Deny warnings for doc tests

[taiki-e]

We cleaned this up with some PRs (#1590, #1381, etc.), so we are not warned at the moment, but it will be useful to maintain the quality of the examples. (EDIT: I missed unused being allowed by default.)

It would have been nice if this was supported as a cargo doc or clippy option (maybe like cargo doc --test -- --Dwarnings?), but it does not seem to be supported yet (rust-lang/rust#56232).

[taiki-e]

Also, this seems to have already been fixed.

[cramertj]

It would have been nice if this was supported as a cargo doc or clippy option (maybe like cargo doc --test -- --Dwarnings?)

Could we maybe bypass it by setting RUSTFLAGS directly?

[taiki-e]

Could we maybe bypass it by setting RUSTFLAGS directly?

I'll try it.

UPDATE: RUSTFLAGS did not go well.

[Nemo157]

I found this while looking through the rustdoc source trying to figure out why RUSTFLAGS doesn't actually apply (docs here):

#![doc(test(attr(deny(warnings))))]

adding that at the crate root will inject the contained attribute into all tests automatically.

EDIT: Although that has what I consider the wrong semantics still, there's no way to trigger it from just CI, it will make the examples fail when building locally. The only workaround I can think of is to use something like #![cfg_attr(feature = "ci-only-fail-doc-test-warnings", doc(test(attr(deny(warnings)))))], but then you won't be shown the warnings when building locally (rustdoc suppresses compiler output if the test passes). So probably better to have them denied everywhere until it's possible to run the lints against them independently.

[taiki-e]

@Nemo157 Thanks for finding it out.

#![doc(test(attr(deny(warnings))))]

It seems that it also denies the lints that rustdoc allows by default, so I feel that it is better to allow dead_code and so on. Probably like:

#![doc(test(attr(deny(warnings), allow(dead_code, unused_assignments, unused_variables))))]

I also agree that it is preferable to deny everywhere until it's possible to run the lints against them independently.

[cramertj]

Nice! Thanks to both of you for the investigation and the fixes.