Detect enums with cfg controlled variants that aren't non_exhaustive

What it does

If an enum has variants controlled by cfg (e.g. feature flags), it should almost certainly be non_exhaustive. This lint would flag exhaustive enums controlled by cfg.

This should not flag enums with cfg variants that nonetheless have the same variants in all cfgs.

Lint Name

cfg_enum_exhaustive

Category

suspicious

Advantage

• Avoid requiring cfg in every match
• Avoid having unexpected errors that only arise in some configurations

Drawbacks

Possible false positives.

Example

enum E {
    V1,
    #[cfg(feature = "xyz")]
    V2,
}

Could be written as:

#[non_exhaustive]
enum E {
    V1,
    #[cfg(feature = "xyz")]
    V2,
}

No implementation files, entry points, or tests are named. Start by locating the Clippy lint registration and test pattern for enum or cfg-related lints, then use the examples in this issue to define the cases. Done means exhaustive enums with cfg-controlled variants are flagged, while enums with identical variants in every configuration are not.