`missing_safety_doc` lint doesn't trigger on `unsafe extern "C" fn`

Summary

By definition, unsafe extern "C" fns are exposed to the outside world, and can therefore be regarded as if they're pub unsafe fns. It is very desirable to be able to require unsafe extern "C" fns be annotated with safety docs. Currently, the missing_safety_doc lint does not trigger for unsafe extern "C" fns, wheras it does trigger for pub unsafe extern "C" fns.

Lint Name

missing_safety_doc

Reproducer

I tried this code:

unsafe extern "C" fn say_hello(msg: *const Cstr) {
   todo!()
}

I expected to see this happen:

warning: unsafe function's docs are missing a `# Safety` section
   --> trie_rs/src/ffi.rs:329:1
    |
329 | unsafe extern "C" fn say_hello(msg: *const u8) {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc
    = note: `#[warn(clippy::missing_safety_doc)]` on by default

Instead, this happened:

No complaints by Clippy whatsoever

Version

rustc 1.85.0 (4d91de4e4 2025-02-17)
binary: rustc
commit-hash: 4d91de4e48198da2e33413efdcd9cd2cc0c46688
commit-date: 2025-02-17
host: aarch64-apple-darwin
release: 1.85.0
LLVM version: 19.1.7

Start by locating the missing_safety_doc lint and run the unsafe extern "C" fn reproducer from the issue. Trace how the lint decides which functions require safety documentation, then add coverage so this case reports a missing # Safety section while existing behavior remains unchanged.