kbuild: rust_is_available: handle failures calling $RUSTC/$BINDGEN

The script already checks if `$RUSTC` and `$BINDGEN` exists via
`command`, but the environment variables may point to a
non-executable file, or the programs may fail for some other reason.
While the script successfully exits with a failure as it should,
the error given can be quite confusing depending on the shell and
the behavior of its `command`. For instance, with `dash`:

    $ RUSTC=./mm BINDGEN=bindgen CC=clang scripts/rust_is_available.sh
    scripts/rust_is_available.sh: 19: arithmetic expression: expecting primary: "100000 *  + 100 *  + "

Thus detect failure exit codes when calling `$RUSTC` and `$BINDGEN` and
print a better message, in a similar way to what we do when extracting
the `libclang` version found by `bindgen`.

Link: https://lore.kernel.org/rust-for-linux/CAK7LNAQYk6s11MASRHW6oxtkqF00EJVqhHOP=5rynWt-QDUsXw@mail.gmail.com/
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20230616001631.463536-10-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

scripts/rust_is_available.sh

@@ -81,8 +81,20 @@ fi
 # Check that the Rust compiler version is suitable.
 #
 # Non-stable and distributions' versions may have a version suffix, e.g. `-dev`.
+rust_compiler_output=$( \
+	LC_ALL=C "$RUSTC" --version 2>/dev/null
+) || rust_compiler_code=$?
+if [ -n "$rust_compiler_code" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Running '$RUSTC' to check the Rust compiler version failed with"
+	echo >&2 "*** code $rust_compiler_code. See output and docs below for details:"
+	echo >&2 "***"
+	echo >&2 "$rust_compiler_output"
+	echo >&2 "***"
+	exit 1
+fi
 rust_compiler_version=$( \
-	LC_ALL=C "$RUSTC" --version 2>/dev/null \
+	echo "$rust_compiler_output" \
 		| sed -nE '1s:.*rustc ([0-9]+\.[0-9]+\.[0-9]+).*:\1:p'
 )
 rust_compiler_min_version=$($min_tool_version rustc)
@@ -108,8 +120,20 @@ fi
 # Check that the Rust bindings generator is suitable.
 #
 # Non-stable and distributions' versions may have a version suffix, e.g. `-dev`.
+rust_bindings_generator_output=$( \
+	LC_ALL=C "$BINDGEN" --version 2>/dev/null
+) || rust_bindings_generator_code=$?
+if [ -n "$rust_bindings_generator_code" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Running '$BINDGEN' to check the Rust bindings generator version failed with"
+	echo >&2 "*** code $rust_bindings_generator_code. See output and docs below for details:"
+	echo >&2 "***"
+	echo >&2 "$rust_bindings_generator_output"
+	echo >&2 "***"
+	exit 1
+fi
 rust_bindings_generator_version=$( \
-	LC_ALL=C "$BINDGEN" --version 2>/dev/null \
+	echo "$rust_bindings_generator_output" \
 		| sed -nE '1s:.*bindgen ([0-9]+\.[0-9]+\.[0-9]+).*:\1:p'
 )
 rust_bindings_generator_min_version=$($min_tool_version bindgen)