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 ${drv}/tests/my-test
''
```
This cuts down the dependency tree on some rust builds where a crate not
just exposes a binary but also a library. `$out/lib` contained a bunch
of extra support files that among other information carry linker flags
(including the full path to link-time dependencies). Worst case this led
to some binary outputs depending on the full build closure of rust
crates.
Moving all the `$out/lib` files to `$lib/lib` solves this nicely.
`lib` might be a bit weird here as they are most of the time just rlib
files (rust libraries). Those are essential only required during
compilation but they can also be shared objects (like with traditional
C-style packages). Which is why I went with `lib` for the new output.
One of the caveats we are running into here is that we do not (always)
know ahead of time of a crate produces just a library or just a binary.
Cargo allows for some ambiguity regarding whether or not a crate
provides one, two, … binaries and libraries as it's outputs. Ideally we
would be able to rely on the `crateType` entirely but so far that isn't
the case. More work on that area might show how difficult that actually
is.
The build expression got quiet large over time and to make it a bit
easier to grasp the different scripts involved in the build are now
separated from the nix file.
