Have an option to output warnings from doctests

[Susurrus]

Running cargo test runs doctests by default, and shows warnings for regular code, but not for the doctests themselves. This can be enabled by running cargo test -- --nocapture. I'd like to have warnings also show in our CI tests, even for doctests, but setting -- --nocapture shows all the output which we don't want (and can't control, because they come from the underlying libraries complaining about our tests that are supposed to fail).

I'd think having warnings on for doctests by default would be good, but that wouldn't be backwards compatible. Maybe a --doctest-warnings option could be added in lieu of that?

I believe this was done in 5d145c1, so discussion there might be worth reading. It seems that there are a bunch of warnings that come up with doctests that should be ignored, but there are also ones that can appear that shouldn't be. The question is how do we address both needs?

[GuillaumeGomez]

I can put it back if you want. Just waiting for @rust-lang/docs team confirmation.

[frewsxcv]

This might be a tools team question, less-so a docs team one.

[GuillaumeGomez]

Let's make them join the issue then!

cc @rust-lang/tools

[carols10cents]

I've updated the title to better reflect what I think this request is, please correct it if I've misunderstood.

Many doc tests are small and would have warnings for unused variables, etc. I wouldn't want to see these by default, but I'd be ok with a flag to turn on just output of doc test warnings. I don't think I'd use it, but I wouldn't mind it being there.

Is there any way to get a sense of how many people would use this if it existed and the kinds of problems it would prevent, in order to prioritize this?

[alexcrichton]

The standard library handles this by denying all warnings, but it seems reasonable to have a flag. I don't think we can change the default behavior.

[GuillaumeGomez]

Ok, so I think we can consider this as acceptable. I'll add the flag.