stream doc tests are now disabled

[montekki]

Looks like they are no longer run since 73d7fea but they probably should be. At least in CI they are gone.

[yoshuawuyts]

Tagging in @stjepang

So the problem here is that if #[cfg(feature = "docs")], then we're using the following signature:

fn read_line<'a>(&'a mut self, buf: &'a mut String) -> ImplFuture<'a, io::Result<usize>>;

Otherwise, the signature is:

fn read_line<'a>(&'a mut self, buf: &'a mut String) -> ReadLine<'a, Self>;

We only wrote docs for the former but not the latter. The doc tests are not run on Travis because we're running tests with docs feature disabled, and we can't run them with the feature enabled because the code is broken in that case.

This is unfortunate and I'm not sure what's the best solution here. What if we copied examples in docs into a test file so that they get tested?

[montekki]

What's handy about rust docs especially with the cases like stdlib is that you:

1. are always sure that that example works (unless it's marked as non-run)
2. can press a "run" button and jump into the playground and run/modify it yourself however you like
3. Probably something else I'm missing

I am not sure that can be achieved with copying to testfile or some similar non-mainstream way, can it?