cargo add should warn on binary crates

[ghost]

Problem

It was suggested that I should open this issue. Currently if you try to add a binary crate, it allows you to do that:

cargo add gitoxide

when it should probably fail, or at least warn the user that this is not something they should be doing.

Proposed Solution

No response

Notes

No response

[narayan-krishna]

OP are you planning to tackle this? Can I claim it?

[ghost]

@narayan-krishna I would like to work on it, but I am quite busy. If you have time you are welcome to work on it 😊

[narayan-krishna]

Awesome! Would love to! @rustbot claim

[epage]

Note that the fix for this is not straight forward.

cargo add is relying solely on the Index. This has a limited set of data. There other issues that would benefit from having more data in the Index, like rust-lang/crates.io#5882 (which also links out to others). We'd likely need to do some planning before moving forward with adding this to the Index to make sure it handles the different needs.

It could be worked around by, at the end of cargo add, causing the relevant crates to be downloaded if we aren't in offline mode like a cargo check would and then use the other code within cargo to determine if a [lib] is present in the crate. The cargo install code (src/cargo/ops) is most similar to this as we error if there are no bins so hunting through that code will most likely yield what you need for this.

[narayan-krishna]

Understood, I'll then look into how the cargo install binary checks works, and repurposing it to look for libs (if it's necessary while the Index changes are being planned out)