Only apply default-members when building root manifest

[Nemo157]

Fixes #5932

The first commit includes all actual changes, the second commit is just running Asciidoctor on the already updated man-pages source, so I suggest looking at the individual commits.

[rust-highfive]

r? @ehuss

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[ehuss]

Thanks for updating the documentation. ❤️

I'm slightly positive to this change. I want to see what others at @rust-lang/cargo think about it. The summary is:

This changes default-members to be ignored when in a non-root member to only build that member.

Without default-members, nothing changes:

Command	Virtual	Packages Selected
cargo in ws/	✓	All members
cargo in ws/		Root crate only
cargo in ws/foo/	✓	foo only
cargo in ws/foo/		foo only

With default members, this PR changes the last two lines in this table from default-members to "foo only":

Command	Virtual	Packages Selected
cargo in ws/	✓	default-members only
cargo in ws/		default-members only
cargo in ws/foo/	✓	default-members only
cargo in ws/foo/		default-members only

I'm a little concerned this may disrupt existing workflows. It may be a little more consistent with the non-default-members case.

I'm also curious what @SimonSapin thinks. He added it for servo, though it appears servo no longer uses it. I don't use any projects with this feature, so I don't have any experience with it.

[alexcrichton]

This seems like a reasonable change in behavior to me as well!

[Nemo157]

I've rebased your doc suggestions in.

[bors]

☔ The latest upstream changes (presumably #6791) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

I'm gonna close this since it's been inactive for some time now, but it can of course be resubmitted! (just trying to help clear out the queue)

[plauche]

@Nemo157 any plans to pick this back up? I've run into the same unexpected behaviour when using default-members. I may restart this work if you aren't planning on continuing with it.

[Nemo157]

I've rebased the branch, and the tests still pass.

I was waiting on the feedback that @ehuss requested above, if they think that consensus on merging this can be had I'll happily open another PR for it. I'm not working on the project where I was having this issue much anymore so I didn't have the motivation to try and push for getting this merged.

[plauche]

@ehuss do you have any input on moving forward with this PR? I'd like to use the default-members feature, however the current behaviour is when working in member crates is not very desirable.

[ehuss]

I have posted a request for feedback at https://internals.rust-lang.org/t/cargo-change-in-workspace-default-members/10649. If there aren't any significant objections, then I'll go ahead and approve this PR.

[ehuss]

I haven't seen any objections, so I'm going to go ahead and merge this. However, github won't let me reopen, so I will open a new PR and merge that (#7270).