Cargo doc may fail no_std builds if a dev-dependency uses std

[ryan-summers]

Problem
When cargo doc is run on a no_std crate, where a dev-dependency uses std, cargo-doc will fail the build.

Since dev-dependencies are not required for the actual build, this is actually acceptable for cargo build. I would expect cargo doc to not fail the compilation if cargo build is successful.

Steps

1. Create a crate that is no_std
2. Utilize a dev-dependency that uses std
3. Run cargo build, note that build succeeds
4. Run cargo doc, note that cargo doc fails and indicates no crate found: std

Note: A sample of this issue can be found in the crate here: https://github.com/quartiq/stabilizer/tree/3a3cbce005a41e53ac8fac1dc68d107502b19511/dsp

nd-array uses std in this build. If the nd-array dev-depency is updated to use default-features = false, cargo-doc works just fine.

Possible Solution(s)
It's possible that cargo-doc shouldn't be building documentation for dev-dependencies unless explicitly asked, or it should build the dev-dependencies identically to how cargo-build does it.

Notes

$ cargo version
cargo 1.52.0 (69767412a 2021-04-21)

[ryan-summers]

Closing. This was actually unintended feature unification between dependencies and dev-dependencies that only came up when running cargo doc. Updating the cargo resolver to version 2 resolved this issue.