[cfe-dev] Silencing "unused argument" for injected default arguments

[Martin Storsjö via cfe-dev]

Hi,

In my setups with clang, I'm using a thin frontend wrapper which passes a 
bunch of custom defaults (allowing using any stock clang executable, 
without requiring it to be built with CLANG_DEFAULT_* set to my desired 
state). The main set of flags I currently use is this:

-target $TARGET -rtlib=compiler-rt -stdlib=libc++ -fuse-ld=lld 
-fuse-cxa-atexit


Many of them are silent (like -fuse-ld=lld despite only compiling, not 
linking), but e.g. -stdlib=libc++ is considered unused if compiling C 
code. (Just looking at the driver name, clang vs clang++, isn't enough for 
deciding to omit it; it could also be "clang -x c++" etc, and I'd prefer 
not to try to deduce that in the wrapper.)

In some cases it also adds a few -D and -Wl,-l<lib>, where the -Wl,-l also 
warns while compiling.

To mitigate this, I'm also adding -Qunused-arguments, which works fine, 
but is a bit too overreaching.

Would it make sense to add some mechanism to flag these arguments as 
default ones that clang shouldn't warn about being unused?

That could be something like --start-no-unused-arguments <args> 
--end-no-unused-arguments.

I know about the clang config files, which also allows setting default 
flags, which are exempt from unused warnings, and I've tried that, but for 
various reasons I'm still more comfortable with setting them via a 
wrapper. (E.g. on windows, the config file setup would require having N 
copies of clang.exe, one for each triple combination, while I currently 
can do with just copies of the wrapper and one single clang.exe.)

// Martin