feat: add argument feature_group

[duskmoon314]

Add new feature feature_group

• When off, do nothing else
• When on
  • generate cfg attribute for every groupName
  • generate features.toml containing all groupName

close #614

[rust-highfive]

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[adamgreig]

Thanks for this PR! We discussed it a bit further at the meeting this evening and a couple of points were raised:

• toml_edit adds quite a bit to the compile time (and modifying Cargo.toml may surprise some users?), perhaps outputting a features.toml snippet that crate authors can copy into their Cargo.toml is simpler? We don't otherwise generate or interact with the crate's Cargo.toml.
• The command line flag seems a bit long-winded, maybe --group-features or --feature-group would be more obvious?

[duskmoon314]

perhaps outputting a features.toml snippet that crate authors can copy into their Cargo.toml is simpler

I agree. Previously, I just wanted to make it easier to use. But I think it is unexpected behavior for most users when trying to use this feature. I will change it soon.

maybe --group-features or --feature-group would be more obvious

Sure. It is indeed long. 🤣

[burrbull]

bors r+

[bors[bot]]

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