Support for excluding rust compiler_builtin from staticlib

[Andarwinux]

Some targets use llvm's compiler-rt by default, in which case cargo-c should exclude rust's compiler-rt from staticlib in order to use what the C toolchain provides. Otherwise it will increase the size or cause symbol conflicts.

Here's one way to hack it, but it's ugly.

llvm-ar t staticlib.a | grep '^compiler_builtins.*\.o$' | xargs -I {} llvm-ar d staticlib.a {}

[lu-zero]

Could you please provide more information? which are the targets? is there a rustc and cargo issue for the problem? Staticlibs are produced by rustc using its provided information.

[Andarwinux]

Hi, the target is x86_64-pc-windows-gnullvm.

I noticed when building libdovi with -Zbuild-std and -Clinker-plugin-lto -Ccodegen-units=1 that there is still a non-LTO bitcode object compiler_builtins-e758a9b796fd24a6.compiler_builtins.45bebbe1a3522e0e-cgu.0.rcgu.o inside libdovi.a that defines some of the same symbols as libclang-rt.builtins.a.

Some old issues mentioned that this would cause symbol conflicts, but I did not encounter such a phenomenon. After searching, I found that for my platform, rustc has defined all compiler_builtins symbols as weak symbols to avoid conflicts, but if I use llvm-ar to remove compiler_builtins from libdovi.a, I can still link it and the final exe/dll becomes smaller.

[lu-zero]

you can tell the final linker invocation to gc what's redundant, but this is an issue for rustc to solve.