nix: add flake

.gitea/template

@@ -1,2 +1,3 @@
 README.md
 Cargo.toml
+flake.nix

flake.nix

@@ -0,0 +1,34 @@
+{
+  description = "A flake for building ${REPO_NAME}.";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
+    flake-utils.url = "github:numtide/flake-utils";
+
+    treefmt-nix.url = "github:numtide/treefmt-nix";
+    treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, treefmt-nix, ... }@inputs:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.$${system};
+
+        cargoToml = builtins.fromTOML (builtins.readFile ./Cargo.toml);
+
+        treefmtEval = treefmt-nix.lib.evalModule pkgs (pkgs: {
+          projectRootFile = "flake.nix";
+          programs.rustfmt.enable = true;
+          programs.nixfmt.enable = true;
+        });
+      in {
+        packages.default = pkgs.rustPlatform.buildRustPackage {
+          inherit (cargoToml.package) name version;
+          src = ./.;
+          cargoLock.lockFile = ./Cargo.lock;
+        };
+
+        formatter = treefmtEval.config.build.wrapper;
+      });
+}