[PATCH] rust: kbuild: pass `-Zunstable-options` for Rust 1.95.0

[Miguel Ojeda <ojeda@kernel.org>]

Custom target specifications are unstable, but starting with Rust 1.95.0,
`rustc` requires to explicitly pass `-Zunstable-options` to use them [1]:

    error: error loading target specification: custom targets are unstable and require `-Zunstable-options`
      |
      = help: run `rustc --print target-list` for a list of built-in targets

David (Rust compiler team lead), writes:

   "We're destabilising custom targets to allow us to move forward with
    build-std without accidentally exposing functionality that we'd like
    to revisit prior to committing to. I'll start a thread on Zulip to
    discuss with the RfL team how we can come up with an alternative
    for them."

Thus pass it.

Cc: David Wood <david@davidtw.co>
Cc: Wesley Wiser <wwiser@gmail.com>
Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Link: https://github.com/rust-lang/rust/pull/151534 [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/rust/Makefile b/rust/Makefile
index 4dcc2eff51cb..725158740fc6 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -552,6 +552,8 @@ $(obj)/$(libpin_init_internal_name): private rustc_target_flags = --cfg kernel
 $(obj)/$(libpin_init_internal_name): $(src)/pin-init/internal/src/lib.rs FORCE
 	+$(call if_changed_dep,rustc_procmacro)
 
+# `rustc` requires `-Zunstable-options` to use custom target specifications
+# since Rust 1.95.0 (https://github.com/rust-lang/rust/pull/151534).
 quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L $@
       cmd_rustc_library = \
 	OBJTREE=$(abspath $(objtree)) \
@@ -562,6 +564,7 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
 		--crate-type rlib -L$(objtree)/$(obj) \
 		--crate-name $(patsubst %.o,%,$(notdir $@)) $< \
 		--sysroot=/dev/null \
+		-Zunstable-options \
 	$(if $(rustc_objcopy),;$(OBJCOPY) $(rustc_objcopy) $@) \
 	$(cmd_objtool)
 

base-commit: 18f7fcd5e69a04df57b563360b88be72471d6b62
-- 
2.53.0