False positive warning on if keyword for unused_qualifications

[djc]

Code

See here:

https://github.com/hickory-dns/hickory-dns/blob/ac67742d64717cf61299871554590beb7c0c296c/crates/proto/src/multicast/mdns_stream.rs#L456

(After enabling unused_qualifications = "warn" via Cargo.toml.)

Current output

warning: unnecessary qualification
   --> crates/proto/src/multicast/mdns_stream.rs:456:9
    |
456 |         if let Err(error) = result {
    |         ^^
    |
    = note: requested on the command line with `-W unused-qualifications`

warning: `hickory-proto` (lib test) generated 1 warning (run `cargo fix --lib -p hickory-proto --tests` to apply 1 suggestion)

Desired output

No warning.

Rationale and extra context

Clearly the lint should not fire on language keywords.

Other cases

Rust Version

rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: aarch64-apple-darwin
release: 1.92.0
LLVM version: 21.1.3

Anything else?

No response

[mu001999]

I can't reproduce this by executing cargo check --lib -p hickory-proto --tests with the same rust version, what I got is:

warning: unnecessary qualification
   --> crates/proto/src/rr/domain/name.rs:945:6
    |
945 | impl core::fmt::Debug for Name {
    |      ^^^^^^^^^^^^^^^^
    |
    = note: requested on the command line with `-W unused-qualifications`
help: remove the unnecessary path segments
    |
945 - impl core::fmt::Debug for Name {
945 + impl fmt::Debug for Name {
    |

warning: unnecessary qualification
   --> crates/proto/src/rr/domain/name.rs:946:27
    |
946 |     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^
    |
help: remove the unnecessary path segments
    |
946 -     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
946 +     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> core::fmt::Result {
    |

warning: unnecessary qualification
   --> crates/proto/src/rr/domain/name.rs:946:56
    |
946 |     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
    |                                                        ^^^^^^^^^^^^^^^^^
    |
help: remove the unnecessary path segments
    |
946 -     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
946 +     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> fmt::Result {
    |

warning: unnecessary qualification
   --> crates/proto/src/runtime.rs:342:28
    |
342 |     ) -> Result<F::Output, std::io::Error>;
    |                            ^^^^^^^^^^^^^^
    |
help: remove the unnecessary path segments
    |
342 -     ) -> Result<F::Output, std::io::Error>;
342 +     ) -> Result<F::Output, io::Error>;
    |

warning: unnecessary qualification
   --> crates/proto/src/runtime.rs:370:28
    |
370 |     ) -> Result<F::Output, std::io::Error> {
    |                            ^^^^^^^^^^^^^^
    |
help: remove the unnecessary path segments
    |
370 -     ) -> Result<F::Output, std::io::Error> {
370 +     ) -> Result<F::Output, io::Error> {
    |

warning: unnecessary qualification
   --> crates/proto/src/runtime.rs:373:31
    |
373 |             .map_err(move |_| std::io::Error::new(std::io::ErrorKind::TimedOut, "future timed out"))
    |                               ^^^^^^^^^^^^^^^^^^^
    |
help: remove the unnecessary path segments
    |
373 -             .map_err(move |_| std::io::Error::new(std::io::ErrorKind::TimedOut, "future timed out"))
373 +             .map_err(move |_| io::Error::new(std::io::ErrorKind::TimedOut, "future timed out"))
    |

warning: unnecessary qualification
   --> crates/proto/src/runtime.rs:373:51
    |
373 |             .map_err(move |_| std::io::Error::new(std::io::ErrorKind::TimedOut, "future timed out"))
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
help: remove the unnecessary path segments
    |
373 -             .map_err(move |_| std::io::Error::new(std::io::ErrorKind::TimedOut, "future timed out"))
373 +             .map_err(move |_| std::io::Error::new(io::ErrorKind::TimedOut, "future timed out"))
    |

warning: unnecessary qualification
   --> crates/proto/src/xfer/dns_multiplexer.rs:271:35
    |
271 |             return NetError::from(crate::NetErrorKind::Busy).into();
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
help: remove the unnecessary path segments
    |
271 -             return NetError::from(crate::NetErrorKind::Busy).into();
271 +             return NetError::from(NetErrorKind::Busy).into();
    |

warning: unnecessary qualification
   --> crates/proto/src/xfer/mod.rs:484:6
    |
484 | impl fmt::Display for Protocol {
    |      ^^^^^^^^^^^^
    |
help: remove the unnecessary path segments
    |
484 - impl fmt::Display for Protocol {
484 + impl Display for Protocol {
    |

warning: `hickory-proto` (lib) generated 9 warnings (run `cargo fix --lib -p hickory-proto` to apply 9 suggestions)
    Checking hickory-resolver v0.26.0-alpha.1 (/Users/mu00/repos/crates/hickory-dns/crates/resolver)
    Checking test-support v0.26.0-alpha.1 (/Users/mu00/repos/crates/hickory-dns/tests/test-support)
warning: unnecessary qualification
  --> tests/test-support/src/lib.rs:59:40
   |
59 |     fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
   |                                        ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: requested on the command line with `-W unused-qualifications`
help: remove the unnecessary path segments
   |
59 -     fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
59 +     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
   |

warning: unnecessary qualification
  --> tests/test-support/src/lib.rs:64:28
   |
64 |     fn flush(&mut self) -> std::io::Result<()> {
   |                            ^^^^^^^^^^^^^^^^^^^
   |
help: remove the unnecessary path segments
   |
64 -     fn flush(&mut self) -> std::io::Result<()> {
64 +     fn flush(&mut self) -> io::Result<()> {
   |

warning: `test-support` (lib) generated 2 warnings (run `cargo fix --lib -p test-support` to apply 2 suggestions)
warning: unnecessary qualification
    --> crates/proto/src/rr/record_data.rs:1314:49
     |
1314 |     fn record_type_from_rdata(rdata: &RData) -> crate::rr::record_type::RecordType {
     |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
help: remove the unnecessary path segments
     |
1314 -     fn record_type_from_rdata(rdata: &RData) -> crate::rr::record_type::RecordType {
1314 +     fn record_type_from_rdata(rdata: &RData) -> RecordType {
     |

warning: unnecessary qualification
   --> crates/proto/src/udp/udp_client_stream.rs:573:49
    |
573 |     async fn retry_handler_test() -> Result<(), std::io::Error> {
    |                                                 ^^^^^^^^^^^^^^
    |
help: remove the unnecessary path segments
    |
573 -     async fn retry_handler_test() -> Result<(), std::io::Error> {
573 +     async fn retry_handler_test() -> Result<(), io::Error> {
    |

warning: unnecessary qualification
   --> crates/proto/src/xfer/dns_multiplexer.rs:462:10
    |
462 |     impl fmt::Display for MockClientStream {
    |          ^^^^^^^^^^^^
    |
help: remove the unnecessary path segments
    |
462 -     impl fmt::Display for MockClientStream {
462 +     impl Display for MockClientStream {
    |

warning: `hickory-proto` (lib test) generated 12 warnings (9 duplicates) (run `cargo fix --lib -p hickory-proto --tests` to apply 3 suggestions)

[djc]

Can you try against the branch from this PR (after removing the suppression)?

• Configure lints at the workspace level hickory-dns/hickory-dns#3390

[mu001999]

the branch from this PR (after removing the suppression)

@djc It has been merged, could you provide a specific commit which can reproduce this directly?

[djc]

Here's how I can reproduce it:

djc-2021 main hickory-dns $ git checkout 4897611bcf2d6e6490708986ec66d464b5efe895
djc-2021 heads/main hickory-dns $ cd crates/proto
djc-2021 heads/main proto $ git diff
diff --git a/crates/proto/src/multicast/mdns_stream.rs b/crates/proto/src/multicast/mdns_stream.rs
index 1cd1d8b09..66d81c616 100644
--- a/crates/proto/src/multicast/mdns_stream.rs
+++ b/crates/proto/src/multicast/mdns_stream.rs
@@ -441,7 +441,7 @@ pub(crate) mod tests {
 
     // one_shot tests are basically clones from the udp tests
     #[tokio::test]
-    #[allow(unused_qualifications)] // https://github.com/rust-lang/rust/issues/149873
+    //#[allow(unused_qualifications)] // https://github.com/rust-lang/rust/issues/149873
     async fn test_next_random_socket() {
         subscribe();
 
djc-2021 heads/main proto $ rustc -V
rustc 1.92.0 (ded5c06cf 2025-12-08)
djc-2021 heads/main proto $ cargo c --tests --all-features
warning: unnecessary qualification
   --> crates/proto/src/multicast/mdns_stream.rs:457:9
    |
457 |         if let Err(error) = result {
    |         ^^
    |
    = note: requested on the command line with `-W unused-qualifications`

warning: `hickory-proto` (lib test) generated 1 warning (run `cargo fix --lib -p hickory-proto --tests` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.32s

[mu001999]

@djc Thanks! I reproduced it. I will take a look.

@rustbot claim

[mu001999]

MCVE:

pub use tokio::runtime;

#[tokio::test]
pub async fn foo() {
    if true {}
}

And we will get:

warning: unnecessary qualification
 --> src/lib.rs:5:5
  |
5 |     if true {}
  |     ^^
  |
  = note: requested on the command line with `-W unused-qualifications`

[mu001999]

This is related to setting span in proc macros. I think there may be more potential issues, not just related to unused_qualifications. I will provide a brief summary and possible solutions when I am free.

[mu001999]

For this issue, it's because #[tokio::test] will expand the input into things like (using the above example in #149873 (comment)):

pub fn foo() {
    let body = async { if true {} };
    let mut body = body;
    #[allow(unused_mut)]
    let mut body = unsafe { ::tokio::macros::support::Pin::new_unchecked(&mut body) };
    let body: ::core::pin::Pin<&mut dyn ::core::future::Future<Output = ()>> = body;
    #[allow(
        clippy::expect_used,
        clippy::diverging_sub_expression,
        clippy::needless_return,
        clippy::unwrap_in_result
    )]
    {
        return tokio::runtime::Builder::new_current_thread()
            .enable_all()
            .build()
            .expect("Failed building the Runtime")
            .block_on(body);
    }
}

So when we have use tokio::runtime; outer, we will get the warning for unused_qualifications.

But #[tokio::test] will use the span of the last stmt for the generated stmt, see see https://github.com/tokio-rs/tokio/blob/master/tokio-macros/src/entry.rs#L395-L422. That's why we will get the warning reported on if.

---

SO I think this position is expected in fact. The first we can do to fix this may be using ::tokio::runtime in the implementation of #[tokio::test].

🤔 And furthermore, considering the flexibility of proc-macro, we can even set empty spans if we want, which may lead to ICEs. And we even cannot tell such spans are from proc-macro or not (IIRC).

[djc]

SO I think this position is expected in fact. The first we can do is to fix this may be using ::tokio::runtime in the implementation of #[tokio::test].

Did you actually try whether that helps? It doesn't seem obvious that it would...