Generated Clone implementation triggers clippy lint about being incorrect for a Copy type

[stefunctional]

This clippy lint is new in Rust 1.72.0.

Reproduction

Create a project with the following:

[package]
name = "playffi"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["staticlib", "lib"]

[dependencies]
safer-ffi = { version = "0.1.2", features = ["proc_macros"] }

use safer_ffi::derive_ReprC;

#[derive_ReprC]
#[repr(opaque)]
pub struct Foo {
    _x: i32,
}

Run the command below:

cargo clippy

Expected result

No lints are emitted.

Actual result

error: incorrect implementation of `clone` on a `Copy` type
 --> src/lib.rs:3:1
  |
3 | #[derive_ReprC]
  | ^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incorrect_clone_impl_on_copy_type
  = note: `#[deny(clippy::incorrect_clone_impl_on_copy_type)]` on by default
  = note: this error originates in the attribute macro `derive_ReprC` (in Nightly builds, run with -Z macro-backtrace for more info)

Analysis

The macro-expanded code is:

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
use safer_ffi::derive_ReprC;
pub struct Foo {
    _x: i32,
}
const _: () = {
    unsafe impl ::safer_ffi::ඞ::ReprC for Foo {
        type CLayout = __opaque_Foo;
        fn is_valid(it: &'_ Self::CLayout) -> ::safer_ffi::ඞ::bool {
            match it._void {}
        }
    }
    #[allow(nonstandard_style)]
    pub struct __opaque_Foo {
        _marker: *mut Self,
        _void: ::safer_ffi::ඞ::convert::Infallible,
    }
    impl ::safer_ffi::ඞ::marker::Copy for __opaque_Foo {}
    impl ::safer_ffi::ඞ::clone::Clone for __opaque_Foo {
        fn clone(self: &'_ Self) -> Self {
            match self._void {}
        }
    }
    unsafe impl ::safer_ffi::ඞ::CType for __opaque_Foo {
        type OPAQUE_KIND = ::safer_ffi::ඞ::OpaqueKind::Opaque;
    }
    unsafe impl ::safer_ffi::ඞ::ReprC for __opaque_Foo {
        type CLayout = Self;
        fn is_valid(_: &'_ Self::CLayout) -> ::safer_ffi::ඞ::bool {
            true
        }
    }
};

The lint is likely due to the Clone::clone implementation being different from *self.

Potential fixes are:

1. Implementing Clone::clone as returning *self.
2. Allowing the clippy lint in the generated code.

(2) may be tricky without bumping the minimum Rust version to 1.72.0.

Let me know if you would like me to open a PR to fix this.

[stefunctional]

Suggested fix (1) is implemented in stefunctional@7db6bc6. If you think this solution is appropriate and a PR would help, please let me know.

[cdata]

Just came here to report this issue 👍 ran into it here: https://github.com/subconsciousnetwork/noosphere/actions/runs/6004740385/job/16285965451?pr=608

[danielhenrymantilla]

Thanks for the report! stefunctional@7db6bc6 does LGTM, so if you submit a PR with it I'll merge it 🙂

[stefunctional]

I opened #184 as suggested. Thank you!