Add Nix flake

This commit adds a Nix flake to just, to make it possible to build just
locally using the Nix flake infrastructure and distribute just as a
flake input.

Author: neunenak

.gitignore

@@ -15,3 +15,4 @@
 /test-utilities/Cargo.lock
 /test-utilities/target
 /tmp
+result

README.md

@@ -346,6 +346,27 @@ most Windows users.)
   </tbody>
 </table>
 
+
+#### Nix flake
+
+The `just` repository has a `flake.nix` file defining the repository as a [nix flake](https://nix.dev/concepts/flakes.html)
+
+You can specify the `just` repository as an input to another nix flake:
+
+```nix
+{
+  inputs = {
+    ...
+    just.url = "github:casey/just";
+  }
+
+  outputs = {self, nixpkgs, just}: {
+    ...
+  }
+}
+
+```
+
 ![just package version table](https://repology.org/badge/vertical-allrepos/just.svg)
 
 ### Pre-Built Binaries

flake.lock

@@ -0,0 +1,135 @@
+{
+  description = "Just a command runner";
+
+  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 = import nixpkgs {
+          inherit system;
+        };
+
+        # Read version from Cargo.toml
+        cargoToml = builtins.fromTOML (builtins.readFile ./Cargo.toml);
+        version = cargoToml.package.version;
+
+        # Common build inputs
+        buildInputs = with pkgs;
+          lib.optionals stdenv.hostPlatform.isDarwin [
+            libiconv
+            darwin.apple_sdk.frameworks.Security
+          ];
+
+        nativeBuildInputs = with pkgs; [
+          installShellFiles
+          pkg-config
+        ];
+
+        # The main just package
+        just = pkgs.rustPlatform.buildRustPackage {
+          pname = "just";
+          inherit version;
+
+          src = ./.;
+
+          cargoLock = {
+            lockFile = ./Cargo.lock;
+          };
+
+          inherit nativeBuildInputs buildInputs;
+
+          # Don't check during build since we run tests separately
+          doCheck = false;
+
+          # Generate shell completions and man pages
+          postInstall = ''
+            # Generate and install shell completions
+            for shell in bash fish zsh; do
+              $out/bin/just --completions $shell > just.$shell
+              installShellCompletion just.$shell
+            done
+
+            # Generate and install man page
+            $out/bin/just --man > just.1
+            installManPage just.1
+          '';
+
+          # Setup hook for runtime dependencies
+          setupHook = pkgs.writeText "setup-hook.sh" ''
+            export JUST_PATH_PREFIX="${pkgs.lib.makeBinPath [pkgs.coreutils pkgs.bashInteractive]}''${JUST_PATH_PREFIX:+:$JUST_PATH_PREFIX}"
+          '';
+
+          meta = with pkgs.lib; {
+            description = cargoToml.package.description;
+            homepage = cargoToml.package.homepage;
+            changelog = "${cargoToml.package.repository}/blob/master/CHANGELOG.md";
+            license = licenses.cc0;
+            mainProgram = "just";
+          };
+        };
+
+        # Development shell with additional tools
+        devShell = pkgs.mkShell {
+          inputsFrom = [just];
+
+          packages = with pkgs; [
+            # Rust toolchain
+            rustc
+            cargo
+            clippy
+            rustfmt
+            rust-analyzer
+
+            # Development tools
+            cargo-watch
+            cargo-fuzz
+            cargo-outdated
+            cargo-udeps
+            mdbook
+            mdbook-linkcheck
+            shellcheck
+
+            # Runtime dependencies
+            bashInteractive
+            coreutils
+
+            # Additional utilities from justfile
+            python3
+            nodejs
+            perl
+            ruby
+          ];
+
+          shellHook = ''
+            echo "Just development environment"
+            echo "Version: ${version}"
+          '';
+        };
+      in {
+        packages = {
+          default = just;
+          just = just;
+        };
+
+        apps = {
+          default = flake-utils.lib.mkApp {
+            drv = just;
+            exePath = "/bin/just";
+          };
+        };
+
+        devShells.default = devShell;
+
+        # Formatter for `nix fmt`
+        formatter = pkgs.nixpkgs-fmt;
+      }
+    );
+}