Motivation for this change

buildRustCrate: add buildTests flag to tell rustc to build tests instead of binaries

This helps us instruct rustc to build tests instead of binaries. The
actual build will then ONLY produce test executables. This is a first
step towards having rust crate tests within nixpkgs.

We default back to only a single output in test cases since that is the
only reasonable thing to do here.

Producing libraries or binaries in addition to tests would theoretically
be feasible but usually generates different dependency trees. It is very
common to have some libraries in [dev-depdendencies] within Cargo.toml
just for your tests. To not start mixing things up going with a
dedicated derivation for the test build sounds like the best choice for
now.

To use this you must provide a proper test dependency chain to
buildRustCrate (as you would usually do with your non-test inputs).
And then set the buildTests attribute to true. The derivation will
then contain all tests that were built in $out/tests. All common test
patterns and directories should be supported and tested by this change.

Below is an example how you would run a single test from the derivation.
This commit contains some more examples in the buildRustCrateTests
attribute set that might be helpful.

   let
     drv = buildRustCrate {
        …
        buildTests true;
     };
   in runCommand "test-my-crate" {} ''
     touch $out
     exec ${drv}/tests/my-test
   ''

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @Mic92