Error instead of panic! when running Ruff from a deleted directory (#16903) (#17054)

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Closes #16903

## Summary
Check if the current working directory exist. If not, provide an error
instead of panicking.

Fixed a stale comment in `resolve_default_files`.

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan
I added a test to the `resolve_files.rs`. 

Manual testing follows steps of #16903 :
- Terminal 1
```bash
mkdir tmp
cd tmp
```
- Terminal 2
```bash
rm -rf tmp
```
- Terminal 1
```bash
ruff check
```

## Open Issues / Questions to Reviewer
All tests pass when executed with `cargo nextest run`.

However, with `cargo test` the parallelization makes the other tests
fail as we change the `pwd`.

Serial execution with `cargo test` seems to require [another dependency
or some
workarounds](https://stackoverflow.com/questions/51694017/how-can-i-avoid-running-some-tests-in-parallel).

Do you think an additional dependency or test complexity is worth
testing this small edge case, do you have another implementation idea,
or should i rather remove the test?
  
---
P.S.: I'm currently participating in a batch at the [Recurse
Center](https://www.recurse.com/) and would love to contribute more for
the next six weeks to improve my Rust. Let me know if you're open to
mentoring/reviewing and/or if you have specific areas where help would
be most valued.

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff/src/lib.rs

@@ -112,7 +112,7 @@ fn is_stdin(files: &[PathBuf], stdin_filename: Option<&Path>) -> bool {
     file == Path::new("-")
 }
 
-/// Returns the default set of files if none are provided, otherwise returns `None`.
+/// Returns the default set of files if none are provided, otherwise returns provided files.
 fn resolve_default_files(files: Vec<PathBuf>, is_stdin: bool) -> Vec<PathBuf> {
     if files.is_empty() {
         if is_stdin {

crates/ruff/src/resolve.rs

@@ -1,6 +1,6 @@
 use std::path::Path;
 
-use anyhow::Result;
+use anyhow::{bail, Result};
 use log::debug;
 use path_absolutize::path_dedot;
 
@@ -21,10 +21,14 @@ pub fn resolve(
     config_arguments: &ConfigArguments,
     stdin_filename: Option<&Path>,
 ) -> Result<PyprojectConfig> {
+    let Ok(cwd) = std::env::current_dir() else {
+        bail!("Working directory does not exist")
+    };
+
     // First priority: if we're running in isolated mode, use the default settings.
     if config_arguments.isolated {
         let config = config_arguments.transform(Configuration::default());
-        let settings = config.into_settings(&path_dedot::CWD)?;
+        let settings = config.into_settings(&cwd)?;
         debug!("Isolated mode, not reading any pyproject.toml");
         return Ok(PyprojectConfig::new(
             PyprojectDiscoveryStrategy::Fixed,
@@ -58,11 +62,7 @@ pub fn resolve(
     // that directory. (With `Strategy::Hierarchical`, we'll end up finding
     // the "closest" `pyproject.toml` file for every Python file later on,
     // so these act as the "default" settings.)
-    if let Some(pyproject) = pyproject::find_settings_toml(
-        stdin_filename
-            .as_ref()
-            .unwrap_or(&path_dedot::CWD.as_path()),
-    )? {
+    if let Some(pyproject) = pyproject::find_settings_toml(stdin_filename.unwrap_or(&cwd))? {
         debug!(
             "Using configuration file (via parent) at: {}",
             pyproject.display()
@@ -130,17 +130,13 @@ pub fn resolve(
         // `pyproject::find_settings_toml`.)
         // However, there may be a `pyproject.toml` with a `requires-python`
         // specified, and that is what we look for in this step.
-        let fallback = find_fallback_target_version(
-            stdin_filename
-                .as_ref()
-                .unwrap_or(&path_dedot::CWD.as_path()),
-        );
+        let fallback = find_fallback_target_version(stdin_filename.unwrap_or(&cwd));
         if let Some(version) = fallback {
             debug!("Derived `target-version` from found `requires-python`: {version:?}");
         }
         config.target_version = fallback.map(ast::PythonVersion::from);
     }
-    let settings = config.into_settings(&path_dedot::CWD)?;
+    let settings = config.into_settings(&cwd)?;
     Ok(PyprojectConfig::new(
         PyprojectDiscoveryStrategy::Hierarchical,
         settings,

crates/ruff/tests/direxist_guard.rs

@@ -0,0 +1,30 @@
+//! Test to verify Ruff's behavior when run from deleted directory.
+//! It has to be isolated in a separate module.
+//! Tests in the same module become flaky under `cargo test`s parallel execution
+//! due to in-test working directory manipulation.
+
+#![cfg(target_family = "unix")]
+
+use std::env::set_current_dir;
+use std::process::Command;
+
+use insta_cmd::{assert_cmd_snapshot, get_cargo_bin};
+const BIN_NAME: &str = "ruff";
+
+#[test]
+fn check_in_deleted_directory_errors() {
+    let temp_dir = tempfile::tempdir().unwrap();
+    let temp_path = temp_dir.path().to_path_buf();
+    set_current_dir(&temp_path).unwrap();
+    drop(temp_dir);
+
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME)).arg("check"), @r###"
+            success: false
+            exit_code: 2
+            ----- stdout -----
+
+            ----- stderr -----
+            ruff failed
+              Cause: Working directory does not exist
+            "###);
+}