Clippy showsunreachable-patterns in #[frame_support::pallet] after update to Rust 1.81.0

[AndreiEres]

During the update of Rust in runtimes, many warnings about unreachable-patterns in #[frame_support::pallet] appeared, as noted in polkadot-fellows/runtimes#473. These warnings were silenced, but it is better to fix the bug in the macros.

error: unreachable pattern
   --> relay/kusama/src/governance/origins.rs:22:1
    |
22  | / pub mod pallet_custom_origins {
23  | |     use crate::{Balance, GRAND, QUID};
24  | |     use frame_support::pallet_prelude::*;
...   |
[19](https://github.com/polkadot-fellows/runtimes/actions/runs/11389304108/job/31688274427#step:7:20)6 | |     }
197 | | }
    | |_^ matches no values because `governance::origins::pallet_custom_origins::Call<T>` is uninhabited
    |
    = note: to learn more about uninhabited types, see https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types
    = note: `-D unreachable-patterns` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unreachable_patterns)]`

[bkontur]

I encountered the same issue here when we were using cargo +nightly clippy in bridges repo CI. This warning seems to be related to +nightly, which is likely why we haven't seen it in the polkadot-sdk repo - most, if not all, CI jobs were switched from +nightly to stable at some point (not sure why and I cannot find that PR).

Now, polkadot-fellows uses cargo +nightly clippy. Shouldn't we switch and align that to stable as well? Giving it a try here: polkadot-fellows/runtimes#489

Anyway, if we can fix it regardless of stable or +nightly, let's do it.

[bkchr]

@AndreiEres could you reproduce this on master? (I tried and failed) As I said already in the pr, there was already a similar issue that is now closed: #5697

[AndreiEres]

• Stable: cargo clippy -p pallet-alliance shows no warnings
• Latest nightly: cargo +nightly clippy -p pallet-alliance shows warnings but not unreachable pattern
• CI nightly: cargo +nightly-2024-09-11 clippy -p pallet-alliance shows unreachable pattern

warning: unreachable pattern
   --> substrate/frame/system/src/lib.rs:672:12
    |
672 |     #[pallet::call]
    |               ^^^^ matches no values because `frame_support::Never` is uninhabited
    |
    = note: to learn more about uninhabited types, see https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types
    = note: `#[warn(unreachable_patterns)]` on by default

I suppose we can be OK with current situation, and since in runtimes we moved to a stable rust in CI we can close the issue.
@bkchr, @bkchr WDYT?