src/librustc_error_codes/error_codes/E0015.md

A constant item was initialized with something that is not a constant
expression.

Erroneous code example:

```compile_fail,E0015
fn create_some() -> Option<u8> {
    Some(1)
}

const FOO: Option<u8> = create_some(); // error!
```

The only functions that can be called in static or constant expressions are
`const` functions, and struct/enum constructors.

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

```
const fn create_some() -> Option<u8> { // declared as a const function
    Some(1)
}

const FOO: Option<u8> = create_some(); // ok!

// These are also working:
struct Bar {
    x: u8,
}

const OTHER_FOO: Option<u8> = Some(1);
const BAR: Bar = Bar {x: 1};
```