I think it's because the deps are not built with this cfg, only when rustdoc is run on them do we have them.

Yes, exactly.

This is essentially a Cargo issue. If you were to manually use rustc and rustdoc to document your projects, you could just pass --cfg doc to each rustc invocation to avoid this issue entirely.

If I understand cargo doc correctly, for a project with N crates, it calls rustc N-1 times (to produce .rmeta files for cross-crate reexports) and rustdoc N times¹ assuming a clean target directory. Now, we could in theory update cargo doc to pass --cfg doc to each rustc invocation. However that would force Cargo to recompile every single dependency whereas presently it can just reuse .rmeta files produced in a previous cargo <check|build|run> run which greatly reduces the time it takes to document your project.

Correct me if I'm wrong. cargo doc --verbose tells you the rustc and rustdoc invocations.

I think cargo doc doesn't set this config at all, it is injected by rustdoc in the generate_config function if I'm not mistaken. I'd post a link but copy link is not working correctly on my phone.

I though rustdoc was responsible for recursing through the dependencies, but if it gets information from executions of rust then this should be added to cargo doc

You might be right about generate_config, although that shouldn't contradict anything I was saying. Cargo doesn't pass --cfg doc to rustc which is the problem. The .rmeta files used by cross-crate reexports represent crates that weren't compiled with --cfg doc while ideally they would but that would require recompilation (cfgs have changed) drastically worsening the build times of docs in incremental settings.

Disclaimer: This is just what I can remember, I haven't double-checked any of this.

I though rustdoc was responsible for recursing through the dependencies

No, neither rustc nor rustdoc do that, that's the task of Cargo (or of any other Rust package manager).

Should we open an issue about this on cargo then? (Or someone did that already? If yes could share the link here pls)

I opened an issue there: rust-lang/cargo#12533

I found a few possibly related issues, but not specific for this:

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

Since the doc config is currently added by rustdoc itself it seems this is not handled by cargo at all.