Pass `doc` cfg to rustc when running cargo doc

[Fryuni]

Problem

From the issue originally reported to rustdoc by @notriddle:

I tried this code:

// crate a
pub struct MyStruct;
#[cfg(doc)]
impl MyStruct {
    pub fn doc_only_method() {}
}

// crate b
#[doc(inline)]
pub use a::MyStruct;

I expected to see this happen: I expected doc_only_method to appear in crate b

Instead, this happened: It didn't.

---

Proposed Solution

Rustdoc adds the doc cfg before generating the docs for a crate (here), but the metadata from dependencies are generated using rustc without such flag, causing the problem above.

cargo doc should add the cfg to the rustc calls that generate the metadata passed to rustdoc.

Notes

A few possibly related issues:

• Pass --cfg doc to check builds of dependencies as part of cargo doc/rustdoc #8811
• Cargo doc does not pass --cfg doc for build scripts #8944
• #[cfg(doc)] does not work in Cargo.toml #11570

[Fryuni]

I learned about this on a mostly unrelated issue, but this inconsistency on when and where the doc config is present causes problems when you make a crate re-exporting something using the exact intended, and documented, use case for this cfg.

[fmease]

I pretty sure that this issue is a duplicate of #8811 which you have already linked. Would you agree?

[Fryuni]

I might be wrong, but it seems to me like it is two different use cases that could be solved by the same solution (passing --cfg doc to rustc).

That one seems to be about dependencies breaking because they behave differently when generating docs while this issue is about the dependent crate breaking (or having incorrect docs) due to a difference in the configs passed to it and its dependencies.

If I understood that incorrectly and they are actually the same problem than sure, we can close this one as duplicate.

[ehuss]

These do appear to be the same issue to me, so closing in favor of #8811.