A yield expression was used outside of the generator literal.
Erroneous code example:
```compile_fail,E0627
#![feature(generators, generator_trait)]
fn fake_generator() -> &'static str {
yield 1;
return "foo"
}
fn main() {
let mut generator = fake_generator;
}
```
The error occurs because keyword `yield` can only be used inside the generator
literal. This can be fixed by constructing the generator correctly.
```
#![feature(generators, generator_trait)]
fn main() {
let mut generator = || {
yield 1;
return "foo"
};
}
```