Detect cfg based on an undeclared feature

[gendx]

What it does

Detect and warn on uses of cfg(feature = "foo") where the feature ("foo") isn't declared in the Cargo.toml file.

Advantage

The cfg always evaluates to false, which is likely not intended (e.g. typo, feature that was renamed in the Cargo.toml file but not everywhere in the code, etc.).

Drawbacks

I don't see any reason to have a cfg based on a feature that doesn't exist.

Example

# Cargo.toml
[features]
bar = []

let x = 0;
// Feature "foo" isn't defined in the Cargo.toml file.
#[cfg(feature = "foo")]
x = 42;

[y21]

Closing as a duplicate of #11649. This already exists unstably in cargo as -Zcheck-cfg, and is close to being stabilized it looks like (tracking issue: rust-lang/rust#82450)

warning: unexpected `cfg` condition value: `foo`
 --> src/main.rs:4:11
  |
4 |     #[cfg(feature = "foo")]
  |           ^^^^^^^^^^^^^^^ help: remove the condition
  |
  = note: no expected values for `feature`
  = help: consider adding `foo` as a feature in `Cargo.toml`

Thanks for opening this regardless!