[PATCH] Kbuild: add RUSTC_BOOTSTRAP to rustc-option

[Alice Ryhl <aliceryhl@google.com>]

When using unstable features with the Rust compiler, you must either use
a nightly compiler release or set the RUSTC_BOOTSTRAP environment
variable. Otherwise, the compiler will emit a compiler error. This
environment variable is missing when rustc-option is executed, so add
the environment variable.

This change is necessary to avoid two kinds of problems:

1. When using rustc-option to test whether a -Z flag is available, the
   check will always fail, even if the flag is available.
2. If KBUILD_RUSTFLAGS happens to contain any -Z flags from the
   environment, even if unrelated to the flag being tested, then all
   invocations of rustc-option everywhere will fail.

I was not actually able to trigger the second kind of problem with the
makefiles that exist today, but it seems like it could easily start
being a problem due to complicated interactions between changes. It is
better to fix this now so it doesn't surprise us later.

I added the flag under try-run as this seemed like the easiest way to
make sure that the fix applies to all variations of rustc-option,
including new ones added in the future.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 scripts/Makefile.compiler | 1 +
 1 file changed, 1 insertion(+)

diff --git a/scripts/Makefile.compiler b/scripts/Makefile.compiler
index 057305eae85c..50eada69aed9 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;		\
+	RUSTC_BOOTSTRAP=1;		\
 	trap "rm -rf $(TMPOUT)" EXIT;	\
 	mkdir -p $(TMPOUT);		\
 	if ($(1)) >/dev/null 2>&1;	\

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

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