[PATCH v3] Kbuild: fix issues with rustc-option

[Alice Ryhl]

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

Note that RUSTC_BOOTSTRAP is also defined in the top-level Makefile, but
its value is unfortunately *not* inherited from the environment. That
said, this is changing as of commit 98da874c4303 ("[SV 10593] Export
variables to $(shell ...) commands"), which is part of Make 4.4.

The probe may also fail with the error message below. To fix it, the
/dev/null argument is replaced with a file containing the crate
attribute #![no_core]. The #![no_core] attribute ensures that rustc does
not look for the standard library. It's not possible to instead supply a
standard library to rustc, as we need `rustc-option` before the Rust
standard library is compiled.

	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

The --sysroot flag is provided as we would otherwise require it to be
present in KBUILD_RUSTFLAGS. The --emit=obj flag is used to write the
resulting rlib to /dev/null instead of writing it to a file in
$(TMPOUT).

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

Fixes: c42297438aee ("kbuild: rust: Define probing macros for rustc")
Co-developed-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
Based on top of:
https://lore.kernel.org/r/20241009102821.2675718-1-masahiroy@xxxxxxxxxx
---
Changes in v3:
- Use stdin instead of a rust/probe.rs file.
- Fix --out-dir argument.
- Move RUSTC_BOOTSTRAP to __rustc-option.
- Add --sysroot and --emit=obj flags.
- Rebase on top of [PATCH] kbuild: refactor cc-option-yn, cc-disable-warning, rust-option-yn macros
- Link to v2: https://lore.kernel.org/r/20241008-rustc-option-bootstrap-v2-1-e6e155b8f9f3@xxxxxxxxxx

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@xxxxxxxxxx
---
 scripts/Makefile.compiler | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/scripts/Makefile.compiler b/scripts/Makefile.compiler
index 73d611d383b2..e0842496d26e 100644
--- a/scripts/Makefile.compiler
+++ b/scripts/Makefile.compiler
@@ -73,8 +73,11 @@ 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)
+# TODO: remove RUSTC_BOOTSTRAP=1 when we raise the minimum GNU Make version to 4.4
 __rustc-option = $(call try-run,\
-	$(1) $(2) $(3) --crate-type=rlib /dev/null --out-dir=$$TMPOUT -o "$$TMP",$(3),$(4))
+	echo '#![allow(missing_docs)]#![feature(no_core)]#![no_core]' | RUSTC_BOOTSTRAP=1\
+	$(1) --sysroot=/dev/null $(filter-out --sysroot=/dev/null,$(2)) $(3)\
+	--crate-type=rlib --out-dir=$(TMPOUT) --emit=obj=- - >/dev/null,$(3),$(4))
 
 # rustc-option
 # Usage: rustflags-y += $(call rustc-option,-Cinstrument-coverage,-Zinstrument-coverage)

---
base-commit: 1ba227507e8459788bf0e192700347c941e3e218
change-id: 20241008-rustc-option-bootstrap-607e5bf3114c

Best regards,
-- 
Alice Ryhl <aliceryhl@xxxxxxxxxx>