[PATCH] kbuild: rust: add `rustupoverride` target

[Miguel Ojeda <ojeda@kernel.org>]

When setting up the Rust support via `rustup`, one may use an override
in order to select the right version of the Rust toolchain.

The current instructions at Documentation/rust/quick-start.rst assume
one is using an in-tree kernel build (i.e. no `O=`) [1]. We would like
to provide also the way to do so for `O=` builds, but ideally in a way
that keeps the one-liner copy-pastable and without duplication [2].

Thus provide a new Make target, `rustupoverride`, that sets it up for
the user given their build options/variables.

Link: https://lore.kernel.org/rust-for-linux/20231207084846.faset66xzuoyvdlg@vireshk-i7/ [1]
Link: https://lore.kernel.org/rust-for-linux/CANiq72=mvca8PXoxwzSao+QFbAHDCecSKCDtV+ffd+YgZNFaww@mail.gmail.com/ [2]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
Viresh may send a patch on top of this to refer to `rustupoverride`
from the Quick Start guide in `Documentation/rust` -- that one should
probably be applied right after this one.

 Makefile | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 70fc4c11dfc0..7fe82dd4dc6f 100644
--- a/Makefile
+++ b/Makefile
@@ -276,7 +276,8 @@ no-dot-config-targets := $(clean-targets) \
 			 cscope gtags TAGS tags help% %docs check% coccicheck \
 			 $(version_h) headers headers_% archheaders archscripts \
 			 %asm-generic kernelversion %src-pkg dt_binding_check \
-			 outputmakefile rustavailable rustfmt rustfmtcheck
+			 outputmakefile rustavailable rustfmt rustfmtcheck \
+			 rustupoverride
 no-sync-config-targets := $(no-dot-config-targets) %install modules_sign kernelrelease \
 			  image_name
 single-targets := %.a %.i %.ko %.lds %.ll %.lst %.mod %.o %.rsi %.s %.symtypes %/
@@ -1611,6 +1612,7 @@ help:
 	@echo  '                    (requires kernel .config; downloads external repos)'
 	@echo  '  rust-analyzer	  - Generate rust-project.json rust-analyzer support file'
 	@echo  '		    (requires kernel .config)'
+	@echo  '  rustupoverride  - Set up a rustup override for the build directory'
 	@echo  '  dir/file.[os]   - Build specified target only'
 	@echo  '  dir/file.rsi    - Build macro expanded source, similar to C preprocessing.'
 	@echo  '                    Run with RUSTFMT=n to skip reformatting if needed.'
@@ -1735,6 +1737,11 @@ rustfmt:
 rustfmtcheck: rustfmt_flags = --check
 rustfmtcheck: rustfmt

+# `rustup override` setup target
+PHONY += rustupoverride
+rustupoverride:
+	$(Q)rustup override set $(shell $(srctree)/scripts/min-tool-version.sh rustc)
+
 # Misc
 # ---------------------------------------------------------------------------


base-commit: a39b6ac3781d46ba18193c9dbb2110f31e9bffe9
--
2.43.0