[PATCH v2] Kbuild: fix issues with rustc-option

[Alice Ryhl <aliceryhl@google.com>]

Fix a few different compiler errors that cause rustc-option to give
wrong results.

If KBUILD_RUSTFLAGS or the flags being tested contain any -Z flags, then
the error below is generated. The RUSTC_BOOTSTRAP environment variable
is added to fix this error.

	error: the option `Z` is only accepted on the nightly compiler
	help: consider switching to a nightly toolchain: `rustup default nightly`
	note: selecting a toolchain with `+toolchain` arguments require a rustup proxy;
	      see <https://rust-lang.github.io/rustup/concepts/index.html>
	note: for more information about Rust's stability policy, see
	      <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html#unstable-features>
	error: 1 nightly option were parsed

The probe may also fail incorrectly with the below error message. To fix
it, the /dev/null argument is replaced with a new rust/probe.rs file
that doesn't need even the core part of the standard library.

error[E0463]: can't find crate for `std`
  |
  = note: the `aarch64-unknown-none` target may not be installed
  = help: consider downloading the target with `rustup target add aarch64-unknown-none`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

The -o and --out-dir parameters are altered to fix this warning:

	warning: ignoring --out-dir flag due to -o flag

I verified that the Kconfig version of rustc-option doesn't have the
same issues.

Fixes: c42297438aee ("kbuild: rust: Define probing macros for rustc")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v2:
- Add `export` to RUSTC_BOOTSTRAP.
- Fix error about core being missing.
- Fix warning about -o flag.
- Link to v1: https://lore.kernel.org/r/20241008-rustc-option-bootstrap-v1-1-9eb06261d4f7@google.com
---
 rust/probe.rs             | 7 +++++++
 scripts/Makefile.compiler | 5 +++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/rust/probe.rs b/rust/probe.rs
new file mode 100644
index 000000000000..bf024e394408
--- /dev/null
+++ b/rust/probe.rs
@@ -0,0 +1,7 @@
+//! Nearly empty file passed to rustc-option by Make.
+//!
+//! The no_core attribute is needed because rustc-option otherwise fails due to
+//! not being able to find the core part of the standard library.
+
+#![feature(no_core)]
+#![no_core]
diff --git a/scripts/Makefile.compiler b/scripts/Makefile.compiler
index 057305eae85c..08d5b7177ea8 100644
--- a/scripts/Makefile.compiler
+++ b/scripts/Makefile.compiler
@@ -21,6 +21,7 @@ TMPOUT = $(if $(KBUILD_EXTMOD),$(firstword $(KBUILD_EXTMOD))/).tmp_$$$$
 # automatically cleaned up.
 try-run = $(shell set -e;		\
 	TMP=$(TMPOUT)/tmp;		\
+	export RUSTC_BOOTSTRAP=1;	\
 	trap "rm -rf $(TMPOUT)" EXIT;	\
 	mkdir -p $(TMPOUT);		\
 	if ($(1)) >/dev/null 2>&1;	\
@@ -76,7 +77,7 @@ ld-option = $(call try-run, $(LD) $(KBUILD_LDFLAGS) $(1) -v,$(1),$(2),$(3))
 # __rustc-option
 # Usage: MY_RUSTFLAGS += $(call __rustc-option,$(RUSTC),$(MY_RUSTFLAGS),-Cinstrument-coverage,-Zinstrument-coverage)
 __rustc-option = $(call try-run,\
-	$(1) $(2) $(3) --crate-type=rlib /dev/null --out-dir=$$TMPOUT -o "$$TMP",$(3),$(4))
+	$(1) $(2) $(3) --crate-type=rlib $(srctree)/rust/probe.rs --out-dir=$$TMP,$(3),$(4))
 
 # rustc-option
 # Usage: rustflags-y += $(call rustc-option,-Cinstrument-coverage,-Zinstrument-coverage)
@@ -86,4 +87,4 @@ rustc-option = $(call __rustc-option, $(RUSTC),\
 # rustc-option-yn
 # Usage: flag := $(call rustc-option-yn,-Cinstrument-coverage)
 rustc-option-yn = $(call try-run,\
-	$(RUSTC) $(KBUILD_RUSTFLAGS) $(1) --crate-type=rlib /dev/null --out-dir=$$TMPOUT -o "$$TMP",y,n)
+	$(RUSTC) $(KBUILD_RUSTFLAGS) $(1) --crate-type=rlib $(srctree)/rust/probe.rs --out-dir=$$TMP,y,n)

---
base-commit: 8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b
change-id: 20241008-rustc-option-bootstrap-607e5bf3114c

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>