Add test to validate the config loading rules

Configs are loaded in the following order:
1. Load configs from the `--config-path` option
2. Travers the directory hierarchy looking for a config file
3. Check the user's `HOME` directory for a config file
4. Check the user's config directory for a config file

tests/rustfmt/main.rs

@@ -174,3 +174,122 @@ fn rustfmt_emits_error_on_line_overflow_true() {
         "line formatted, but exceeded maximum width (maximum: 100 (see `max_width` option)"
     ))
 }
+
+/// helper function to write a new rustfmt.toml to a file
+fn write_rustfmt_toml<P: AsRef<Path>>(path: P) {
+    let dir_path = if path.as_ref().is_dir() {
+        path.as_ref()
+    } else {
+        path.as_ref().parent().unwrap()
+    };
+
+    rustfmt(&[
+        "--print-config=default",
+        &dir_path.join("rustfmt.toml").display().to_string(),
+    ]);
+}
+
+/// helper function to turn an AsRef<Path> into a String
+fn path_arg<P: AsRef<Path>>(path: P) -> String {
+    path.as_ref().display().to_string()
+}
+
+macro_rules! assert_using_expected_config {
+    ($file_path:expr) => {
+        // case where we don't specify a config
+        // No configs should exist in the file system when calling this case
+        let (stdout, _stderr) = rustfmt(&["-v", &path_arg($file_path)]);
+        assert!(!stdout.contains("rustfmt.toml"));
+    };
+
+    ($file_path:expr, $config_dir:expr) => {
+        // case where we expect a config to be implicitly loaded
+        let (stdout, _stderr) = rustfmt(&["-v", &path_arg($file_path)]);
+        assert!(stdout.contains(&path_arg($config_dir)));
+    };
+
+    ($file_path:expr, "--config-path", $config_path:expr) => {
+        // case where we explictly set a config and ensure it gets loaded
+        let (stdout, _stderr) = rustfmt(&[
+            "-v",
+            "--config-path",
+            &path_arg($config_path),
+            &path_arg($file_path),
+        ]);
+        assert!(stdout.contains(&path_arg($config_path)));
+    };
+}
+
+/// Ensure that we're loading the correct config when running rustfmt
+///
+/// Configs are loaded in the following order:
+/// 1. Load configs from the `--config-path` option.
+/// 2. Travers the directory hierarchy looking for a config file
+/// 3. Check the user's `HOME` directory for a config file (could vary by platform)
+/// 4. Check the user's config directory for a config file (could vary by platform)
+///
+/// When no configs are found rustfmt is run with just the default options.
+#[cfg(not(windows))]
+#[test]
+fn test_load_config_logic() {
+    // Create a temporary directory and set it as the new $HOME.
+    // This sets up a clean environment that we'll use to test the config loading logic
+    let tmpdir = tempfile::tempdir().unwrap();
+    let _home_env = tmp_env::set_var("HOME", tmpdir.as_ref());
+
+    // Sanity check to make sure that we set the $HOME directory
+    let home_dir = dirs::home_dir().unwrap();
+    assert_eq!(tmpdir.as_ref(), home_dir.as_path());
+
+    // Create a working directory nested a few levels deep inside the temporary $HOME.
+    // We want a few directory levels so we can properly test case #2 listed above.
+    // Set the current working directory to the new path so we don't pick up any rustfmt.toml
+    // files defined outside our clean environment.
+    let work_dir = home_dir.join("path").join("to").join("file");
+    std::fs::create_dir_all(&work_dir).unwrap();
+    let _current_dir = tmp_env::set_current_dir(&work_dir).unwrap();
+
+    // Set up the user's config directory
+    let mut config_dir = dirs::config_dir().unwrap();
+    config_dir.push("rustfmt");
+    std::fs::create_dir_all(&config_dir).unwrap();
+
+    // Write a hello world file used for formatting checks in the working directory
+    let file_path = work_dir.join("test.rs");
+    std::fs::write(
+        &file_path,
+        "fn main() {\n    println!(\"Hello world!\");\n}",
+    )
+    .unwrap();
+
+    // 1. Run rustfmt and make sure we don't load any configs
+    assert_using_expected_config!(&file_path);
+
+    // Write a rustfmt.toml to the users config directory
+    // 2. Run rustfmt and make sure we load the config from the user's config dir.
+    //    Sinces no other configs exist this one should be used.
+    write_rustfmt_toml(&config_dir);
+    assert_using_expected_config!(&file_path, &config_dir);
+
+    // Write a rustfmt.toml to the users $HOME directory
+    // 3. Run rustmft and make sure we load the config from the user's $HOME dir
+    //    Configs in the $HOME dir take precedence over those in the config dir
+    write_rustfmt_toml(&home_dir);
+    assert_using_expected_config!(&file_path, &home_dir);
+
+    // write a rustfmt.toml to some directory in the `work_dir` hierarchy.
+    // 4. Run rustfmt and make sure we load the config from the file hierarcy.
+    //    Configs found in the file hierarcy take precedence to those in $HOME and the config dir.
+    let config_along_path = work_dir.parent().unwrap().parent().unwrap();
+    write_rustfmt_toml(&config_along_path);
+    assert_using_expected_config!(&file_path, &config_along_path);
+
+    // write a rustfmt.toml outside the working directory hierarchy.
+    // This ensures it isn't automatically picked up.
+    // 5. run rustfmt and explicitly set the `--config-path` option to this config file.
+    //    Configs that are explicity set take precedence over all other configs.
+    let explicit_config_path = home_dir.join("new").join("config").join("path");
+    std::fs::create_dir_all(&explicit_config_path).unwrap();
+    write_rustfmt_toml(&explicit_config_path);
+    assert_using_expected_config!(&file_path, "--config-path", &explicit_config_path);
+}