the rules of const functions to avoid further errors.