[lld-macho] Add -fatal-warnings alias

[keith]

Summary

This behaves the same as -fatal_warnings but provides a consistent
interface with ld.lld and ld.gold. This is useful for build tools like
bazel to be able to provide a consistent interface for 'warnings as
errors' to the linker.

[MaskRay]

Options across binary formats are just so different. I think a user seeing -option-name may probably be more confused.
Not playing the house rule can probably just lead to confusion.

BTW: for ELF, I'd like users specify --fatal-warnings instead of -fatal-warnings.

[keith]

In D113068#3104756, @MaskRay wrote:

Options across binary formats are just so different. I think a user seeing -option-name may probably be more confused.

I did hide this by default in help to steer folks towards the default. For users who don't manually pass a lot of linker flags (vs clang passing them), this might be one of the few they do pass themselves, so having that unified across linkers seems valuable to me.

BTW: for ELF, I'd like users specify --fatal-warnings instead of -fatal-warnings.

I think either option would be fine in this case if that's the recommendation

[int3]

I'm with MaskRay here, I think each platform differs too much to have a consistent linker flag API

[thakis]

+1; lld-link doesn't have this either, and even if we add this alias there's a ton of other options that are different. And if bazel (or similar) _requires_ this flag, it won't work with link.exe and ld64, and the point of the compatible drivers is to make linkers mostly interchangeable on each os.

(Otoh if we do end up adding it, please use two -- as prefix and move it up into the "lld-only" section further up )

[keith]

Thanks for the feedback

lld/MachO/Options.td

def application_extension : Flag<["-"], "application_extension">,
     HelpText<"Mark output as safe for use in an application extension, and validate that linked dylibs are safe">,
     Group<grp_rare>;
 def no_application_extension : Flag<["-"], "no_application_extension">,
     HelpText<"Disable application extension functionality (default)">,
     Group<grp_rare>;
 def fatal_warnings : Flag<["-"], "fatal_warnings">,
     HelpText<"Treat warnings as errors">,
     Group<grp_rare>;
+def _fatal_warnings : Flag<["-"], "fatal-warnings">,
+    Alias<fatal_warnings>,
+    HelpText<"Alias for -fatal_warnings to match other platforms">,
+    Flags<[HelpHidden]>,
+    Group<grp_rare>;
 def no_eh_labels : Flag<["-"], "no_eh_labels">,
     HelpText<"In -r mode, suppress .eh labels in the __eh_frame section">,
     Flags<[HelpHidden]>,
     Group<grp_rare>;
 def warn_compact_unwind : Flag<["-"], "warn_compact_unwind">,
     HelpText<"Warn for each FDE that cannot compact into the __unwind_info section and must remain in the __eh_frame section">,
     Flags<[HelpHidden]>,
     Group<grp_rare>;

lld/test/MachO/fatal-warnings.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t1.o
 
 # RUN: %no_fatal_warnings_lld %t1.o -o /dev/null -single_module 2>&1 \
 # RUN:     | FileCheck -check-prefix=WARNING %s
+# RUN: %no_fatal_warnings_lld %t1.o -o /dev/null -fatal_warnings
+# RUN: %no_fatal_warnings_lld %t1.o -o /dev/null -fatal-warnings
 # RUN: not %no_fatal_warnings_lld %t1.o -fatal_warnings -o /dev/null \
 # RUN:     -single_module 2>&1 | FileCheck -check-prefix=ERROR %s
+# RUN: not %no_fatal_warnings_lld %t1.o -fatal-warnings -o /dev/null \
+# RUN:     -single_module 2>&1 | FileCheck -check-prefix=ERROR %s
 
 # ERROR: error: Option `-single_module' is deprecated
 # WARNING: warning: Option `-single_module' is deprecated
 
 .globl _main
 _main: