doc: Add section about overriding the linker

[eureka-cpu]

[Cyberboss]

We're experiencing issues building a .so with --target=i686-unknown-linux-gnu on our x86_64 system.

= note: rust-lld: error: /nix/store/j4kybm2h5l6ing0i494zwdswp672pmgv-xgcc-14.3.0-libgcc/lib/libgcc_s.so.1 is incompatible with elf32-i386

Would an override help resolve this?

[eureka-cpu]

Hm, not sure, I would have to work backwards from the error. I haven't had to build for i686 before. Are you using a library like naersk for cross compilation?

[Cyberboss]

No just cargo from fenix

with fenix.packages.x86_64-linux; combine [minimal.toolchain targets.i686-unknown-linux-gnu.minimal.rust-std]

[eureka-cpu]

Right, but the error is that the linker is incompatible during the build process. Fenix is just the toolchain, so I was curious how you are cross compiling, eg. using nix to build a package or in a devShell and running cargo locally. If it's the latter, I would try looking at the available linker options in the rustup/cargo book and seeing if overriding the linker works. For instance, maybe give llvm a try. You'll need to provide either gcc lib or clang lib as well as llvm bintools from nixpkgs in the nativeBuildInputs. Let me see if I can find how I did this before.

[eureka-cpu]

So, I was using crane and added this to my commonArgs for each cargo derivation:

    nativeBuildInputs = with prev; [
      llvmPackages.bintools
      libclang.lib
    ] ++ optional stdenv.isLinux prev.autoPatchelfHook;
    buildInputs = optional stdenv.isDarwin prev.libiconv ++ optionals stdenv.isLinux (with prev; [
      gcc.cc
      gcc.cc.lib
    ]);

This may be overkill for your situation, and not sure if you want to use llvm. Also this is from an overlay, so prev here would just be pkgs.

[eureka-cpu]

Then I basically did this: https://doc.rust-lang.org/cargo/guide/build-performance.html?highlight=linker#use-an-alternative-linker

Mine looks like:

# .cargo/config.toml
[build]
rustflags = ["-Clink-arg=-fuse-ld=lld"]

[eureka-cpu]

You can set these in your cargo derivation, but I chose to just include this file in the fileset of my derivation's source.