Add feature dependencies to all targets but lib

[tsurai]

This small PR adds the ability to use feature dependencies for all targets but lib fixing issue #1570

# ...

[features]
a = []
b = []

[[example]]
name = "ex_5"
features = ["a", "b"]

[[bin]]
name = "foobar"
path = "src/foo.rs"
features = ["a"]

Please tell me if and how the user should be notified about ignored targets as I am not sure how verbose cargo should be. Big thanks to @alexcrichton for guiding me.

As always, critique is much appreciated.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @wycats (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[alexcrichton]

The set of activated features for a package is actually best learned through Resolve which should be available by the time this gets down below I believe.

[alexcrichton]

Thanks! Can you be sure to add some tests for this as well?

[bors]

☔ The latest upstream changes (presumably #2068) made this pull request unmergeable. Please resolve the merge conflicts.

[tsurai]

I've made the changes you suggested and added a few tests. Please tell me if those are insufficient.

[alexcrichton]

Hm perhaps this changed from before? It looks like the map part of filter_map isn't being used, so this may be able to just be a filter.

It may also be more clear here to explicitly use if/else instead of a match for the logic, the multi-line guard for one of the arms was a little jarring.

[alexcrichton]

Also, instead of having targets be of the type CargoResult<Vec<_>> it's probably best to have it just be Vec<_> (e.g. remove all the Ok above) and just wrap it in Ok after the filtering is done.

Ah and as a final point I think this can use retain instead of iter + filter + collect

[bors]

☔ The latest upstream changes (presumably #2126) made this pull request unmergeable. Please resolve the merge conflicts.

[JanLikar]

Is this still being worked on?

[alexcrichton]

@JanLikar I think this may be going a bit stale, so if you want to pick it up feel free to!

[ebkalderon]

I am really looking forward to this feature. Any progress on this?

[alexcrichton]

I think this should be good to go with comments addressed and a rebase, but @ebkalderon if you'd like to spearhead that feel free!

[JanLikar]

I'm working on this.

[alexcrichton]

Closing in favor of the rebase over at #2325, but thanks for the PR regardless @tsurai!