Improvement of strange lint warnings

[Enyium]

I have #![warn(clippy::pedantic)] in my crate's lib.rs. In CXX bridges, this can lead to warnings about things that are not my business. If you could please use #[allow(clippy::pedantic)] in the relevant, not all-encompassing locations, this would solve these warnings. If you aren't sure where exactly, here's concrete warnings I get:

• Without #![allow(clippy::ptr_as_ptr)] in my bridge, I get `as` casting between raw pointers without changing its mutability on non-self parameters in the extern "C++" block that have the pattern &T or Pin<&mut T>, where T is a C++ bridge type. See https://rust-lang.github.io/rust-clippy/master/index.html#ptr_as_ptr.
• Without #![allow(clippy::expl_impl_clone_on_copy)] in my bridge, I get you are implementing `Clone` explicitly on a `Copy` type in enums that work together with an extern "C++" type of the same name (in that sense). See https://rust-lang.github.io/rust-clippy/master/index.html#expl_impl_clone_on_copy.

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Not related to #![warn(clippy::pedantic)] is the following kind of warning: Without #![allow(dead_code)] in my bridge, I get associated constant `FOO_BAR` is never used on enums. Is there a better allowable warning possible, something with "unused"? The worst is, I can't use #[allow(dead_code)] on the whole enum, but only on every single currently unused variant, which are many in that large enum. (Hence, I'm temporarily using it on the whole bridge, which works again.)

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Related: #1061

[edward-shen]

The clippy::use_self lint also seems to be emitted when defining a shared enum type that has an existing c++ enum implementation:

Repro:

Rust side

#[cxx::bridge]
mod ffi {
    #[derive(Debug)]
    enum SomeErrorType {
        EMPTY,
    }

    unsafe extern "C++" {
        include!("mylibrary.h");
        type SomeErrorType;
    }
}

Cpp side

enum class SomeErrorType : unsigned char {
  EMPTY,
};