[PATCH 4/6] kbuild: rust_is_available: check if the script was invoked from Kbuild

[Miguel Ojeda <ojeda@kernel.org>]

Sometimes [1] users may attempt to setup the Rust support by
checking what Kbuild does and they end up finding out about
`scripts/rust_is_available.sh`. Inevitably, they run the script
directly, but unless they setup the required variables,
the result of the script is not meaningful.

We could add some defaults to the variables, but that could be
confusing for those that may override the defaults (compared
to their kernel builds), and `$CC` would not be a simple default
in any case.

Therefore, instead, print a warning when the script detects
the user may be invoking it, by testing `$MAKEFLAGS`.

Link: https://lore.kernel.org/oe-kbuild-all/Y6r4mXz5NS0+HVXo@zn.tnic/ [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 scripts/rust_is_available.sh | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/scripts/rust_is_available.sh b/scripts/rust_is_available.sh
index cd87729ca3bf..0c082a248f15 100755
--- a/scripts/rust_is_available.sh
+++ b/scripts/rust_is_available.sh
@@ -35,6 +35,16 @@ print_docs_reference()
 warning=0
 trap 'if [ $? -ne 0 ] || [ $warning -ne 0 ]; then print_docs_reference; fi' EXIT
 
+# Check whether the script was invoked from Kbuild.
+if [ -z "${MAKEFLAGS+x}" ]; then
+	echo >&2 "***"
+	echo >&2 "*** This script is intended to be called from Kbuild."
+	echo >&2 "*** Please use the 'rustavailable' target to call it instead."
+	echo >&2 "*** Otherwise, the results may not be meaningful."
+	echo >&2 "***"
+	warning=1
+fi
+
 # Check that the Rust compiler exists.
 if ! command -v "$RUSTC" >/dev/null; then
 	echo >&2 "***"
-- 
2.39.0