Enhance flake with more robust setup

flake.nix

@@ -67,39 +67,43 @@
             ];
         };
 
-        my-crate = craneLib.buildPackage (
+        # Build *just* the cargo dependencies, so we can reuse
+        # all of that work (e.g. via cachix) when running in CI
+        cargoArtifacts = craneLib.buildDepsOnly (commonArgs);
+
+        # Run clippy (and deny all warnings) on the crate source,
+        # reusing the dependency artifacts (e.g. from build scripts or
+        # proc-macros) from above.
+        #
+        # Note that this is done as a separate derivation so it
+        # does not impact building just the crate by itself.
+        myCrateClippy = craneLib.cargoClippy (
           commonArgs
           // {
-            cargoArtifacts = craneLib.buildDepsOnly commonArgs;
-
-            # Additional environment variables or build phases/hooks can be set
-            # here *without* rebuilding all dependency crates
-            # MY_CUSTOM_VAR = "some value";
+            # Again we apply some extra arguments only to this derivation
+            # and not every where else. In this case we add some clippy flags
+            inherit cargoArtifacts;
+            cargoClippyExtraArgs = "--all-targets -- --deny warnings";
           }
         );
+
+        # Build the actual crate itself, reusing the dependency
+        # artifacts from above.
+        myCrate = craneLib.buildPackage (commonArgs // { inherit cargoArtifacts; });
+
+        # Also run the crate tests under cargo-tarpaulin so that we can keep
+        # track of code coverage
+        myCrateCoverage = craneLib.cargoTarpaulin (commonArgs // { inherit cargoArtifacts; });
       in
       {
+        packages.default = myCrate;
         checks = {
-          inherit my-crate;
-        };
-
-        packages.default = my-crate;
-
-        apps.default = flake-utils.lib.mkApp { drv = my-crate; };
-
-        devShells.default = craneLib.devShell {
-          # Inherit inputs from checks.
-          checks = self.checks.${system};
-
-          # Additional dev-shell environment variables can be set directly
-          # MY_CUSTOM_DEVELOPMENT_VAR = "something else";
-
-          # Extra inputs can be added here; cargo and rustc are provided by default.
-          packages = with pkgs; [
-            # rustPackages.clippy
-            # rustfmt
-            # pre-commit
-          ];
+          inherit
+            # Build the crate as part of `nix flake check` for convenience
+            myCrate
+            myCrateClippy
+            myCrateCoverage
+            ;
         };
       }
     );