An undeclared type or module was used.
Erroneous code example:
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!