diff --git a/hacking/nix/rust-utils/build-crate-in-layers.nix b/hacking/nix/rust-utils/build-crates-in-layers.nix
similarity index 86%
rename from hacking/nix/rust-utils/build-crate-in-layers.nix
rename to hacking/nix/rust-utils/build-crates-in-layers.nix
index 531bae09c..b753433d0 100644
--- a/hacking/nix/rust-utils/build-crate-in-layers.nix
+++ b/hacking/nix/rust-utils/build-crates-in-layers.nix
@@ -32,7 +32,11 @@ in
 , rustEnvironment ? defaultRustEnvironment
 , targetTriple ? defaultRustTargetTriple
 
-, rootCrate
+
+, name ? if rootCrate != null then rootCrate.name else "build-crates"
+
+, rootCrate ? null
+, rootCrates ? if rootCrate != null then [ rootCrate ] else throw "must supply 'rootCrates' argument"
 
 , layers ? [ crateUtils.defaultIntermediateLayer ] # default to two building in two steps (external then local)
 , commonModifications ? {}
@@ -45,6 +49,7 @@ in
 , noDefaultFeatures ? false
 
 , test ? false
+, justBuildTests ? false
 
 , verifyWithVerus ? false
 , extraVerusArgs ? []
@@ -74,7 +79,7 @@ let
 
   prunedLockfile = pruneLockfile {
     inherit (rustEnvironment) rustToolchain vendoredSuperLockfile;
-    rootCrates = [ rootCrate ];
+    inherit rootCrates;
     extraManifest = elaboratedCommonModifications.modifyManifest {}; # TODO
   };
 
@@ -87,7 +92,7 @@ let
 
   elaboratedCommonModifications = elaborateModifications commonModifications;
 
-  closure = crateUtils.getClosureOfCrate rootCrate;
+  closure = crateUtils.getClosureOfCrates rootCrates;
 
   accumulatedLayers =
     let
@@ -116,7 +121,7 @@ let
 
   baseManifest = {
     workspace.resolver = "2";
-    workspace.members = [ "src/${rootCrate.name}" ];
+    workspace.members = lib.forEach rootCrates (crate: "src/${crate.name}");
   };
 
   baseConfig = denyWarnings: crateUtils.clobber [
@@ -137,8 +142,8 @@ let
   baseFlags = [
     "--offline"
     "--frozen"
-    "-p" rootCrate.name
-  ] ++ lib.optionals (lib.length features > 0) [
+  ] ++ lib.concatMap (crate: [ "-p" crate.name ]) rootCrates
+    ++ lib.optionals (lib.length features > 0) [
     "--features" (lib.concatStringsSep "," features)
   ] ++ lib.optionals noDefaultFeatures [
     "--no-default-features"
@@ -158,12 +163,12 @@ let
       else "build"
     );
 
-  mkCargoInvocation = runClippy: commonArgs: subcommandArgs:
+  mkCargoInvocation = isLastLayer: runClippy: commonArgs: subcommandArgs:
     let
       joinedCommonArgs = lib.concatStringsSep " " commonArgs;
       joinedSubcommandArgs = lib.concatStringsSep " " (
         subcommandArgs
-          ++ lib.optionals test [ "--no-run" ]
+          ++ lib.optionals (test && (!isLastLayer || justBuildTests)) [ "--no-run" ]
           ++ lib.optionals verifyWithVerus ([ "--" ] ++ extraVerusArgs)
       );
     in ''
@@ -211,7 +216,7 @@ let
         runClippyThisLayer = runClippy && layer.reals != {};
       in
         modifications.modifyDerivation (stdenv.mkDerivation (baseArgs // {
-          name = "${rootCrate.name}-intermediate";
+          name = "${name}-intermediate";
 
           phases = [ "buildPhase" ];
 
@@ -221,7 +226,7 @@ let
             cp -r --preserve=timestamps ${prev} $out
             chmod -R +w $out
 
-            ${mkCargoInvocation runClippyThisLayer (flags ++ [
+            ${mkCargoInvocation false runClippyThisLayer (flags ++ [
               "--config" "${config}"
               "--manifest-path" "${workspace}/Cargo.toml"
               "--target-dir" "$out"
@@ -256,7 +261,7 @@ in let
   ];
 
   final = modifications.modifyDerivation (stdenv.mkDerivation (baseArgs // {
-    name = rootCrate.name;
+    name = name;
 
     phases = [ "buildPhase" ];
 
@@ -267,7 +272,7 @@ in let
       cp -r --preserve=timestamps ${lastIntermediateLayer} $target_dir
       chmod -R +w $target_dir
 
-      ${mkCargoInvocation runClippy (flags ++ [
+      ${mkCargoInvocation true runClippy (flags ++ [
         "--config" "${config}"
         "--manifest-path" "${workspace}/Cargo.toml"
         "--target-dir" "$target_dir"
@@ -275,15 +280,21 @@ in let
         "--out-dir" "$out/bin"
       ])}
 
-      ${lib.optionalString test (lib.concatStringsSep " " (findTestsCommandPrefix "$target_dir" ++ [
-        "-exec" "install" "-D" "-t" "$out/bin" "'{}'" "';'"
-      ]))}
+      ${lib.optionalString test (
+        if justBuildTests
+        then (lib.concatStringsSep " " (findTestsCommandPrefix "$target_dir" ++ [
+          "-exec" "install" "-D" "-t" "$out/bin" "'{}'" "';'"
+        ]))
+        else ''
+          touch $out
+        ''
+      )}
 
       runHook postBuild
     '';
 
     passthru = {
-      inherit rootCrate workspace lastIntermediateLayer;
+      inherit rootCrate rootCrates workspace lastIntermediateLayer;
     };
   }));
 
diff --git a/hacking/nix/rust-utils/default.nix b/hacking/nix/rust-utils/default.nix
index 0c429d16c..e796e57eb 100644
--- a/hacking/nix/rust-utils/default.nix
+++ b/hacking/nix/rust-utils/default.nix
@@ -18,7 +18,7 @@ let
 in
 
 {
-  buildCrateInLayers = callPackage ./build-crate-in-layers.nix {};
+  buildCratesInLayers = callPackage ./build-crates-in-layers.nix {};
 
   buildSysroot = callPackage ./build-sysroot.nix {};
 
diff --git a/hacking/nix/scope/default.nix b/hacking/nix/scope/default.nix
index 8a220d388..8b9af45f5 100644
--- a/hacking/nix/scope/default.nix
+++ b/hacking/nix/scope/default.nix
@@ -127,7 +127,7 @@ superCallPackage ../rust-utils {} self //
 
   mkMkCustomTargetPathForEnvironment = { rustEnvironment }:
     let
-      tool = buildCrateInLayers rec {
+      tool = buildCratesInLayers rec {
         inherit rustEnvironment;
         rootCrate = crates.sel4-generate-target-specs;
         lastLayerModifications = crateUtils.elaborateModifications {
@@ -182,7 +182,7 @@ superCallPackage ../rust-utils {} self //
 
   ### local tools
 
-  mkTool = rootCrate: buildCrateInLayers {
+  mkTool = rootCrate: buildCratesInLayers {
     inherit rootCrate;
   };
 
@@ -197,6 +197,21 @@ superCallPackage ../rust-utils {} self //
   shellForMakefile = callPackage ./shell-for-makefile.nix {};
   shellForHacking = callPackage ./shell-for-hacking.nix {};
 
+  ### unit tests
+
+  someUnitTests = buildCratesInLayers {
+    name = "some-unit-tests";
+    test = true;
+    rootCrates = with crates; [
+      sel4-bitfield-ops
+      sel4-kernel-loader-embed-page-tables
+      sel4-backtrace-types
+    ];
+    features = [
+      "sel4-backtrace-types/full"
+    ];
+  };
+
   ### kernel
 
   mkSeL4 = callPackage ./sel4 {};
diff --git a/hacking/nix/scope/world/instances/default.nix b/hacking/nix/scope/world/instances/default.nix
index 531ecbd1d..fe6bf3d86 100644
--- a/hacking/nix/scope/world/instances/default.nix
+++ b/hacking/nix/scope/world/instances/default.nix
@@ -220,6 +220,7 @@ in rec {
         rootTask = mkTask {
           rootCrate = crates.tests-root-task-default-test-harness;
           test = true;
+          justBuildTests = true;
         };
         extraPlatformArgs = lib.optionalAttrs canSimulate {
           canAutomateSimply = true;
@@ -231,6 +232,7 @@ in rec {
           rootTask = lib.makeOverridable mkTask {
             rootCrate = crates.ring;
             test = true;
+            justBuildTests = true;
             features = [
               "less-safe-getrandom-custom-or-rdrand"
               # "slow_tests"
diff --git a/hacking/nix/scope/world/mk-task.nix b/hacking/nix/scope/world/mk-task.nix
index 680037e59..cc9a8e255 100644
--- a/hacking/nix/scope/world/mk-task.nix
+++ b/hacking/nix/scope/world/mk-task.nix
@@ -6,7 +6,7 @@
 
 { lib, buildPackages
 , runCommand, runCommandCC
-, buildCrateInLayers, buildSysroot, crateUtils
+, buildCratesInLayers, buildSysroot, crateUtils
 , crates
 , defaultRustEnvironment
 , defaultRustTargetTriple
@@ -107,7 +107,7 @@ let
 
 in
 
-buildCrateInLayers (prunedArgs // {
+buildCratesInLayers (prunedArgs // {
   commonModifications = crateUtils.composeModifications
     (crateUtils.elaborateModifications commonModifications)
     theseCommonModifications
diff --git a/hacking/nix/scope/world/sel4-kernel-loader.nix b/hacking/nix/scope/world/sel4-kernel-loader.nix
index 1107af5bc..b0bab1dd8 100644
--- a/hacking/nix/scope/world/sel4-kernel-loader.nix
+++ b/hacking/nix/scope/world/sel4-kernel-loader.nix
@@ -5,7 +5,7 @@
 #
 
 { lib, buildPackages, writeText
-, buildCrateInLayers, buildSysroot, crateUtils
+, buildCratesInLayers, buildSysroot, crateUtils
 , crates, bareMetalRustTargetTriple
 , libclangPath
 , seL4RustEnvVars, seL4ForBoot, seL4ForUserspace
@@ -44,7 +44,7 @@ let
   };
 
 in
-buildCrateInLayers {
+buildCratesInLayers {
 
   inherit rootCrate;
   inherit targetTriple;
diff --git a/hacking/nix/top-level/default.nix b/hacking/nix/top-level/default.nix
index b344e3f17..f09e2e99f 100644
--- a/hacking/nix/top-level/default.nix
+++ b/hacking/nix/top-level/default.nix
@@ -56,6 +56,8 @@ in {
       map (instance: instance.links) world.instances.all
     ))
 
+    pkgs.build.this.someUnitTests
+
     someConfigurationBuildTests
 
     sel4testInstancesList