librustc_llvm: call llvm-config with correct linking mode

[leahneukirchen]

In 1.12, src/librustc_llvm/build.rs figures out which libraries it needs for external LLVM using llvm-config --libs which breaks if you have LLVM with -DLLVM_BUILD_LLVM_DYLIB=ON and -DLLVM_LINK_LLVM_DYLIB=OFF (which is needed for clang 3.9 these days), since then there is a dynamic libLLVM3.9.so and lots of libLLVM*.a, but the code sees the libLLVM and decides it has to link statically, which fails.

The code should decide whether to link static or dynamic first, and then call llvm-config --libs --link-shared resp --link-static, to get the correct library name.

[alexcrichton]

Thanks for the report! Sounds like it should be a relatively easy bug to tackle as well!

[leahneukirchen]

In Void, we work around it by just adding --link-static, I guess for a general fix you need some policy how to link it.

https://github.com/voidlinux/void-packages/blob/79600b7b416c46191aefa079cd544c9a3b6779d7/srcpkgs/rust/patches/link-llvm-static.patch

[alexcrichton]

Oh we basically always want to link LLVM statically, so we can probably just always pass it.

[cuviper]

Oh we basically always want to link LLVM statically, so we can probably just always pass it.

We emphatically don't want that in distro builds. Would you accept a patch to make this conditional?

[alexcrichton]

@cuviper certainly!