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+An undeclared crate, module, or type was used.
+
+Erroneous code example:
+
+```compile_fail,E0433
+let map = HashMap::new();
+// error: failed to resolve: use of undeclared type `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!
+```
+
+If you've expected to use a crate name:
+
+```compile_fail
+use ferris_wheel::BigO;
+// error: failed to resolve: use of undeclared crate or module `ferris_wheel`
+```
+
+Make sure the crate has been added as a dependency in `Cargo.toml`.
+
+To use a module from your current crate, add the `crate::` prefix to the path.