Set `cc` for rustdoc

[nmattia]

Describe the problem you are trying to solve

I can't find a way to set the cc for rustdoc.

CARGO_TARGET_XXX_LINKER is not passed, and it appears that rustdoc reacts neither to CARGO_BUILD_RUSTFLAGS=-C linker=/path/to/cc nor to CC=/path/to/cc. One solution may be to set RUSTDOCFLAGS=-Z unstable-options --linker=/path/to/cc but this is only enabled on nightly, which I don't have.

Describe the solution you'd like

It would be great for cargo to forward CARGO_TARGET_XX_LINKER to rustdoc, where XXX is the CARGO_BUILD_TARGET.

[ehuss]

1.39 will now allow -C linker=cc (see rust-lang/rust#63834). Cargo doesn't issue that flag, though, so you'll still need to use RUSTDOCFLAGS.

What is your use case for passing a custom linker? Doctests don't support cross-compiling without the unstable -Zdoctest-xcompile, so I'm curious why the default host linker is not working for you.

With -Zdoctest-xcompile, it seems like a reasonable idea to pass that flag, but I'm not too familiar with the use cases for using custom linkers with doc tests.

[nmattia]

Thanks for the feedback! Looking forward to 1.39.

What is your use case for passing a custom linker?

I work on systems that unfortunately do not have a cc in the path or in any standard location.

[benesch]

Also, sometimes linkers have bugs. For example binutils 2.30 (shipped with Ubuntu 18.04 LTS) mishandles some compressed debug info sections: https://sourceware.org/bugzilla/show_bug.cgi?id=23919.

We work around this at Materialize by setting CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_LINKER appropriately, to a toolchain that does not exhibit the bug, but the fact that this setting does not apply to rustdoc links is very surprising! Here's a build failure that took me well over an hour to track down: https://buildkite.com/materialize/tests/builds/7669#e8398bc6-91d4-4130-b417-be4233dfcfd8.

So even when you're not cross compiling, I think it's important that CARGO_TARGET_<HOST TRIPLE>_LINKER apply to rustdoc too.