I should be working by now.

flake.nix

@@ -25,13 +25,21 @@
               # something that work.. ish
               rustc
               cargo
-              rustPlatform.rustcSrc gcc
+              rustfmt
+              rustPlatform.rustcSrc
+              gcc
+              cargo-tarpaulin
               ];
           };
 
 
         devShell = with pkgs; mkShell {
-          buildInputs = [ cargo rustc rustfmt pre-commit rustPackages.clippy ];
+          buildInputs = [cargo
+                         rustc
+                         rustfmt
+                         pre-commit
+                         rustPackages.clippy
+                         cargo-tarpaulin];
           RUST_SRC_PATH = rustPlatform.rustLibSrc;
         };
       });