feature request: have --features flag check available features

[savente93]

cargo add by default already queries crates.io for the latest versions which is great. I'm wondering if it's also possible to have it query available feature flags at the same time? This comes from a slight frustration in the following scenario:

> cargo add -p common csv --features serde
    Updating 'https://github.com/rust-lang/crates.io-index' index
      Adding csv v1.1.6 to dependencies with features: ["serde"]
> cargo build
    the package `common` depends on `csv`, with features: `serde` but `csv` does not have these features.
    It has a required dependency with that name, but only optional dependencies can be used as features.

This is a frequent occurrence since a lot of crates have common features like serde or derive but they're often used interchangeably so it can be frustrating to try and find out which one it is. I personally use cargo add all the time with this, and it would be a really nice quality of life feature to have cargo add check for this. Would this be possible? In any case, thanks for all the great work!

[epage]

The registry contains features, we also have logic for getting Cargo.toml from git repos and paths. The main issue is plugging all of this together to get the list of features to validate them.

[epage]

#193 is related

[savente93]

The main grievance I had when opening this issue is mostly addressed now that #538 et. al. is merged, so I just wanted to check if anyone thinks this would be a substantial enough improvement to add? I guess it would guard against typos etc. I'm happy to work on it sometime if people still think it would be worth it, otherwise I'm happy to close it.

[epage]

Oh, I think validating inputs would be a big help! I see showing the list as something to help with discoverability. For myself, if I thought I knew what the features were already, I'm unlikely to check the list. This is on top of identifying typos, as you mentioned.