Revert "Remove unit tests from old build system"

`make check` was reverted too soon. The hacking guide wasn't brought
up to date with the new workflow, and it's not clear how to use
meson for everything.

This reverts commit 6f3045c2a2.

package.nix

@@ -53,6 +53,10 @@
 # Whether to build Nix. Useful to skip for tasks like testing existing pre-built versions of Nix
 , doBuild ? true
 
+# Run the unit tests as part of the build. See `installUnitTests` for an
+# alternative to this.
+, doCheck ? __forDefaults.canRunInstalled
+
 # Run the functional tests as part of the build.
 , doInstallCheck ? test-client != null || __forDefaults.canRunInstalled
 
@@ -85,6 +89,11 @@
 # - readline
 , readlineFlavor ? if stdenv.hostPlatform.isWindows then "readline" else "editline"
 
+# Whether to install unit tests. This is useful when cross compiling
+# since we cannot run them natively during the build, but can do so
+# later.
+, installUnitTests ? doBuild && !__forDefaults.canExecuteHost
+
 # For running the functional tests against a pre-built Nix. Probably
 # want to use in conjunction with `doBuild = false;`.
 , test-daemon ? null
@@ -108,7 +117,7 @@ let
   # things which should instead be gotten via `finalAttrs` in order to
   # work with overriding.
   attrs = {
-    inherit doBuild doInstallCheck;
+    inherit doBuild doCheck doInstallCheck;
   };
 
   mkDerivation =
@@ -124,11 +133,16 @@ in
 mkDerivation (finalAttrs: let
 
   inherit (finalAttrs)
+    doCheck
     doInstallCheck
     ;
 
   doBuild = !finalAttrs.dontBuild;
 
+  # Either running the unit tests during the build, or installing them
+  # to be run later, requiresthe unit tests to be built.
+  buildUnitTests = doCheck || installUnitTests;
+
 in {
   inherit pname version;
 
@@ -162,6 +176,8 @@ in {
           ./scripts/local.mk
         ] ++ lib.optionals enableManual [
           ./doc/manual
+        ] ++ lib.optionals buildUnitTests [
+          ./tests/unit
         ] ++ lib.optionals doInstallCheck [
           ./tests/functional
         ]));
@@ -174,6 +190,8 @@ in {
     # If we are doing just build or just docs, the one thing will use
     # "out". We only need additional outputs if we are doing both.
     ++ lib.optional (doBuild && enableManual) "doc"
+    ++ lib.optional installUnitTests "check"
+    ++ lib.optional doCheck "testresults"
     ;
 
   nativeBuildInputs = [
@@ -212,6 +230,9 @@ in {
     ({ inherit readline editline; }.${readlineFlavor})
   ] ++ lib.optionals enableMarkdown [
     lowdown
+  ] ++ lib.optionals buildUnitTests [
+    gtest
+    rapidcheck
   ] ++ lib.optional stdenv.isLinux libseccomp
     ++ lib.optional stdenv.hostPlatform.isx86_64 libcpuid
     # There have been issues building these dependencies
@@ -226,16 +247,22 @@ in {
   );
 
   dontBuild = !attrs.doBuild;
+  doCheck = attrs.doCheck;
 
   configureFlags = [
     (lib.enableFeature doBuild "build")
+    (lib.enableFeature buildUnitTests "unit-tests")
     (lib.enableFeature doInstallCheck "functional-tests")
     (lib.enableFeature enableManual "doc-gen")
     (lib.enableFeature enableGC "gc")
     (lib.enableFeature enableMarkdown "markdown")
+    (lib.enableFeature installUnitTests "install-unit-tests")
     (lib.withFeatureAs true "readline-flavor" readlineFlavor)
   ] ++ lib.optionals (!forDevShell) [
     "--sysconfdir=/etc"
+  ] ++ lib.optionals installUnitTests [
+    "--with-check-bin-dir=${builtins.placeholder "check"}/bin"
+    "--with-check-lib-dir=${builtins.placeholder "check"}/lib"
   ] ++ lib.optionals (doBuild) [
     "--with-boost=${boost}/lib"
   ] ++ lib.optionals (doBuild && stdenv.isLinux) [
@@ -316,6 +343,10 @@ in {
     platforms = lib.platforms.unix ++ lib.platforms.windows;
     mainProgram = "nix";
     broken = !(lib.all (a: a) [
+      # We cannot run or install unit tests if we don't build them or
+      # Nix proper (which they depend on).
+      (installUnitTests -> doBuild)
+      (doCheck -> doBuild)
       # The build process for the manual currently requires extracting
       # data from the Nix executable we are trying to document.
       (enableManual -> doBuild)