One problem with non-mainstream languages (such as Scala) is that tools are hard to find, because they are hard to build.
This technical paper Branch Coverage for Arbitrary Languages Made Easy (I'm the author) describes how to build test coverage tools for langauges in systematic way to help get around this problem, using a generic tool-building infrastructure.
We've implemented test coverage tools for Java, C#, COBOL, C, C++, PL/SQL, ... this way, including instrumenters, data collection and test coverage display and reporting.
It would be straightforward to implement Scala like this.
The solutions posed by other answers produces confusing information from the implementation of Scala ("auto genreated classes"). What developers want to see is coverage data in terms of their code. The approach we use instruments the source code, so the results are stated entirely and only in terms of the source code; even the test coverage viewer shows the source code covered with coverage information.
