Propagated coverage flags cause `gcov` link errors

[cuviper]

When compiling code like the example and RUSTFLAGS=-Cprofile-generate=$PWD, it fails to link with errors like this:

foo.c:1:(.text._sub_I_00100_0+0xf): undefined reference to `__gcov_init'

(and many other __gcov_* symbols)

That's with a default CC that is GCC; it's okay with Clang because that uses the same profiling library as Rust.

Maybe -Cprofile-generate (and -Cprofile-use) should not be translated for ToolFamily::Gnu at all? It seems unlikely that someone would use this and try to juggle both LLVM profraw and GCC gcov files, even if the link error is fixed.

[madsmtm]

CC @mrkajetanp.

Maybe -Cprofile-generate (and -Cprofile-use) should not be translated for ToolFamily::Gnu at all? It seems unlikely that someone would use this and try to juggle both LLVM profraw and GCC gcov files, even if the link error is fixed.

I think that'd be a reasonable fix.

[mrkajetanp]

Huh interesting, I didn't realise gcc and clang actually used different formats for the profiling. Yeah in this case there's no point passing it to gcc I agree, would you like me to write up a PR?

[cuviper]

@mrkajetanp If you're inclined, please go ahead! Otherwise I'll try to get to it later this weekend.