`cargo doc` with `build-std` should document the requested sysroot crates

[PatchMixolydic]

Problem

I'm currently working on a project that uses a custom target spec, so I need to use build-std to build core, alloc, and compiler_builtins. Naturally, this means that my target doesn't have precompiled documentation for core and alloc, which breaks intra-doc links and hides which APIs are available on my target (including the elusive AtomicU128).

Proposed Solution

If the project is already using build-std (eg. in .cargo/config.toml), cargo doc should automatically build the documentation for the requested sysroot crates. This would require rust-src, but build-std already requires that anyway.

As a workaround for this issue, I've written a script that makes the appropriate calls to rustdoc for my project. It's hacky (it depends on the layout of the sysroot and uses the exact names of build artifacts, including hashes), but I hope it may prove useful in informing any implementation efforts.

Notes

@rustbot label +Z-build-std

[ehuss]

Thanks for the report! This is a known issue tracked in rust-lang/wg-cargo-std-aware#88. I'm going to close as a duplicate of that. rust-lang/rfcs#3874 also has some things to say about it.