Add rust-toolchain and nix flake (#20)

rust-toolchain will allow us to ensure that everyone developing on the
repository is using the same version of rust.

Nix flake will provide out-of-the-box development environment for nix
users with the right rust version.

.gitignore

@@ -4,3 +4,10 @@
 
 .vscode/*
 .DS_Store
+
+# Allow everyone to customize .envrc
+/.envrc
+# Output / cache from direnv
+/.direnv
+# Output from nix
+/result

flake.nix

@@ -0,0 +1,60 @@
+{
+  # This is a Nix Flake that defines development environment for this project.
+  #
+  # Using Nix is NOT required to develop this project.
+  #
+  # Nix provides a declariative way to define reproducible development environment
+  # and aims to remove the issue of "works on my machine".
+  # 
+  # If you use Nix, you can install and activate all dependencies required to
+  # develop on this project with:
+  #
+  # $ nix develop
+  #
+  # If in addition you have `direnv` installed, you can create `.envrc` file with
+  # `use flake` content, so that development environment activates automatically
+  # when the project root directory is entered. 
+  # 
+  # More information:
+  # - Nix: https://nixos.org
+  # - Nix flakes:  https://wiki.nixos.org/wiki/Flakes
+  # - Direnv: https://direnv.net
+  description = "The New Nushell Parser";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    {
+      nixpkgs,
+      rust-overlay,
+      flake-utils,
+      ...
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        overlays = [ (import rust-overlay) ];
+        pkgs = import nixpkgs { inherit system overlays; };
+        rustToolchain = pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          nativeBuildInputs = [
+            (rustToolchain.override {
+              extensions = [
+                "rust-src"
+                "rust-analyzer"
+              ];
+            })
+          ];
+        };
+      }
+    );
+}

rust-toolchain.toml

@@ -3,4 +3,5 @@
 # The default profile includes rustc, rust-std, cargo, rust-docs, rustfmt and clippy.
 # https://rust-lang.github.io/rustup/concepts/profiles.html
 profile = "default"
+# When updating, it might be necessary to run `nix flake update`.
 channel = "1.81.0"