Compile errors when building tests for a crate that depends on its own library

[pikajude]

You can check the example here: https://github.com/pikajude/cargo2nix/tree/circular-dep-demonstration/examples/4-circular-dependency

The dependency "graph" looks like lib-crate cfg(test) => util-crate => lib-crate (lib).

If you run nix-build examples/4-circular-dependency:

building
   Compiling lib-crate v0.1.0 (/build/lib-crate)
error[E0308]: mismatched types
 --> src/lib.rs:6:33
  |
6 |     assert_eq!(util_crate::show(&LibFoo), "Libfoo");
  |                                 ^^^^^^^ expected struct `lib_crate::LibFoo`, found struct `LibFoo`
  |
  = note: expected reference `&lib_crate::LibFoo`
             found reference `&LibFoo`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `lib-crate`.

cargo2nix strips out all the dependency info from Cargo.toml when it builds crates, since the tool itself is the one managing dependency resolution. However, when building a crate like this, we can't prevent Cargo from discovering that lib-crate's source code is available alongside the tests, meaning that, even though the version we intend to use is already being picked up by the rustc wrapper, Cargo then recompiles the lib before building the tests.

The error above reads strangely but just means that it can't unify lib_crate-$metadata_for_lib::LibFoo with lib_crate-$metadata_for_test::LibFoo.

[pikajude]

Sorry, my suggested repro here is actually meaningless, because it has the same error with regular cargo builds. I'm going to close this for now and reopen once I can get a proper repro.