feat: treating required-feature as default feature for cli

[cathaysia]

What does this PR try to resolve?

fix #4663

This PR changes the semantics of required-features for binary targets, treating them as default features. When --no-default-features is not specified, the features listed in required-features are automatically enabled.

How to test and review this PR?

$ cargo install xidlc # success
$ cargo install xidlc@0.38.0 --no-default-features # failed
$ cargo install xidlc@0.38.0 --no-default-features -F cli -F fmt #  success

[rustbot]

r? @weihanglo

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The reviewer was selected based on:

• Owners of files modified in this PR: @ehuss, @epage, @weihanglo
• @ehuss, @epage, @weihanglo expanded to ehuss, epage, weihanglo
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[epage]

I'm going to close this pending design discussions per our contrib guide. #4663 is marked as S-needs-design and not S-accepted. In particular, this is going in a completely new discussion from previous discussions and needs more work on the proposal.

[cathaysia]

I noticed that issue dates back about nine years, and there doesn't seem to have been any significant progress. Is there really any chance of further progress?

[cathaysia]

I have no objections to the process itself; however, given that it has been shelved for so long—and rather than getting bogged down in endless disputes—why not implement a version first to see what the response is?

[weihanglo]

Please see #4663 (comment) and the RFC rust-lang/rfcs#3374. There are some technical and design problems needing to resolve and experiment.

why not implement a version first to see what the response is?

I am not sure whether your first version referring to a stable version or not. A PR that aims for insta-stable is unlikely to get merge. Cargo follows a strong compatibility guarantee. That said, you are welcome to experiment and if it is working good we can start from a nightly feature.