How do I change the default rustc / Cargo linker?

19
votes

I would like to make rustc use lld as a linker instead of ld in a particular crate. So I create .cargo/config in my project directory with the following:

[target.x86_64-unknown-linux-gnu]                                                                   
linker = "ld.lld"

Which leads to linker errors:

$ cargo build
...
  = note: ld.lld: error: unable to find library -ldl
          ld.lld: error: unable to find library -lrt
          ld.lld: error: unable to find library -lpthread
          ld.lld: error: unable to find library -lgcc_s
          ld.lld: error: unable to find library -lc
          ld.lld: error: unable to find library -lm
          ld.lld: error: unable to find library -lrt
          ld.lld: error: unable to find library -lpthread
          ld.lld: error: unable to find library -lutil
          ld.lld: error: unable to find library -lutil

Same thing with rust-lld. If I set linker = "ld" (which should be the default, right?), I just get

  = note: ld: cannot find -lgcc_s

I tried to resolve all the missing libraries manually (with -C link-arg=--library-path=/usr/lib/x86_64-linux-gnu and the like), but it only lead to wrong linkage and a segfaulting binary.

Interestingly enough, if I replace /usr/bin/ld with a symlink to /usr/bin/ld.lld, it works great (no errors, and from the compiled binary I see that it was indeed linked with lld). However, I don't want to make lld my system-wide linker, I just want to use it in a particular Rust crate.

So what is the proper way to change the default rustc linker?

2
rustrust-cargolld
What happens if you put clang as the linker? – Jmb

2 Answers

16
votes

Thanks to @Jmb comment, I found a solution. Turns out that the default linker that rustc uses is actually cc (which makes sense - it supplies all the needed defaults to compile/link C code, which also work for Rust). We can pass an argument to cc to make it link with lld:

[target.x86_64-unknown-linux-gnu]
rustflags = [
    "-C", "link-arg=-fuse-ld=lld",
]

Now cargo build links with lld.

2
votes

This also works and is what I think @Jmb actually asked.

rustflags = [
  "-C", "linker=clang-12",  # change the version as needed
]