Add undefined feature diagnostic

[jplatte]

Since this isn't really possible to solve in Clippy, I was wondering whether rust-analyzer could check for existance of a feature or optional dependency named foo in Cargo.toml when encountering cfg(feature = "foo"), and raise a warning if there isn't one.

[bjorn3]

There is an accepted rfc for doing this in rustc. It hasn't been implemented yet though: rust-lang/rust#82450

[jplatte]

Oh, awesome! I'll close this then.

[lnicola]

In the long run we might still want to implement this, just as with the other rustc diagnostics.

[jplatte]

Okay, in that case I'll reopen.