clippy::empty_enums: Checks for enums with no variants, which therefore are uninhabited types (cannot be instantiated). As of this writing, the never_type is still a nightly-only experimental API. Therefore, this lint is only triggered if #![feature(never_type)] is enabled

Does it matter?

If you only want a type which can’t be instantiated, you should use ! (the primitive type “never”), because ! has more extensive compiler support (type inference, etc.) and implementations of common traits. If you need to introduce a distinct type, consider using a newtype struct containing ! instead (struct MyType(pub !)), because it is more idiomatic to use a struct rather than an enum when an enum is unnecessary. If you do this, note that the visibility of the ! field determines whether the uninhabitedness is visible in documentation, and whether it can be pattern matched to mark code unreachable. If the field is not visible, then the struct acts like any other struct with private fields. For further information, visit the never type’s documentation.

New lints still being finished. Off by default and may have false positives.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

enum CannotExist {}

Write this instead

#![feature(never_type)]

/// Use the `!` type directly...
type CannotExist = !;

/// ...or define a newtype which is distinct.
struct CannotExist2(pub !);

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

[lints.clippy]
empty_enums = "warn"

#![warn(clippy::empty_enums)]

cargo clippy -- -W clippy::empty-enums

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.