Enable doc_auto_cfg on docs.rs builds

[phil-opp]

This flag includes #[cfg] bounds in the generated documentation.

For example, the docs for the x86_64::structures::paging::mapper module now look like this:

We see that OffsetPageTable is now annotated with 64-bit because the module has #![cfg(target_pointer_width = "64")]. Similarly, the RecursivePageTable struct is now annotated with instructions because it is #[cfg(feature = "instructions")].

[Freax13]

What's the relationship between doc_cfg and doc_auto_cfg? The nightly features enables the doc_cfg feature, should it also enable doc_auto_cfg?

Overall I really like this change.

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The failing CI check is unrelated to this pr, though we should take a look at it nonetheless.

[phil-opp]

What's the relationship between doc_cfg and doc_auto_cfg?

See https://doc.rust-lang.org/rustdoc/unstable-features.html#doccfg-recording-what-platforms-or-features-are-required-for-code-to-be-present. The doc_cfg feature allows the same thing, but through manual annotations. Looks like we used in the past for annotating nightly-only macros that require inline assembly: 17ca2b0. Now that inline assembly is stable, we don't use that feature anymore, so I think we can remove it.

The nightly features enables the doc_cfg feature, should it also enable doc_auto_cfg?

It does make the generated docs nicer, but it increases the chance of nightly breakage. Given that cargo doc automatically resolves cfg flags when building the documentation of dependencies, the value for users of this crate would be limited. So I think it's better to keep it disabled by default.

[phil-opp]

I opened #408 to remove the doc_cfg feature.

[Freax13]

What's the relationship between doc_cfg and doc_auto_cfg?

See https://doc.rust-lang.org/rustdoc/unstable-features.html#doccfg-recording-what-platforms-or-features-are-required-for-code-to-be-present. The doc_cfg feature allows the same thing, but through manual annotations. Looks like we used in the past for annotating nightly-only macros that require inline assembly: 17ca2b0. Now that inline assembly is stable, we don't use that feature anymore, so I think we can remove it.

The nightly features enables the doc_cfg feature, should it also enable doc_auto_cfg?

It does make the generated docs nicer, but it increases the chance of nightly breakage. Given that cargo doc automatically resolves cfg flags when building the documentation of dependencies, the value for users of this crate would be limited. So I think it's better to keep it disabled by default.

Thanks for the explanations!