missing_safety_doc should catch unsafe impl

[sffc]

I tried this code:

struct ExampleStruct {}
unsafe trait ExampleUnsafeTrait {}

unsafe impl ExampleUnsafeTrait for ExampleStruct {}

I expected to see this happen: The missing_safety_doc should complain about the unsafe impl not having safety docs.

Instead, this happened: This code passes the clippy check.

Meta

Rust version (rustc -Vv):

rustc 1.56.0-beta.3 (deef86610 2021-09-17)
binary: rustc
commit-hash: deef866109022248b2506587b377c6b1c881d963
commit-date: 2021-09-17
host: x86_64-unknown-linux-gnu
release: 1.56.0-beta.3
LLVM version: 13.0.0

CC @Manishearth

[Manishearth]

missing_safety_doc is about documenting safety invariants for library types in API docs, not for documenting unsafe code in comments. As such it's irrelevant when it comes to unsafe impls (or unsafe blocks). However, it should absolutely warn on unsafe trait so we can fix that.

[Manishearth]

#7734