Change to use self-contained lld by default in rustup nightly artifacts breaks native linking

[Nemo157]

rust-lang/rust#71515 (comment) mentions that the next nightly will switch to using the self-contained lld by default instead of the system linker. Trying to do some pre-testing of this setup by manually enabling the new defaults in a project with a native dependency indicates that this causes issues with library loading (presumably something rpath related, but I've never dug into the details of how that is handled normally):

> cargo rustc -- -Clink-self-contained=+linker -Clinker-flavor=gnu-lld-cc
   Compiling u2f-touch-detector v0.1.0 (/home/nemo157/sources/u2f-touch-detector)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.58s

> $CARGO_HOME/target/shared/x86_64-unknown-linux-gnu/debug/u2f-touch-detector --help
/run/user/1000/cargo-home/target/shared/x86_64-unknown-linux-gnu/debug/u2f-touch-detector: error while loading shared libraries: libudev.so.1: cannot open shared object file: No such file or directory

The PR linked from the tracking issue mentions that the workaround will be to use -Zlinker-features=-lld.

[szlend]

I assume we'll probably need to wrap rust-lld, similar to NixOS/nixpkgs#24744 (comment)

[Nemo157]

Hmm, finally got round to testing it on the latest nightly and it seems like it's not active by default when installed via the overlay. I'm not sure why though, rustc --print target-spec-json -Z unstable-options | jq -r '.["link-self-contained"]' shows that it is enabled in the target-spec. Running RUSTC_LOG=rustc_codegen_ssa::back::link=info cargo rustc -- -Clink-arg=-Wl,-v with/without -Clink-self-contained=+linker shows that by default it's using GNU ld (GNU Binutils) 2.40 and only uses LLD 18.1.4 (compatible with GNU linkers) when explicitly requested.

[Nemo157]

I found out why, I use cranelift as my codegen backend which disables default-active lld. Testing with that overridden successfully fails:

> CARGO_PROFILE_DEV_CODEGEN_BACKEND=llvm cargo run -- --help
   Compiling u2f-touch-detector v0.1.0 (/home/nemo157/sources/u2f-touch-detector)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.61s
     Running `/run/user/1000/cargo-home/target/shared/x86_64-unknown-linux-gnu/debug/u2f-touch-detector --help`
/run/user/1000/cargo-home/target/shared/x86_64-unknown-linux-gnu/debug/u2f-touch-detector: error while loading shared libraries: libudev.so.1: cannot open shared object file: No such file or directory

[Nemo157]

I tried to figure out how to wrap the binaries, but bintools-wrapper is really not setup to wrap arbitrary binaries within a derivation, it wants something that looks at least somewhat like a bintools and wraps the whole thing.

[szlend]

The wrap function is fairly simple, maybe worth vendoring it:

wrap() {
  local dst="$1"
  local wrapper="$2"
  export prog="$3"
  export use_response_file_by_default=${if isCCTools then "1" else "0"}
  substituteAll "$wrapper" "$out/bin/$dst"
  chmod +x "$out/bin/$dst"
}

You could then reference the ld wrapper like so:

mv $out/bin/rust-lld $out/bin/.rust-lld-unwrapped
wrap $out/bin/rust-lld \
  ${pkgs.path + /pkgs/build-support/bintools-wrapper/ld-wrapper.sh} \
  $out/bin/.rust-lld-unwrapped

Though there's a bunch of other things it expects substituted:

@coreutils_bin@
@suffixSalt@

coreutils_bin = lib.getBin coreutils; # pkgs.coreutils but probably unnecessary? 
suffixSalt = replaceStrings ["-" "."] ["_" "_"] targetPlatform.config; # pkgs.stdenv.targetPlatform

Edit: Ah, this needs more bintools stuff in nix-support. Might be more difficult than I first thought.

[szlend]

Alternatively, maybe we could extend stdenv.cc.bintools so it also includes and wraps rust-lld.

[oxalica]

The wrap function is fairly simple, maybe worth vendoring it:

Thanks for the investment of the ld-wrapper. I ends up copying part of the wrapper implementation to make it actually work. I created #171 and it works for me now. I'm looking for tests and feedback from people involving here.