Incorrect `unnecessary_cast` on a raw pointer with an unknown pointee type

Summary

The lint should not warn in this case

Lint Name

unnecessary_cast

Reproducer

I tried this code:

fn src<A>() -> *mut *mut A {
    unimplemented!()
}

unsafe fn error_repo<A>() -> bool {
    let val = src();
    (*val as *mut A).is_null()
}

I saw this happen:

warning: casting raw pointers to the same type and constness is unnecessary (`*mut A` -> `*mut A`)
   --> src/functions.rs:622:5
    |
622 |     (*val as *mut A).is_null()
    |     ^^^^^^^^^^^^^^^^ help: try: `*val`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
    = note: `#[warn(clippy::unnecessary_cast)]` on by default

I expected not to see this error, because removing as *mut A fails to compile with this error:

error[E0699]: cannot call a method on a raw pointer with an unknown pointee type
   --> src/functions.rs:622:12
    |
622 |     (*val).is_null()
    |            ^^^^^^^

Version

rustc 1.73.0 (cc66ad468 2023-10-03)
binary: rustc
commit-hash: cc66ad468955717ab92600c770da8c1601a4ff33
commit-date: 2023-10-03
host: x86_64-unknown-linux-gnu
release: 1.73.0
LLVM version: 17.0.2

Additional Labels

No response

Start by reproducing the unnecessary_cast warning with the Rust example and the expression at src/functions.rs:622. Read the unnecessary_cast lint entry point and its existing tests, then verify that the raw-pointer cast is not warned about when removing it produces the unknown-pointee-type error.