Re: [PATCH] rust: kbuild: give `--config-path` to `rustfmt` in `.rsi` target

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Jan 15, 2026 at 07:38:32PM +0100, Miguel Ojeda wrote:
> `rustfmt` is configured via the `.rustfmt.toml` file in the source tree,
> and we apply `rustfmt` to the macro expanded sources generated by the
> `.rsi` target.
> 
> However, under an `O=` pointing to an external folder (i.e. not just
> a subdir), `rustfmt` will not find the file when checking the parent
> folders. Since the edition is configured in this file, this can lead to
> errors when it encounters newer syntax, e.g.
> 
>     error: expected one of `!`, `.`, `::`, `;`, `?`, `where`, `{`, or an operator, found `"rust_minimal"`
>       --> samples/rust/rust_minimal.rsi:29:49
>        |
>     28 | impl ::kernel::ModuleMetadata for RustMinimal {
>        |                                               - while parsing this item list starting here
>     29 |     const NAME: &'static ::kernel::str::CStr = c"rust_minimal";
>        |                                                 ^^^^^^^^^^^^^^ expected one of 8 possible tokens
>     30 | }
>        | - the item list ends here
>        |
>        = note: you may be trying to write a c-string literal
>        = note: c-string literals require Rust 2021 or later
>        = help: pass `--edition 2024` to `rustc`
>        = note: for more on editions, read https://doc.rust-lang.org/edition-guide
> 
> A workaround is to use `RUSTFMT=n`, which is documented in the `Makefile`
> help for cases where macro expanded source may happen to break `rustfmt`
> for other reasons, but this is not one of those cases.
> 
> One solution would be to pass `--edition`, but we want `rustfmt` to
> use the entire configuration, even if currently we essentially use the
> default configuration.
> 
> Thus explicitly give the path to the config file to `rustfmt` instead.
> 
> Reported-by: Alice Ryhl <aliceryhl@google.com>
> Fixes: 2f7ab1267dc9 ("Kbuild: add Rust support")
> Cc: stable@vger.kernel.org
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

I assume you will take this via the Rust tree?

> ---
>  scripts/Makefile.build | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 5037f4715d74..0c838c467c76 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -356,7 +356,7 @@ $(obj)/%.o: $(obj)/%.rs FORCE
>  quiet_cmd_rustc_rsi_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
>        cmd_rustc_rsi_rs = \
>  	$(rust_common_cmd) -Zunpretty=expanded $< >$@; \
> -	command -v $(RUSTFMT) >/dev/null && $(RUSTFMT) $@
> +	command -v $(RUSTFMT) >/dev/null && $(RUSTFMT) --config-path $(srctree)/.rustfmt.toml $@
>  
>  $(obj)/%.rsi: $(obj)/%.rs FORCE
>  	+$(call if_changed_dep,rustc_rsi_rs)
> 
> base-commit: 74e15ac34b098934895fd27655d098971d2b43d9
> -- 
> 2.52.0
>