[RFC PATCH] rust: kbuild: support global per-version flags

[Miguel Ojeda <ojeda@kernel.org>]

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.

Link: https://lore.kernel.org/rust-for-linux/CANiq72mWdFU11GcCZRchzhy0Gi1QZShvZtyRkHV2O+WA2uTdVQ@mail.gmail.com/ [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
> Doing it globally for certain versions (i.e. `rust_common_flags`, not
> just `KBUILD_RUSTFLAGS`) requires a bit of reorganization,

If we wanted to go for a per-version global one, we we would need
something like this patch -- sending it here so that it gets archived
in case we need it.

 Makefile | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/Makefile b/Makefile
index 2446085983f7..a49cddb0599b 100644
--- a/Makefile
+++ b/Makefile
@@ -833,6 +833,14 @@ 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

base-commit: 11439c4635edd669ae435eec308f4ab8a0804808
--
2.53.0