An undeclared type or module was used.

Erroneous code example:

```compile_fail,E0433
let map = HashMap::new();
// error: failed to resolve: use of undeclared type or module `HashMap`
```

Please verify you didn't misspell the type/module's name or that you didn't
forget to import it:


```
use std::collections::HashMap; // HashMap has been imported.
let map: HashMap<u32, u32> = HashMap::new(); // So it can be used!
```