add flag to optionally document dev dependencies

[yaahc]

This is working. One issue that I noticed right away is that a second run of cargo doc --open without the --dev option after it has already been run once with dev documentation enabled it will still show the documentation items from the last run.

closes #3475

[rust-highfive]

r? @alexcrichton

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[ehuss]

it will still show the documentation items from the last run.

This is normal. The rustdoc code is written such that it is additive, it doesn't remove previously documented crates (unless you run cargo clean).

I just had a couple observations:

• Why not also support build-dependencies? Maybe the flag could be changed to accommodate different dependency kinds? I don't want it to be too complicated, but it seems strange to leave that out.
• Implementation-wise, there are now two places where cargo doc options are handled: CompileMode::Doc{deps} and BuildConfig::document_dev_dependencies. I would like to see that unified into one place. My personal preference would be to remove the deps field from CompileMode::Doc, it is ignored 99% of the time. This could also tie into the first point, where the field in BuildConfig could be a single field that indicates which kinds of dependencies to include.

[ehuss]

Oh, and this will need a test when it is ready.

[bors]

☔ The latest upstream changes (presumably #7216) made this pull request unmergeable. Please resolve the merge conflicts.

[yaahc]

Gonna close this because I doubt I'll get to it any time soon, but if someone else wants to pick up where I left off they're more than welcome to.