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};
```