Describe the bug

cargoSetupHook:70
compares stdenv.buildPlatform to stdenv.hostPlatform when writing the desired config.toml, but cargoSetupHook is host tooling, so buildPlatorm and hostPlatform are going to be the same value even when cross compiling; we should be comparing hostPlatform and targetPlatform to see if our target package is being cross compiled.

The same goes for line 74: stdenv.hostPlatform.isStatic should be targetPlatform.isStatic.

Steps To Reproduce

Steps to reproduce the behavior:

1. cross compile a rust package to a host not already supported in the target's .cargo/config.toml file, and keep the failed result e.g.

nix build .#nixosConfigurations.<SYSTEM>.pkgs.pkgsCross.aarch64-multiplatform-musl.uutils-coreutils --keep-failed

2. output the resulting cargo.toml file:

[root@jordan:/tmp/nix-build-uutils-coreutils-aarch64-unknown-linux-musl-0.0.27.drv-1]# cat .cargo/config.toml
[source."crates-io"]
"replace-with" = "vendored-sources"

[source."vendored-sources"]
"directory" = "/build/uutils-coreutils-0.0.27-vendor.tar.gz"


    [target."x86_64-unknown-linux-gnu"]
"linker" = "/nix/store/n89l3mzh49hgwvd4dmgmd90h59a7j7lq-gcc-wrapper-13.3.0/bin/cc"

"rustflags" = [ "-C", "target-feature=-crt-static" ]

Expected behavior

Target information should also be contained in the cargo.toml file, like so:

[root@jordan:/tmp]# cat nix-build-uutils-coreutils-aarch64-unknown-linux-musl-0.0.27.drv-2/.cargo/config.toml
[source."crates-io"]
"replace-with" = "vendored-sources"

[source."vendored-sources"]
"directory" = "/build/uutils-coreutils-0.0.27-vendor.tar.gz"


    [target."x86_64-unknown-linux-gnu"]
"linker" = "/nix/store/n89l3mzh49hgwvd4dmgmd90h59a7j7lq-gcc-wrapper-13.3.0/bin/cc"
[target."aarch64-unknown-linux-musl"]
"linker" = "/nix/store/0655rcnl111my4fyyrhrkk9hdw6r7yrf-aarch64-unknown-linux-musl-gcc-wrapper-13.3.0/bin/aarch64-unknown-linux-musl-cc"

"rustflags" = [ "-C", "target-feature=-crt-static" ]

Additional context

• an overlay, modifying the cargoSetupHook nativeBuildInput to
  nativeBuildInputs = [ final.pkgsHostHost.rustPlatform.cargoSetupHook ]; works around the problem.
• in order to successfully cross-compile packages using cargoSetupHooks, a fix for ‎cargoSetupPostUnpackHook writes config.toml in incorrect directory #355788 is also needed.
• I am not 100% certain about scenarios where we're using rustPlatform to build rust tooling; where a package's target and host are different, but given cargo is still building the correct target, I'd suggest just changing cargoSetupHook to targetPlatform shouldn't cause any issues.

Metadata

• system: "x86_64-linux"
• host os: Linux 6.11.5, NixOS, 24.11 (Vicuna), 24.11.20241109.76612b1
• multi-user?: yes
• sandbox: yes
• version: nix-env (Lix, like Nix) 2.91.1 System type: x86_64-linux Additional system types: aarch64-linux, i686-linux Features: gc, signed-caches System configuration file: /etc/nix/nix.conf User configuration files: /home/jordan/.config/nix/nix.conf:/etc/xdg/nix/nix.conf:/home/jordan/.nix-profile/etc/xdg/nix/nix.conf:/home/jordan/.local/state/nix/profile/etc/xdg/nix/nix.conf:/home/jordan/.local/state/nix/profile/etc/xdg/nix/nix.conf:/etc/profiles/per-user/jordan/etc/xdg/nix/nix.conf:/nix/var/nix/profiles/default/etc/xdg/nix/nix.conf:/run/current-system/sw/etc/xdg/nix/nix.conf Store directory: /nix/store State directory: /nix/var/nix Data directory: /nix/store/k1iys4jifr01ql4bzafl6ny71qzw32wz-lix-2.91.1/share
• nixpkgs: /nix/store/0sxsfzswhjck7f4sbsvznz285s4i7y64-source

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@mmlb @winterqt @figsoda

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