`unused_async_trait_impl` is not working well with `!`

[nazar-pc]

Summary

unused_async_trait_impl should probably recognize ! type and not trigger on it

Lint Name

unused_async_trait_impl

Reproducer

I tried this code:

impl ReadAtAsync for ! {
    async fn read_at<B>(&self, _buf: B, _offset: u64) -> io::Result<B>
    where
        AsyncReadBytes<B>: From<B>,
        B: AsMut<[u8]> + Unpin + 'static,
    {
        unreachable!("Is never called")
    }
}

I saw this happen:

warning: unused `async` for async trait impl function with no `.await` statements
   --> crates/farmer/ab-farmer-components/src/lib.rs:150:5
    |
150 | /     async fn read_at<B>(&self, _buf: B, _offset: u64) -> io::Result<B>
151 | |     where
152 | |         AsyncReadBytes<B>: From<B>,
153 | |         B: AsMut<[u8]> + Unpin + 'static,
154 | |     {
155 | |         unreachable!("Is never called")
156 | |     }
    | |_____^
    |
    = note: `std::future::ready` creates a `Future` which returns the value immediately when `poll`ed
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_async_trait_impl
    = note: `-W clippy::unused-async-trait-impl` implied by `-W clippy::pedantic`
    = help: to override `-W clippy::pedantic` add `#[allow(clippy::unused_async_trait_impl)]`
help: consider removing the `async` from this function and returning `impl Future<Output = io::Result<B>>` instead
    |
150 ~     fn read_at<B>(&self, _buf: B, _offset: u64) -> impl Future<Output = io::Result<B>>
151 |     where
...
154 |     {
155 ~         std::future::ready(unreachable!("Is never called"))
    |

Applying the suggestion leads to a different warning though:

   --> crates/farmer/ab-farmer-components/src/lib.rs:155:9
    |
155 |         ready(unreachable!("Is never called"))
    |         ^^^^^ ------------------------------- any code following this expression is unreachable
    |         |
    |         unreachable call
    |
    = note: `#[warn(unreachable_code)]` (part of `#[warn(unused)]`) on by default

I think it'd be better to special-case ! and not trigger this lint on it at all.

Version

rustc 1.98.0-nightly (e7815e522 2026-06-04)
binary: rustc
commit-hash: e7815e522ecc746592fee32f50478f521333b503
commit-date: 2026-06-04
host: x86_64-unknown-linux-gnu
release: 1.98.0-nightly
LLVM version: 22.1.6

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