FAILED: Patch "rust: kbuild: pass `-Zunstable-options` for Rust 1.95.0" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 0a9be83e57de0d0ca8ca4ec610bc344f17a8e5e7 Mon Sep 17 00:00:00 2001
From: Miguel Ojeda <[email protected]>
Date: Fri, 6 Feb 2026 21:45:35 +0100
Subject: [PATCH] rust: kbuild: pass -Zunstable-options for Rust 1.95.0

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.

---
 rust/Makefile | 3 +++
 1 file changed, 3 insertions(+)

index 4dcc2eff51cb2..725158740fc6f 100644
@@ -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)
 

2.51.0

There is no Rust in 5.15.y, so this is fine:

Cheers,
Miguel