Add Nix flake for development environment

Author: erning

flake.lock

@@ -0,0 +1,64 @@
+{
+  description = "Advent of Code 2020 in Rust";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            cargo
+            rustc
+            clippy
+            rustfmt
+            rust-analyzer
+
+            # Additional development tools
+            cargo-watch
+            cargo-edit
+            cargo-audit
+
+            # Git and other useful tools
+            git
+            ripgrep
+            fd
+
+            # Optional: Editor support
+            nil # Nix language server
+          ];
+
+          shellHook = ''
+            echo "🎄 Advent of Code 2020 - Rust Development Environment"
+            echo ""
+            echo "Available commands:"
+            echo "  cargo build --release          # Build optimized binary"
+            echo "  cargo run --release --         # Run all days"
+            echo "  cargo run --release -- 5       # Run day 5"
+            echo "  cargo test                     # Run all tests"
+            echo "  cargo test day05               # Test specific day"
+            echo "  cargo clippy                   # Lint code"
+            echo "  cargo fmt                      # Format code"
+            echo "  cargo watch -x check           # Watch for changes"
+            echo ""
+            echo "Happy coding! 🦀"
+          '';
+        };
+
+        # For compatibility with older nix versions
+        devShell = self.devShells.${system}.default;
+      }
+    );
+}