37

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?

Shepmaster
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kreo
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3 Answers3

36

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.

Nosrep
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kreo
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9

Adding this to .cargo/config.toml (in the project) forces using lld

[target.x86_64-pc-windows-msvc]
rustflags = ["-C", "link-arg=-fuse-ld=lld"]

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

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

For more information see this: https://github.com/rust-lang/rust/issues/71515

You can remove "-C", "linker=clang" on Linux, if you have Gcc 9 or newer installed

Amin Ya
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  • I don't think this has any effect if it's placed in the Cargo.toml. It should go into the config file instead: https://doc.rust-lang.org/cargo/reference/config.html – Kevin May 06 '23 at 00:06
5

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

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