kbuild: rust_is_available: print docs reference

People trying out the Rust support in the kernel may get
warnings and errors from `scripts/rust_is_available.sh`
from the `rustavailable` target or the build step.

Some of those users may be following the Quick Start guide,
but others may not (likely those getting warnings from
the build step instead of the target).

While the messages are fairly clear on what the problem is,
it may not be clear how to solve the particular issue,
especially for those not aware of the documentation.

We could add all sorts of details on the script for each one,
but it is better to point users to the documentation instead,
where it is easily readable in different formats. It also
avoids duplication.

Thus add a reference to the documentation whenever the script
fails or there is at least a warning.
Reviewed-by: Finn Behrens <fin@nyantec.com>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20230616001631.463536-5-ojeda@kernel.orgSigned-off-by: Miguel Ojeda <ojeda@kernel.org>

scripts/rust_is_available.sh

@@ -19,6 +19,20 @@ get_canonical_version()
 	echo $((100000 * $1 + 100 * $2 + $3))
 }
 
+# Print a reference to the Quick Start guide in the documentation.
+print_docs_reference()
+{
+	echo >&2 "***"
+	echo >&2 "*** Please see Documentation/rust/quick-start.rst for details"
+	echo >&2 "*** on how to set up the Rust support."
+	echo >&2 "***"
+}
+
+# If the script fails for any reason, or if there was any warning, then
+# print a reference to the documentation on exit.
+warning=0
+trap 'if [ $? -ne 0 ] || [ $warning -ne 0 ]; then print_docs_reference; fi' EXIT
+
 # Check that the Rust compiler exists.
 if ! command -v "$RUSTC" >/dev/null; then
 	echo >&2 "***"
@@ -60,6 +74,7 @@ if [ "$rust_compiler_cversion" -gt "$rust_compiler_min_cversion" ]; then
 	echo >&2 "***   Your version:     $rust_compiler_version"
 	echo >&2 "***   Expected version: $rust_compiler_min_version"
 	echo >&2 "***"
+	warning=1
 fi
 
 # Check that the Rust bindings generator is suitable.
@@ -87,6 +102,7 @@ if [ "$rust_bindings_generator_cversion" -gt "$rust_bindings_generator_min_cvers
 	echo >&2 "***   Your version:     $rust_bindings_generator_version"
 	echo >&2 "***   Expected version: $rust_bindings_generator_min_version"
 	echo >&2 "***"
+	warning=1
 fi
 
 # Check that the `libclang` used by the Rust bindings generator is suitable.
@@ -126,6 +142,7 @@ if [ "$cc_name" = Clang ]; then
 		echo >&2 "***   libclang version: $bindgen_libclang_version"
 		echo >&2 "***   Clang version:    $clang_version"
 		echo >&2 "***"
+		warning=1
 	fi
 fi