rust: kbuild: support global per-version flags

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`.

In addition, move the expansion of `HOSTRUSTFLAGS` to the same place,
so that users can also override per-version flags [2].

Link: https://lore.kernel.org/rust-for-linux/CANiq72mWdFU11GcCZRchzhy0Gi1QZShvZtyRkHV2O+WA2uTdVQ@mail.gmail.com/ [1]
Link: https://lore.kernel.org/rust-for-linux/CANiq72mTaA2tjhkLKf0-2hrrrt9rxWPgy6SfNSbponbGOegQvA@mail.gmail.com/ [2]
Link: https://patch.msgid.link/20260307170929.153892-1-ojeda@kernel.org
Link: https://patch.msgid.link/20260405235309.418950-33-ojeda@kernel.org
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Makefile

@@ -506,7 +506,7 @@ KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) \
 KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS) \
 		       -I $(srctree)/scripts/include
 KBUILD_HOSTRUSTFLAGS := $(rust_common_flags) -O -Cstrip=debuginfo \
-			-Zallow-features= $(HOSTRUSTFLAGS)
+			-Zallow-features=
 KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
 KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
 KBUILD_PROCMACROLDFLAGS := $(or $(PROCMACROLDFLAGS),$(KBUILD_HOSTLDFLAGS))
@@ -836,6 +836,14 @@ 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) $(HOSTRUSTFLAGS)
+KBUILD_RUSTFLAGS += $(rust_common_flags_per_version)
+
 include $(srctree)/arch/$(SRCARCH)/Makefile
 
 ifdef need-config