Without the change `cargo` tests fail the eval as:
$ nix build --no-link -f. cargo.tests
error: attribute 'tests' missing
at pkgs/development/compilers/rust/cargo.nix:22:15:
21|
22| passthru = {
| ^
23| rustc = rustc;
The test teried to pull `tests` attribute from `rustc`, which is a
wrapper without `tests`. Pull tests from unwrapped rust instead.
a3957236ce