I was trying to output LLVM IR directly to the console:

    $ rustc hello.rs --emit=llvm-ir -o /dev/stdout
    LLVM ERROR: IO failure on output stream: Bad file descriptor

Now `LLVMRustPrintModule` returns an error, and we print:

    error: failed to write LLVM IR to /dev/stdout.hello.7rcbfp3g-cgu.0.rcgu.ll: Permission denied

... which is more informative.