add nix flake

.envrc

+use flake

.gitignore

 # rust
 target/
+
+# direnv
+.direnv/
\ No newline at end of file

flake.lock

+{
+  "nodes": {
+    "naersk": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1698420672,
+        "narHash": "sha256-/TdeHMPRjjdJub7p7+w55vyABrsJlt5QkznPYy55vKA=",
+        "owner": "nix-community",
+        "repo": "naersk",
+        "rev": "aeb58d5e8faead8980a807c840232697982d47b9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "ref": "master",
+        "repo": "naersk",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1701268161,
+        "narHash": "sha256-hL4jGGwMHHmyx6G9wi6IrYa8RLkoEtzCb4zWITH1B40=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "67be70a859530f6f7c358568eaa6ab0d84b36b01",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-23.05",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "naersk": "naersk",
+        "nixpkgs": "nixpkgs",
+        "utils": "utils"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    },
+    "utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1694529238,
+        "narHash": "sha256-zsNZZGTGnMOf9YpHKJqMSsa0dXbfmxeoJ7xHlrt+xmY=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "ff7b65b44d01cf9ba6a71320833626af21126384",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

+{
+  description = "AoC 2023";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.05";
+    naersk = {
+      url = "github:nix-community/naersk/master";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, utils, naersk }:
+    utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+        buildInputs = with pkgs; [ perl ];
+
+        naerskBuildPackage = (conf: (
+          (pkgs.callPackage naersk { }).buildPackage (
+            {
+              src = ./.;
+              inherit buildInputs;
+            } // conf
+          )
+        ));
+      in
+      {
+        packages."01" = naerskBuildPackage { src = ./01/.; };
+
+        devShell = with pkgs; mkShell {
+          buildInputs = buildInputs ++ [ cargo rustc rustfmt rustPackages.clippy ];
+          RUST_SRC_PATH = rustPlatform.rustLibSrc;
+        };
+      });
+}