Install rust-src in devshell to fix intellisense issues

flake.nix

@@ -27,7 +27,12 @@
       };
     in {
       devShells.${system}.default = with pkgs; mkShell {
-        packages = [ bacon ];
+        packages = [
+          (pkgs.rust-bin.stable.latest.default.override {
+            extensions = [ "rust-src" ];
+          })
+          bacon
+        ];
         inputsFrom = with self.packages.${system}; [ tatoeba images ];
       };
       packages.${system} = {