Fix clippy warning on FromRef derive macro

[AlphaKeks]

Motivation

Fixes a clippy warning caused by the FromRef derive macro.

See #2826.

Solution

Add the lint to the #[allow(…)] attribute above the generated function that triggers it.

[AlphaKeks]

The test is failing because of the latest serde release where they leverage the new #[diagnostic::on_unimplemented] feature to give better error messages.

[jplatte]

Would you mind running TRYBUILD=overwrite cargo test from axum-macros to fix this?
Just add that as a separate commit to this PR, or open another PR.

[AlphaKeks]

It seems like CI is running a very old version of nightly (from ~4 months ago). Running the tests on a newer release (from the past few days) results in a lot more failing tests, not only in axum-macros. I could fix just this particular case by switching to the exact same release that CI is using, but I think it's noteworthy that these tests and nightly CI should be updated at some point.

[jplatte]

Yeah, I tried to bump nightly in #2805, I guess locally the trybuild tests generated different output from CI because of lockfile differences. Also David isn't currently around much to approve my PRs.

If you care, I would be happy to merge a PR that bumps nightly, and passes CI unlike mine 😄

[jplatte]

💯