Re: [PATCH 32/33] rust: kbuild: support global per-version flags

Gary Guo
2026-04-01T15:27:18Z

On Wed Apr 1, 2026 at 12:45 PM BST, Miguel Ojeda wrote:
> Sometimes it is useful to gate global Rust flags per compiler version.
> For instance, we may want to disable a lint that has false positives in
> a single version [1].
>
> We already had helpers like `rustc-min-version` for that, which we use
> elsewhere, but we cannot currently use them for `rust_common_flags`,
> which contains the global flags for all Rust code (kernel and host),
> because `rustc-min-version` depends on `CONFIG_RUSTC_VERSION`, which
> does not exist when `rust_common_flags` is defined.
>
> Thus, to support that, introduce `rust_common_flags_per_version`,
> defined after the `include/config/auto.conf` inclusion (where
> `CONFIG_RUSTC_VERSION` becomes available), and append it to
> `rust_common_flags`, `KBUILD_HOSTRUSTFLAGS` and `KBUILD_RUSTFLAGS`.
>
> An alternative is moving all those three down, but that would mean
> separating them from the other `KBUILD_*` variables.

I think I would prefer moving these down.

The current approach append the flags to all variables, which will cause the
following equivalence to stop holding after the flag update.

KBUILD_HOSTRUSTFLAGS := $(rust_common_flags) -O -Cstrip=debuginfo \
-Zallow-features= $(HOSTRUSTFLAGS)

(Per version flags doesn't go before -O anymore, it comes after HOSTRUSTFLAGS).

Best,
Gary

>
> Link: https://lore.kernel.org/rust-for-linux/CANiq72mWdFU11GcCZRchzhy0Gi1QZShvZtyRkHV2O+WA2uTdVQ@mail.gmail.com/ [1]
> Link: https://patch.msgid.link/20260307170929.153892-1-ojeda@kernel.org
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
> ---
> Makefile | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index 1a219bf1c771..20c8179d96ee 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -834,6 +834,14 @@ endif # CONFIG_TRACEPOINTS
>
> export WARN_ON_UNUSED_TRACEPOINTS
>
> +# Per-version Rust flags. These are like `rust_common_flags`, but may
> +# depend on the Rust compiler version (e.g. using `rustc-min-version`).
> +rust_common_flags_per_version :=
> +
> +rust_common_flags += $(rust_common_flags_per_version)
> +KBUILD_HOSTRUSTFLAGS += $(rust_common_flags_per_version)
> +KBUILD_RUSTFLAGS += $(rust_common_flags_per_version)
> +
> include $(srctree)/arch/$(SRCARCH)/Makefile
>
> ifdef need-config