A plugin/crate was declared but cannot be found. Erroneous code example:

```compile_fail,E0463
#![feature(plugin)]
#![plugin(cookie_monster)] // error: can't find crate for `cookie_monster`
extern crate cake_is_a_lie; // error: can't find crate for `cake_is_a_lie`
```

You need to link your code to the relevant crate in order to be able to use it
(through Cargo or the `-L` option of rustc example). Plugins are crates as
well, and you link to them the same way.