# Error code E0015

A non-`const` function was called in a `const` context.

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();
    }

All functions used in a `const` context (constant or static expression) must be marked `const`.

To fix this error, you can declare `create_some` as a constant function:

    #![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!
    }