auto merge of #9936 : madjar/rust/master, r=alexcrichton
This should close #9468.
I removed the test stating that nested comments should not be implemented.
I had a little chicken-and-egg problem because a comment of the std contains "/*", and adding support for nested comment creates a backward incompatibility in that case, so I had to use a dirty hack to get stage1 and stage2 to compile. This part should be revert when this commit lands in a snapshot.
This is my first non-typo contribution, so I'm open to any comment.