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@xxxxxxxxxxxxxx/
Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
---
[...]