Re: [PATCH 2/2] rust: enable `no_mangle_with_rust_abi` Clippy lint

[Gary Guo <gary@garyguo.net>]

On Sun, 30 Jul 2023 00:03:17 +0200
Miguel Ojeda <ojeda@kernel.org> wrote:

> Introduced in Rust 1.69.0 [1], this lint prevents forgetting to set
> the C ABI when using `#[no_mangle]` (or thinking it is implied).
> 
> For instance, it would have prevented the issue [2] fixed by commit
> c682e4c37d2b ("rust: kernel: Mark rust_fmt_argument as extern "C"").
> 
>     error: `#[no_mangle]` set on a function with the default (`Rust`) ABI
>       --> rust/kernel/print.rs:21:1  
>        |
>     21 | / unsafe fn rust_fmt_argument(
>     22 | |     buf: *mut c_char,
>     23 | |     end: *mut c_char,
>     24 | |     ptr: *const c_void,
>     25 | | ) -> *mut c_char {
>        | |________________^
>        |
>        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#no_mangle_with_rust_abi
>        = note: requested on the command line with `-D clippy::no-mangle-with-rust-abi`
>     help: set an ABI
>        |
>     21 | unsafe extern "C" fn rust_fmt_argument(
>        |        ++++++++++
>     help: or explicitly set the default
>        |
>     21 | unsafe extern "Rust" fn rust_fmt_argument(
>        |        +++++++++++++
> 
> Thus enable it.
> 
> In rare cases, we may need to use the Rust ABI even with `#[no_mangle]`
> (e.g. one case, before 1.71.0, would have been the `__rust_*`
> functions). In those cases, we would need to `#[allow(...)]` the lint,
> since using `extern "Rust"` explicitly (as the compiler suggests)
> currently gets overwritten by `rustfmt` [3].
> 
> Link: https://github.com/rust-lang/rust-clippy/issues/10347 [1]
> Link: https://github.com/Rust-for-Linux/linux/pull/967 [2]
> Link: https://github.com/rust-lang/rustfmt/issues/5701 [3]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/Makefile b/Makefile
> index 658ec2b8aa74..93bf3eca3ead 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -467,6 +467,7 @@ export rust_common_flags := --edition=2021 \
>  			    -Dclippy::let_unit_value -Dclippy::mut_mut \
>  			    -Dclippy::needless_bitwise_bool \
>  			    -Dclippy::needless_continue \
> +			    -Dclippy::no_mangle_with_rust_abi \
>  			    -Wclippy::dbg_macro
>  
>  KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)