Warn when adding a dependancy that is already an optional dependancy

[lolbinarycat]

Problem

Adding a new required dependency is usually as simple as cargo add pkgname, then use cratename in your code.

However, if you try to do this with a package that is already an optional dependency, it will result in rustc telling you to cargo add cratename, which can be quite confusing.

Proposed Solution

If a cargo add command is run, and all of the following are true:

• All listed crates already are in Cargo.toml
• No new features were enabled by this command
• The --optional flag was not passed
• One of the listed crates has optional = true already set

A warning should be printed:

Warning: `pkgname` is already added as an optional dependency, use `cargo add pkgname --no-optional` to make it required

Notes

In my case this happened when I revisited a project that I abandoned for several months.

I originally added a dep thinking I would provide optional integration with it, but just now decided it would make more sense as part of the core API.

This lead to a moment of confusion when cargo add seemingly refused to add the dep, only realizing what was happening when I looked at Cargo.toml, but in projects with more deps this might not be as obvious.

Another reason this may be confusing is because a similar situation does not happen when doing cargo add --dev pkgname followed by cargo add pkgname. I understand this is because dev deps are a separate table and not just a flag, but it can still be a bit unintuitive.

[epage]

It is unfortunate when diagnostics lead you down a path that don't solve your problem.

Looking at the output, we do call out its an "optional dependency". Granted, that can be easy to miss.

cargo add is intended to be additive so you can do

$ cargo add foo --optional
$ cargo add foo -F bar

I feel like a warning in that case would cause user confusion as well. You do talk about being limited in what situations we provide the message. We'd have to carefully consider what other flags are present and what that might imply. We'd also need to consider what this would do when adding multiple packages.

One option is we change the compiler message to cargo add --no-optional cratename but I worry there might be cases where that could cause problems.