nix: add flake with dev shell

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -1,4 +1,5 @@
 .DS_Store
+.direnv/
 
 /target
 

flake.lock

@@ -0,0 +1,43 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1662096612,
+        "narHash": "sha256-R+Q8l5JuyJryRPdiIaYpO5O3A55rT+/pItBrKcy7LM4=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "21de2b973f9fee595a7a1ac4693efff791245c34",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs",
+        "utils": "utils"
+      }
+    },
+    "utils": {
+      "locked": {
+        "lastModified": 1659877975,
+        "narHash": "sha256-zllb8aq3YO3h8B/U0/J1WBgAL8EX5yWf5pMj3G0NAmc=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "c0e246b9b83f637f4681389ecabcb2681b4f3af0",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,18 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, utils }:
+    utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+      in
+      {
+        devShell = pkgs.mkShell {
+          packages = with pkgs; [ cargo rustc rustfmt clippy rust-analyzer ];
+          RUST_SRC_PATH = pkgs.rustPlatform.rustLibSrc;
+        };
+      });
+}