rustc can't find the linker when `--sysroot` is set to a local build of the sysroot

[RalfJung]

This is a recent regression, starting with #124129: when --sysroot is set, in my case to /home/r/.cache/cargo-careful where I have prepared a locally built sysroot (that's https://github.com/RalfJung/cargo-careful), then rustc now assumes it can find the linker in the sysroot, which does not work. See RalfJung/cargo-careful#31 for some more details.

It is new to me that the sysroot is supposed to contain the linker as well, is that expected behavior?

Also the error message in that case is really unhelpful.^^

  = note: collect2: fatal error: cannot find 'ld'
          compilation terminated.

Maybe rustc should check that the path it sets for -B actually exists before starting cc?

[ChrisDenton]

It is new to me that the sysroot is supposed to contain the linker as well, is that expected behavior?

Yes. The blog post has a summary of that PR https://blog.rust-lang.org/2024/05/17/enabling-rust-lld-on-linux.html

[RalfJung]

That blog post doesn't mention the term "sysroot" so it doesn't help with my issue.

[RalfJung]

FWIW what I would have expected is that rustc searches for the linker binary somewhere around current_exe(). Isn't that how we find some other things as well that are usually installed with rustc by rustup?

[ChrisDenton]

How do you handle windows-gnu? Imo this also uses a self-contained linker by default.

[RalfJung]

No idea. CI passes on Windows, whatever github gives us when we say windows-latest.

[lqd]

Yeah the rust-lld linker has been in the sysroot since #85961, is used by default in e.g. some of the wasm and aarch64 targets, and rustc had flags to opt-into using it since that PR. What is new is that it is now used by x86_64-unknown-linux-gnu nightlies we distribute.

bootstrap puts it in the sysroot when rust.lld is true (and it's now the default on x64 linux under certain conditions), so you can also set that when ./x build-ing if you want it. It works with or without download-ci-llvm.

[onur-ozkan]

FWIW what I would have expected is that rustc searches for the linker binary somewhere around current_exe(). Isn't that how we find some other things as well that are usually installed with rustc by rustup?

We resolve the sysroot from rustc driver, not current executable.

[RalfJung]

Yes, the surprise is just that the linker is searched in the sysroot and thus affected by --sysroot, rather than being searched directly. But I guess this is working as intended, I just need to add some more stuff to these custom sysroots then.

[ehuss]

This broke cargo's CI since the build-std tests try to verify that nothing in the sysroot is required.

Would it be possible to change it so that it will only require rust-lld if it exists in the sysroot (similar to windows-gnu and other targets)?

[lqd]

I don't know what we do on windows-gnu but we can do something similar, yes. I'll open a PR.