`unsafe_derive_deserialize` fires for `unsafe` blocks in associated constants

Summary

unsafe_derive_deserialize is triggered when you have an associated constant that uses unsafe. However, associated constants will not affect a derived serde::Deserialize impl.

To ensure this particular bit of information doesn't get lost:
My original code had something along the lines of const FOO: NonZeroUsize = unsafe { NonZeroUsize::new_unchecked(123) }, because Option::unwrap is not const-stable.

Lint Name

unsafe_derive_deserialize

Reproducer

I tried this code:

#![warn(clippy::unsafe_derive_deserialize)]

#[derive(serde::Deserialize)]
pub struct Foo;

impl Foo {
    #[allow(unused_unsafe)]
    pub const BAR: () = unsafe {};
}

I saw this happen:

warning: you are deriving `serde::Deserialize` on a type that has methods using `unsafe`
 --> src/lib.rs:4:10
  |
4 | #[derive(serde::Deserialize)]
  |          ^^^^^^^^^^^^^^^^^^
  |
  = help: consider implementing `serde::Deserialize` manually. See https://serde.rs/impl-deserialize.html
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unsafe_derive_deserialize
note: the lint level is defined here
 --> src/lib.rs:1:9
  |
1 | #![warn(clippy::unsafe_derive_deserialize)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: this warning originates in the derive macro `serde::Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: `playground` (lib) generated 1 warning

I expected to see this happen:
No output.

Version

Note: this particular instance- this was the MRE- was triggered on the rust playground with the following Clippy: version 0.1.69 (2023-02-14 0416b1a)

However, the version info for my local system, where I originally found this issue is as follows:
rustc 1.66.0-nightly (5e9772042 2022-10-29)
binary: rustc
commit-hash: 5e9772042948002f9c6f60c4c81603170035fffa
commit-date: 2022-10-29
host: aarch64-apple-darwin
release: 1.66.0-nightly
LLVM version: 15.0.2

This also occurs on the latest nightly available on the playground as of posting (2023-02-14)

Additional Labels

No response

Start by reproducing the unsafe_derive_deserialize warning with the associated-constant example and inspect that lint's implementation and existing tests. Done means the reproducer no longer emits a warning while the lint still detects unsafe methods relevant to derived serde::Deserialize implementations.