fix: devshell cargo version

fixes: #4

flake.lock

@@ -1,5 +1,24 @@
 {
   "nodes": {
+    "fenix": {
+      "inputs": {
+        "nixpkgs": "nixpkgs",
+        "rust-analyzer-src": "rust-analyzer-src"
+      },
+      "locked": {
+        "lastModified": 1643437378,
+        "narHash": "sha256-nciShE41PZ5wU54Sm7xSLsZxzgcSlab2UNrf5GGjndc=",
+        "owner": "nix-community",
+        "repo": "fenix",
+        "rev": "af56bbdd36b7644ea466cef5a4b1163d923296a7",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "fenix",
+        "type": "github"
+      }
+    },
     "flakeCompat": {
       "flake": false,
       "locked": {
@@ -33,25 +52,46 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1643119265,
+        "lastModified": 1643347846,
-        "narHash": "sha256-mmDEctIkHSWcC/HRpeaw6QOe+DbNOSzc0wsXAHOZWwo=",
+        "narHash": "sha256-O0tyXF//ppRpe9yT1Uu5n34yI2MWDyY6ZiJ4Qn5zIkE=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "b05d2077ebe219f6a47825767f8bab5c6211d200",
+        "rev": "5bb20f9dc70e9ee16e21cc404b6508654931ce41",
         "type": "github"
       },
       "original": {
         "owner": "nixos",
-        "ref": "nixpkgs-unstable",
+        "ref": "nixos-unstable",
         "repo": "nixpkgs",
         "type": "github"
       }
     },
     "root": {
       "inputs": {
+        "fenix": "fenix",
         "flakeCompat": "flakeCompat",
         "flakeUtils": "flakeUtils",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": [
+          "fenix",
+          "nixpkgs"
+        ]
+      }
+    },
+    "rust-analyzer-src": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1643383912,
+        "narHash": "sha256-QtpTMM54jozLsjpOt3tMvnAoL+nc5bezbwxilHooOzM=",
+        "owner": "rust-analyzer",
+        "repo": "rust-analyzer",
+        "rev": "6634eaf13a7e3a2b30f98f6e69af952f2d760df4",
+        "type": "github"
+      },
+      "original": {
+        "owner": "rust-analyzer",
+        "ref": "nightly",
+        "repo": "rust-analyzer",
+        "type": "github"
       }
     }
   },

flake.nix

@@ -3,7 +3,9 @@
     flakeCompat.url = github:edolstra/flake-compat;
     flakeCompat.flake = false;
     flakeUtils.url = "github:numtide/flake-utils";
-    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    nixpkgs.follows = "fenix/nixpkgs";
+    # add https://app.cachix.org/cache/nix-community to your `nix.conf`
+    fenix.url = "github:nix-community/fenix";
   };
   outputs =
     inputs:
@@ -11,6 +13,7 @@
       (
         system:
         let
+          toolchain = "stable";
           nixpkgs = import inputs.nixpkgs { inherit system; };
           cargoToml = builtins.fromTOML ( builtins.readFile ./Cargo.toml );
         in
@@ -21,7 +24,9 @@
           };
           defaultApp = { type = "app"; program = "${ inputs.self.defaultPackage.${ system } }/bin/alejandra"; };
           defaultPackage =
-            nixpkgs.rustPlatform.buildRustPackage
+            (nixpkgs.makeRustPlatform {
+              inherit (inputs.fenix.packages.${ system }.${ toolchain }) cargo rustc;
+            }).buildRustPackage
               {
                 pname = cargoToml.package.name;
                 version =
@@ -43,11 +48,16 @@
           devShell =
             nixpkgs.mkShell
               {
-                packages = [ nixpkgs.cargo-tarpaulin nixpkgs.rustup ];
+                packages = [
-                shellHook =
+                  inputs.fenix.packages.${ system }.rust-analyzer
-                  ''
+                  (inputs.fenix.packages.${ system }.${ toolchain }.withComponents [
-                  rustup toolchain install nightly
+                    "cargo"
-                  '';
+                    "clippy"
+                    "rust-src"
+                    "rustc"
+                    "rustfmt"
+                  ])
+                ];
               };
           packages = {
             nixpkgsFormatted =