Implement `--isolated` CLI flag (#1727)

Closes #1724.

.github/ISSUE_TEMPLATE.md

@@ -4,7 +4,7 @@ Thank you for taking the time to report an issue! We're glad to have you involve
 If you're filing a bug report, please consider including the following information:
 
 - A minimal code snippet that reproduces the bug.
-- The command you invoked (e.g., `ruff /path/to/file.py --fix`).
+- The command you invoked (e.g., `ruff /path/to/file.py --fix`), ideally including the `--isolated` flag.
 - The current Ruff settings (any relevant sections from your `pyproject.toml`).
 - The current Ruff version (`ruff --version`).
 -->

README.md

@@ -341,6 +341,8 @@ Options:
           Avoid writing any fixed files back; instead, output a diff for each changed file to stdout
   -n, --no-cache
           Disable cache reads
+      --isolated
+          Ignore all configuration files
       --select <SELECT>
           Comma-separated list of error codes to enable (or ALL, to enable all checks)
       --extend-select <EXTEND_SELECT>

src/cli.rs

@@ -20,7 +20,7 @@ pub struct Cli {
     pub files: Vec<PathBuf>,
     /// Path to the `pyproject.toml` or `ruff.toml` file to use for
     /// configuration.
-    #[arg(long)]
+    #[arg(long, conflicts_with = "isolated")]
     pub config: Option<PathBuf>,
     /// Enable verbose logging.
     #[arg(short, long, group = "verbosity")]
@@ -56,6 +56,9 @@ pub struct Cli {
     /// Disable cache reads.
     #[arg(short, long)]
     pub no_cache: bool,
+    /// Ignore all configuration files.
+    #[arg(long, conflicts_with = "config")]
+    pub isolated: bool,
     /// Comma-separated list of error codes to enable (or ALL, to enable all
     /// checks).
     #[arg(long, value_delimiter = ',')]
@@ -240,6 +243,7 @@ impl Cli {
                 explain: self.explain,
                 files: self.files,
                 generate_shell_completion: self.generate_shell_completion,
+                isolated: self.isolated,
                 no_cache: self.no_cache,
                 quiet: self.quiet,
                 show_files: self.show_files,
@@ -301,6 +305,7 @@ pub struct Arguments {
     pub explain: Option<CheckCode>,
     pub files: Vec<PathBuf>,
     pub generate_shell_completion: Option<clap_complete_command::Shell>,
+    pub isolated: bool,
     pub no_cache: bool,
     pub quiet: bool,
     pub show_files: bool,

src/main_native.rs

@@ -5,7 +5,6 @@ use std::sync::mpsc::channel;
 
 use ::ruff::autofix::fixer;
 use ::ruff::cli::{extract_log_level, Cli, Overrides};
-use ::ruff::commands;
 use ::ruff::logging::{set_up_logging, LogLevel};
 use ::ruff::printer::{Printer, Violations};
 use ::ruff::resolver::{resolve_settings, FileDiscovery, PyprojectDiscovery, Relativity};
@@ -14,22 +13,29 @@ use ::ruff::settings::types::SerializationFormat;
 use ::ruff::settings::{pyproject, Settings};
 #[cfg(feature = "update-informer")]
 use ::ruff::updates;
+use ::ruff::{commands, one_time_warning};
 use anyhow::Result;
 use clap::{CommandFactory, Parser};
 use colored::Colorize;
 use notify::{recommended_watcher, RecursiveMode, Watcher};
 use path_absolutize::path_dedot;
-use ruff::one_time_warning;
 
 /// Resolve the relevant settings strategy and defaults for the current
 /// invocation.
 fn resolve(
+    isolated: bool,
     config: Option<&Path>,
     overrides: &Overrides,
     stdin_filename: Option<&Path>,
 ) -> Result<PyprojectDiscovery> {
-    if let Some(pyproject) = config {
-        // First priority: the user specified a `pyproject.toml` file. Use that
+    if isolated {
+        // First priority: if we're running in isolated mode, use the default settings.
+        let mut config = Configuration::default();
+        config.apply(overrides.clone());
+        let settings = Settings::from_configuration(config, &path_dedot::CWD)?;
+        Ok(PyprojectDiscovery::Fixed(settings))
+    } else if let Some(pyproject) = config {
+        // Second priority: the user specified a `pyproject.toml` file. Use that
         // `pyproject.toml` for _all_ configuration, and resolve paths relative to the
         // current working directory. (This matches ESLint's behavior.)
         let settings = resolve_settings(pyproject, &Relativity::Cwd, Some(overrides))?;
@@ -39,7 +45,7 @@ fn resolve(
             .as_ref()
             .unwrap_or(&path_dedot::CWD.as_path()),
     )? {
-        // Second priority: find a `pyproject.toml` file in either an ancestor of
+        // Third priority: find a `pyproject.toml` file in either an ancestor of
         // `stdin_filename` (if set) or the current working path all paths relative to
         // that directory. (With `Strategy::Hierarchical`, we'll end up finding
         // the "closest" `pyproject.toml` file for every Python file later on,
@@ -47,7 +53,7 @@ fn resolve(
         let settings = resolve_settings(&pyproject, &Relativity::Parent, Some(overrides))?;
         Ok(PyprojectDiscovery::Hierarchical(settings))
     } else if let Some(pyproject) = pyproject::find_user_settings_toml() {
-        // Third priority: find a user-specific `pyproject.toml`, but resolve all paths
+        // Fourth priority: find a user-specific `pyproject.toml`, but resolve all paths
         // relative the current working directory. (With `Strategy::Hierarchical`, we'll
         // end up the "closest" `pyproject.toml` file for every Python file later on, so
         // these act as the "default" settings.)
@@ -59,7 +65,6 @@ fn resolve(
         // "closest" `pyproject.toml` file for every Python file later on, so these act
         // as the "default" settings.)
         let mut config = Configuration::default();
-        // Apply command-line options that override defaults.
         config.apply(overrides.clone());
         let settings = Settings::from_configuration(config, &path_dedot::CWD)?;
         Ok(PyprojectDiscovery::Hierarchical(settings))
@@ -84,6 +89,7 @@ pub(crate) fn inner_main() -> Result<ExitCode> {
     // Construct the "default" settings. These are used when no `pyproject.toml`
     // files are present, or files are injected from outside of the hierarchy.
     let pyproject_strategy = resolve(
+        cli.isolated,
         cli.config.as_deref(),
         &overrides,
         cli.stdin_filename.as_deref(),