Comments not allowed in module usage.

[Spatenheinz]

This fails parsing

mod foo #breaking comment

I dont see anything that should be problematic with this approach?
Specifically I can see that modules are only matched when:

          Some(Keyword::Mod)
            if self.next_are(&[Identifier, Identifier, StringToken])
              || self.next_are(&[Identifier, Identifier, Identifier, StringToken])
              || self.next_are(&[Identifier, Identifier, Eof])
              || self.next_are(&[Identifier, Identifier, Eol])
              || self.next_are(&[Identifier, QuestionMark]) =>

Is there a reason to explicitly match on Eof and Eol?
Also what does the triple indentifier, StringToken match?

[casey]

Good catch! Fixed in #2201.

Identifier, Identifier, Identifier, StringToken catches this case, which uses a shell-expanded string:

mod foo x"$HELLO/bar.just"