Fix Nix build & direnv config (#70)

- **flake: Update inputs**
- **fix(direnv): Specify shell w/shebang + use direnv stdlib + use
correct syntax**
- **flake: Simplify input dependencies**
- **refactor(flake): Add formatter for nix + tidy up flake**
- **fix(flake): Nix builds release + always specifies target triple**

## Description of changes
- Updated inputs used in nix flake (tool that defines the nix package of
nufmt)
- Added formatter for nix flake/nix code
- Simplified nix flake's inputs/dependencies (synchronized and
deduplicated)
- Tidied up/fixed .envrc file/direnv
- Fixed build broken by new tests added in #69

## Relevant Issues
None

Author: x10an14

.envrc

@@ -1,11 +1,11 @@
-# If nix is installed hook into it.
-if [ $(which nix) ]
-then
+#!/usr/bin/env bash
+
+if has nix; then
+    # If nix is installed hook into it.
     use flake
 fi
 
 # If nu is the current shell use toolkit.nu
-if [ $(echo $SHELL) == $(which nu) ]
-then    
+if [ $(echo $SHELL) == $(which nu) ]; then
     nu -e "use toolkit.nu"
 fi

flake.lock

@@ -5,37 +5,72 @@
     nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
     flake-utils.url = "github:numtide/flake-utils";
 
-    rust-overlay.url = "github:oxalica/rust-overlay";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
-  outputs = { self, nixpkgs, rust-overlay, flake-utils, ... }:
-    flake-utils.lib.eachDefaultSystem (system:
+  outputs =
+    { self, ... }@inputs:
+    inputs.flake-utils.lib.eachDefaultSystem (
+      system:
       let
-        overlays = [ (import rust-overlay) ];
-        pkgs = import nixpkgs {
+        overlays = [ (import inputs.rust-overlay) ];
+        pkgs = import inputs.nixpkgs {
           inherit system overlays;
         };
+        integrationTestPatch = pkgs.writeTextFile {
+          name = "integration-tests-nix-fix.patch";
+          text =
+            # patch
+            ''
+              diff --git a/tests/main.rs b/tests/main.rs
+              index c3a2b64..173aea0 100644
+              --- a/tests/main.rs
+              +++ b/tests/main.rs
+              @@ -8,7 +8,7 @@ let one = 1
+               const VALID: &str = "# beginning of script comment
+               let one = 1
+               ";
+              -const TEST_BINARY: &'static str = "target/debug/nufmt";
+              +const TEST_BINARY: &'static str = "target/@target_triple@/release/nufmt";
+
+               #[test]
+               fn failure_with_invalid_config() {
+            '';
+        };
       in
       {
         devShells.default = pkgs.mkShell {
-          nativeBuildInputs = with pkgs; [
-            rust-bin.stable.latest.default
-            rust-analyzer
-
+          inputsFrom = [ self.packages.${system}.default ];
+          packages = with pkgs; [
             nushell
-          ];
 
-          buildInputs = with pkgs; [ ];
+            # Not included in the package dependencies, but used for development
+            rust-analyzer
+          ];
         };
 
-        packages.default = pkgs.rustPlatform.buildRustPackage rec {
+        packages.default = self.packages.${system}.nufmt;
+        packages.nufmt = pkgs.rustPlatform.buildRustPackage {
           name = "nufmt";
-
           src = ./.;
+          patches = [
+            (pkgs.replaceVars "${integrationTestPatch}" {
+              target_triple = pkgs.stdenv.hostPlatform.rust.rustcTarget;
+            })
+          ];
+          cargoLock.lockFile = ./Cargo.lock;
+        };
 
-          cargoLock = {
-            lockFile = ./Cargo.lock;
-          };
+        formatter = inputs.treefmt-nix.lib.mkWrapper pkgs {
+          programs.nixfmt.enable = true;
         };
       }
     );

flake.nix

@@ -8,14 +8,15 @@ let one = 1
 const VALID: &str = "# beginning of script comment
 let one = 1
 ";
+const TEST_BINARY: &str = "target/debug/nufmt";
 
 #[test]
 fn failure_with_invalid_config() {
     let dir = tempdir().unwrap();
     let config_file = dir.path().join("nufmt.nuon");
     fs::write(&config_file, r#"{unknown: 1}"#).unwrap();
 
-    let output = Command::new("target/debug/nufmt")
+    let output = Command::new(TEST_BINARY)
         .arg("--config")
         .arg(config_file.to_str().unwrap())
         .arg(dir.path().to_str().unwrap())
@@ -29,7 +30,7 @@ fn failure_with_invalid_config() {
 
 #[test]
 fn failure_with_invalid_config_file() {
-    let output = Command::new("target/debug/nufmt")
+    let output = Command::new(TEST_BINARY)
         .arg("--config")
         .arg("path/that/does/not/exist/nufmt.nuon")
         .output()
@@ -42,7 +43,7 @@ fn failure_with_invalid_config_file() {
 
 #[test]
 fn failure_with_invalid_file_to_format() {
-    let output = Command::new("target/debug/nufmt")
+    let output = Command::new(TEST_BINARY)
         .arg("path/that/does/not/exist/a.nu")
         .output()
         .unwrap();
@@ -56,7 +57,7 @@ fn failure_with_invalid_file_to_format() {
 fn warning_when_no_files_are_detected() {
     let dir = tempdir().unwrap();
 
-    let output = Command::new("target/debug/nufmt")
+    let output = Command::new(TEST_BINARY)
         .arg("--dry-run")
         .arg(dir.path().to_str().unwrap())
         .output()
@@ -75,7 +76,7 @@ fn warning_is_displayed_when_no_files_are_detected_with_excluded_files() {
     fs::write(&config_file, r#"{exclude: ["a*"]}"#).unwrap();
     fs::write(&file_a, INVALID).unwrap();
 
-    let output = Command::new("target/debug/nufmt")
+    let output = Command::new(TEST_BINARY)
         .arg("--config")
         .arg(config_file.to_str().unwrap())
         .arg("--dry-run")
@@ -98,7 +99,7 @@ fn files_are_reformatted() {
     fs::write(&file_a, INVALID).unwrap();
     fs::write(&file_b, INVALID).unwrap();
 
-    let output = Command::new("target/debug/nufmt")
+    let output = Command::new(TEST_BINARY)
         .arg("--config")
         .arg(config_file.to_str().unwrap())
         .arg(dir.path().to_str().unwrap())
@@ -122,7 +123,7 @@ fn files_are_checked() {
     fs::write(&file_a, INVALID).unwrap();
     fs::write(&file_b, INVALID).unwrap();
 
-    let output = Command::new("target/debug/nufmt")
+    let output = Command::new(TEST_BINARY)
         .arg("--config")
         .arg(config_file.to_str().unwrap())
         .arg("--dry-run")