fix pkgsMusl.cargo via cargoSetupHook

[yu-re-ka]

Description of changes

This is a variant of #190796, which removes the need for patching rustc by always applying the unstable host-config flag in cargoSetupHook.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[alyssais]

Do we need to specify the triple here, or can we just do [host]?

[yu-re-ka]

I think both work, do you prefer just [host]?

[alyssais]

I'd have a mild preference for the former, just because it means we don't have to worry about what happens if e.g. using a custom build spec. But I don't think it's too important, and would understand if you wanted it to be consistent with what we were already doing for target.

[yu-re-ka]

done

[alyssais]

Might make sense to rename this, and probably expand the comment a bit to explain what's going on? You could probably just adapt what I wrote in the other PR if you want.

[yu-re-ka]

done, please let me know if the comment is ok

[symphorien]

are -Z flags allowed without RUSTC_BOOTSTRAP=1 ?

[yu-re-ka]

are -Z flags allowed without RUSTC_BOOTSTRAP=1 ?

The way we set it, in .cargo/config, it works. I tested that fd builds, which does not have RUSTC_BOOTSTRAP=1.

[symphorien]

I'm very puzzled as to why you set target-applies-to-host to true: the doc says

It requires the -Zhost-config and -Ztarget-applies-to-host command-line options to be set, and that target-applies-to-host = false is set in the Cargo configuration file.

https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#host-config

[yu-re-ka]

There is two things here:

• Is the target-applies-to-host unstable feature enabled?
• If yes, this unlocks a new target-applies-to-host option

Also see, in the same section:

Setting -Zhost-config changes the default for target-applies-to-host to false from true.

[symphorien]

ok I understand. Then the change looks sensible to me, and if things build with it then go for it.

[alyssais]

I like this better than my approach, and in my testing, all the builds my version fixed are also fixed by this change.

It's still an open question what to do for rustc on Musl when used outside of Cargo, but that's orthogonal to this change. For Cargo this is the way to go.

[lf-]

This PR seems to have regressed packages being able to set target.*.rustflags inside a .cargo/config file. I'm investigating sapling being broken on darwin (see #201798), and that setting is not getting through to the compiler. Since the setup hook sets host.rustflags, it seems that any .cargo/config file inside projects will have its target.aarch64-apple-darwin.rustflags setting ignored. This is the resulting config:

[nix-shell:/tmp/sapling/source/eden/scm]$ RUSTC_BOOTSTRAP=1 cargo config -Z unstable-options get --show-origin
-snip-
source.vendored-sources.directory = "cargo-vendor-dir" # /private/tmp/sapling/.cargo/config
target.aarch64-apple-darwin.linker = "/nix/store/v069wkyb6y6qw2bhpzag5kka19j7clgg-clang-wrapper-11.1.0/
bin/cc" # /private/tmp/sapling/.cargo/config
target.aarch64-apple-darwin.rustflags = [
    "-C", # /private/tmp/sapling/source/eden/scm/.cargo/config
    "link-args=-Wl,-undefined,dynamic_lookup", # /private/tmp/sapling/source/eden/scm/.cargo/config
    "-C", # /private/tmp/sapling/.cargo/config
    "target-feature=-crt-static", # /private/tmp/sapling/.cargo/config
]
target.arm64-apple-darwin.rustflags = [
    "-C", # /private/tmp/sapling/source/eden/scm/.cargo/config
    "link-args=-Wl,-undefined,dynamic_lookup", # /private/tmp/sapling/source/eden/scm/.cargo/config
]
target.i686-apple-darwin.rustflags = [
    "-C", # /private/tmp/sapling/source/eden/scm/.cargo/config
    "link-args=-Wl,-undefined,dynamic_lookup", # /private/tmp/sapling/source/eden/scm/.cargo/config
]
target.x86_64-apple-darwin.rustflags = [
    "-C", # /private/tmp/sapling/source/eden/scm/.cargo/config
    "link-args=-Wl,-undefined,dynamic_lookup", # /private/tmp/sapling/source/eden/scm/.cargo/config
]
unstable.host-config = true # /private/tmp/sapling/.cargo/config
unstable.target-applies-to-host = true # /private/tmp/sapling/.cargo/config

[lf-]

Ah, it's actually specifically the unstable.* options that cause this. Nevertheless, this causes some breakage of project cargo config files.