Confusing error when a fully-qualified class name is expected

[fpoli]

The following macro invocation seems to contain a valid Java class declaration, but duchess fails to parse it and the error message doesn't clearly explain why.

java_package!{
  package foo
  public interface Baz {}
  public interface Bazz {}
  public class Bar implements Baz, Bazz {}
}

error: unexpected token `... ss Bar implements Baz  <<< , >>>  Bazz {}` at offset 33, expected one of `["r#/"[./]/"#"]`

The reason is that the parser currently expects a fully-qualified class name after the extends, and a fully-qualified name contains at least a dot. The fixed version of the macro invocation is the following:

java_package!{
  package foo
  public interface Baz {}
  public interface Bazz {}
  public class Bar implements foo.Baz, foo.Bazz {} // <-- Note the "foo."
}

The error message above is one of the firsts that a user will encounter. Duchess should either report a more helpful error message, or the parser should accept the first code snippet. Note that a similar issue occurs when declaring the type of a field, a method parameter, or a return type. It would be great if a fix for this issue also addressed those cases.

[nikomatsakis]

That is pretty confusing, I agree. We should probably make the first one work.