Suggested adding dep:foo to the foo feature if it exists for unused optional dependencies

[epage]

cargo-features = ["edition2024"]
[package]
name = "cargo-14010"
version = "0.1.0"
edition = "2024"

[dependencies]
serde = { version = "1.0.203", optional = true }

[features]
serde = []

$ cargo +nightly check -Zcargo-lints
warning: unused optional dependency
 --> Cargo.toml:8:1
  |
8 | serde = { version = "1.0.203", optional = true }
  | -----
  |
  = note: `cargo::unused_optional_dependency` is set to `warn` by default
  = help: remove the dependency or activate it in a feature with `dep:serde`
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s

Likely the user intended for dep:serde to be on the serde feature and it might be nice for us to call that out explicitly.

[epage]

We'd likely get this "for free" if/when we show a code suggestion

[epage]

As we don't have unused optional dependencies anymore, I'm going to close this but note it for when we bring it back