Enums with Infallible Variants and Derivative Clone Impls make Clippy Overreact on Nightly

[Brandon H. Gomes]

NB: Not sure if fixing this issue is the responsibility of derivative or clippy, but there may be a proactive solution that derivative can implement that can future-proof against these kinds of bugs.

Describe the bug
On nightly, if we implement Clone using derivative::Derivative on the following enum

pub enum Enum<T> {
    /// First Choice
    First,

    /// Second Choice
    Second,

    /// Parameter Marker
    #[doc(hidden)]
    __(Infallible, PhantomData<T>),
}

clippy overreacts with

warning: unreachable expression
   |
   | #[derive(derivative::Derivative)]
   |          ^^^^^^^^^^^^^^^^^^^^^^
   |          |
   |          unreachable expression
   |          any code following this expression is unreachable
   |
   = note: `#[warn(unreachable_code)]` on by default
note: this expression has type `Infallible`, which is uninhabited
   |
   | #[derive(derivative::Derivative)]
   |          ^^^^^^^^^^^^^^^^^^^^^^
   = note: this warning originates in the derive macro `derivative::Derivative` (in Nightly builds, run with -Z macro-backtrace for more info)

Implementing Clone using the standard library derive macro doesn't give the error but it has the undesirable property that it automatically adds the T: Clone constraint where derivative does not.

To Reproduce
Here's a rust playground example with the standard library derive and derivative::Derivative.

Expected behavior
I expected no warning since it did not give a warning in the past (and the warning is not helpful either).

Errors
N/A

Version (please complete the following information):

rustup 1.24.3 (2021-05-31)
cargo 1.56.0-nightly (18751dd3f 2021-09-01)
rustc 1.57.0-nightly (fdf65053e 2021-09-07)
derivative 2.2.0

Additional context
You can also just try to silence the warning from clippy by adding #[allow(unreachable_code)] but no matter where you put it on the enum, above the variant, above the pub enum Enum<T> line, or above the #[derive(derivative::Derivative)] line, clippy still complains. You can only silence the warning by putting a global #![allow(unreachable_code)] but this is not safe.