rustPlatform.buildRustPackage: packages that support cross compilation fail to build on platforms that are cross compilation targets

[uri-canva]

Describe the bug

If a package supports cross compilation upstream:
https://github.com/bvaisvil/zenith/blob/8f890291284870ea12e065ada0844100ad7aa7dc/.cargo/config
https://github.com/EFForg/apkeep/blob/0.6.0/.cargo/config.toml

then if we build the package with rustPlatform.buildRustPackage on a platform that matches a target then cargo will attempt to use that configuration for building, leading to errors like:

error: linker `aarch64-linux-gnu-gcc` not found
  |
  = note: No such file or directory (os error 2)

   Compiling instant v0.1.10
error: could not compile `encoding_rs` due to previous error
warning: build failed, waiting for other jobs to finish...
error: build failed
builder for '/nix/store/pqx5zpfzj6a90li969dqpw9q51zh6q6c-apkeep-0.6.0.drv' failed with exit code 101
error: build of '/nix/store/pqx5zpfzj6a90li969dqpw9q51zh6q6c-apkeep-0.6.0.drv' failed

https://github.com/NixOS/nixpkgs/pull/145452/checks?check_run_id=4174333214

This is because we always pass --target to it, regardless of whether it's cross compiling or not:

nixpkgs/pkgs/build-support/rust/hooks/cargo-build-hook.sh

Line 24 in 6d3f429

--target @rustTargetPlatformSpec@ \

cargo assumes it is cross compiling when --target is passed: rust-lang/cargo#5754 (comment)

The easiest fix is to delete the cross compilation configuration like in #107237.

Steps To Reproduce

Steps to reproduce the behavior:

1. Build apkeep on aarch64-linux.

Expected behavior

Successful build.

Additional context

#107237
#88616
#145452

Notify maintainers

@retrry

Metadata

• system: "aarch64-darwin"
• host os: Darwin 21.1.0, macOS 12.0.1
• multi-user?: yes
• sandbox: no
• version: nix-env (Nix) 2.3.16
• channels(root): "nixpkgs-21.11pre309670.253aecf69ed"
• channels(uribaghin): "darwin, nixpkgs"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixpkgs

[uri-canva]

Can we generalise this fix? Always delete .cargo/config.toml? Or delete the section that matches the host platform? Or change the way we invoke cargo to avoid --target when we're not cross compiling.

[jyooru]

I've tested that deleting the linker configuration for a target fixes that target, and it does.

Instead of deleting .cargo/config.toml, would it be better to delete the linker configuration of each target? This way other configuration is still there.

I've tested that this script works to fix apkeep, and it does. It simply loads the configuration, removes linker configuration for each target and then saves it.

#!/usr/bin/env nix-shell
#! nix-shell -i python3 -p python3 python3Packages.toml
from os import path
import toml


if path.exists(".cargo/config.toml"):
    config = toml.load(open(".cargo/config.toml"))

    for target in config.get("target",{}).values():
        if "linker" in target:
            del target["linker"]

    toml.dump(config,open(".cargo/config.toml", "w"))

# .config/cargo.toml
[target.aarch64-unknown-linux-gnu]
linker = "aarch64-linux-gnu-gcc"