llvm suggests invalid command

[vks]

$ rustc dummy.rs -C llvm-args=xyz
rustc: Unknown command line argument 'xyz'.  Try: 'rustc -help'

It should be rustc --help or rustc -h.

[jooert]

The problem here is that rustc: Unknown command line argument 'xyz'. Try: 'rustc -help' is actually printed by LLVM (see src/llvm/lib/Support/CommandLine.cpp) which rustc calls with a faked program name (see src/librustc_trans/back/write.rs).

We could change CommandLine.cpp to print Try: 'rustc --help' but that won't help with the real issue as rustc --help doesn't print anything about valid arguments to LLVM. To get information about those you would have to call rustc dummy.rs -C llvm-args=-help (note that you cannot call rustc -C llvm-args=-help without an input file, which is pretty awkward, too).

[mark-i-m]

I played around with this a little, but didn't get very far because of my lack of expertise in how codegen is called. Here's an update for the next person that comes along:

• The offending message is produced from LLVM's argument parser
  • src/llvm/lib/Support/CommandLine.cpp in ParseCommandLineOptions.
• Everything else is unchanged from #25819 (comment)
• It would be great if the fix didn't have to involve landing a patch in LLVM.
• One fix is to add a rustc flag --llvm-help that calls LLVM with the -help flag. Unfortunately, this is hard because the codegen backend is not initialized until after we have parsed flags and handled stuff like --help.
  • Help/usage messages are printed in handle_options in src/librustc_driver/lib.rs which is called from run_compiler (same file).
  • run_compiler calls run_compiler_with_pool after handle_options is done.
  • run_compiler_with_pool builds a whole compiler Session first before building and initializing the codegen backend with get_codegen_backend.
  • get_codegen_backend calls backend.init(&sess) which eventually calls configure_llvm in src/librustc_codegen_llvm/llvm_util.rs`
  • configure_llvm makes the FFI call llvm::LLVMRustSetLLVMOptions which calls the parser in LLVM (in C++)...