`cargo test` requires windows-only dependency on linux

[msrd0]

Problem

I am trying to test a crate that has a windows-only dependency using the --frozen flag. The windows-only dependency in question is ipconfig, locked to 0.3.0. However, cargo gives me this unexpected error instead of running tests, which is problematic (a) because it is not a dependency on my platform, and (b) ignores the version lock:

thread 'main' panicked at src/tools/cargo/src/cargo/ops/cargo_compile/mod.rs:950:14:
packages downloaded: failed to download `ipconfig v0.3.2`

Caused by:
    attempting to make an HTTP request, but --frozen was specified

Stack backtrace:
   0: <anyhow::Error>::msg::<&str>
   1: cargo::util::network::http::http_handle_and_timeout
   2: <cargo::core::package::Downloads>::start_inner
   3: <cargo::core::package::Downloads>::start
   4: <cargo::core::package::PackageSet>::get_one
   5: cargo::ops::cargo_compile::resolve_all_features
   6: <cargo::ops::cargo_compile::unit_generator::UnitGenerator>::proposals_to_units
   7: <cargo::ops::cargo_compile::unit_generator::UnitGenerator>::generate_root_units
   8: cargo::ops::cargo_compile::create_bcx
   9: cargo::ops::cargo_compile::compile_ws
  10: cargo::ops::cargo_compile::compile_with_exec
  11: cargo::ops::cargo_compile::compile
  12: cargo::ops::cargo_test::compile_tests
  13: cargo::ops::cargo_test::run_tests
  14: cargo::commands::test::exec
  15: <cargo::cli::Exec>::exec
  16: cargo::main
  17: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
  18: std::rt::lang_start::<()>::{closure#0}
  19: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/core/src/ops/function.rs:284:13
  20: std::panicking::try::do_call
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/panicking.rs:584:40
  21: std::panicking::try
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/panicking.rs:547:19
  22: std::panic::catch_unwind
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/panic.rs:358:14
  23: std::rt::lang_start_internal::{{closure}}
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/rt.rs:174:48
  24: std::panicking::try::do_call
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/panicking.rs:584:40
  25: std::panicking::try
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/panicking.rs:547:19
  26: std::panic::catch_unwind
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/panic.rs:358:14
  27: std::rt::lang_start_internal
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/rt.rs:174:20
  28: main
  29: <unknown>
  30: __libc_start_main
  31: <unknown>
stack backtrace:
   0: rust_begin_unwind
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/panicking.rs:692:5
   1: core::panicking::panic_fmt
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/core/src/panicking.rs:75:14
   2: core::result::unwrap_failed
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/core/src/result.rs:1704:5
   3: cargo::ops::cargo_compile::resolve_all_features
   4: <cargo::ops::cargo_compile::unit_generator::UnitGenerator>::proposals_to_units
   5: <cargo::ops::cargo_compile::unit_generator::UnitGenerator>::generate_root_units
   6: cargo::ops::cargo_compile::create_bcx
   7: cargo::ops::cargo_compile::compile_ws
   8: cargo::ops::cargo_compile::compile_with_exec
   9: cargo::ops::cargo_compile::compile
  10: cargo::ops::cargo_test::compile_tests
  11: cargo::ops::cargo_test::run_tests
  12: cargo::commands::test::exec
  13: <cargo::cli::Exec>::exec
  14: cargo::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Steps

On a linux computer, run

git clone https://github.com/hickory-dns/hickory-dns
cd hickory-dns
git checkout v0.25.1
mkdir .cargo_home
CARGO_HOME=.cargo_home cargo fetch --locked --target=x86_64-unknown-linux-gnu
CARGO_HOME=.cargo_home cargo test --frozen -p hickory-resolver

Possible Solution(s)

• Don't download unnecessary dependencies
• As a workaround, make cargo fetch download the dependency

Version

On ArchLinux: cargo 1.85.0 (d73d2caf9 2024-12-31) from rustup
On AlpineLinux: cargo 1.85.1 (d73d2caf9 2024-12-31) from their repositories
On ArchLinux: cargo 1.86.0 (adf9b6ad1 2025-02-28) from rustup

[epage]

An important reminder for anyone bisecting this, we changed the hash algorithm, so you have to re-fetch when using an old enough cargo.

Looks like I can see the panic going back to at least 1.74 (MSRV and what I had installed limited me from going back further).

An abbreviated Cargo.toml

[features]
system-config = ["dep:ipconfig", "dep:resolv-conf"]

[dependencies]

[target.'cfg(windows)'.dependencies]
ipconfig = { workspace = true, optional = true }

If I edit it to

[features]
-system-config = ["dep:ipconfig", "dep:resolv-conf"]
+system-config = ["dep:resolv-conf"]

[dependencies]

[target.'cfg(windows)'.dependencies]
ipconfig = { workspace = true, optional = true }

the panic goes away and build continues as normal.

It seems that when we have a dep: on a target dependency, we are looking at it even if we didn't need to.