Allow `uv format` in unmanaged projects (#15553)

Closes #15550

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

crates/uv/src/commands/project/format.rs

@@ -11,7 +11,7 @@ use uv_client::{BaseClientBuilder, retries_from_env};
 use uv_pep440::Version;
 use uv_preview::{Preview, PreviewFeatures};
 use uv_warnings::warn_user;
-use uv_workspace::{DiscoveryOptions, VirtualProject, WorkspaceCache};
+use uv_workspace::{DiscoveryOptions, VirtualProject, WorkspaceCache, WorkspaceError};
 
 use crate::child::run_to_completion;
 use crate::commands::ExitStatus;
@@ -39,9 +39,21 @@ pub(crate) async fn format(
     }
 
     let workspace_cache = WorkspaceCache::default();
-    let project =
-        VirtualProject::discover(project_dir, &DiscoveryOptions::default(), &workspace_cache)
-            .await?;
+    let target_dir =
+        match VirtualProject::discover(project_dir, &DiscoveryOptions::default(), &workspace_cache)
+            .await
+        {
+            // If we found a project, we use the project root
+            Ok(proj) => proj.root().to_owned(),
+            // If there is a problem finding a project, we just use the provided directory,
+            // e.g., for unmanaged projects
+            Err(
+                WorkspaceError::MissingPyprojectToml
+                | WorkspaceError::MissingProject(_)
+                | WorkspaceError::NonWorkspace(_),
+            ) => project_dir.to_owned(),
+            Err(err) => return Err(err.into()),
+        };
 
     // Parse version if provided
     let version = version.as_deref().map(Version::from_str).transpose()?;
@@ -67,8 +79,7 @@ pub(crate) async fn format(
     .with_context(|| format!("Failed to install ruff {version}"))?;
 
     let mut command = Command::new(&ruff_path);
-    // Run ruff in the project root
-    command.current_dir(project.root());
+    command.current_dir(target_dir);
     command.arg("format");
 
     if check {

crates/uv/tests/it/format.rs

@@ -43,6 +43,108 @@ fn format_project() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn format_missing_pyproject_toml() -> Result<()> {
+    let context = TestContext::new_with_versions(&[]);
+
+    // Create an unformatted Python file
+    let main_py = context.temp_dir.child("main.py");
+    main_py.write_str(indoc! {r"
+        x    = 1
+    "})?;
+
+    uv_snapshot!(context.filters(), context.format(), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    1 file reformatted
+
+    ----- stderr -----
+    warning: `uv format` is experimental and may change without warning. Pass `--preview-features format` to disable this warning.
+    ");
+
+    // Check that the file was formatted
+    let formatted_content = fs_err::read_to_string(&main_py)?;
+    assert_snapshot!(formatted_content, @r"
+        x = 1
+    ");
+
+    Ok(())
+}
+
+#[test]
+fn format_missing_project_in_pyproject_toml() -> Result<()> {
+    let context = TestContext::new_with_versions(&[]);
+
+    // Create an empty pyproject.toml with no [project] section
+    context.temp_dir.child("pyproject.toml");
+
+    // Create an unformatted Python file
+    let main_py = context.temp_dir.child("main.py");
+    main_py.write_str(indoc! {r"
+        x    = 1
+    "})?;
+
+    uv_snapshot!(context.filters(), context.format(), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    1 file reformatted
+
+    ----- stderr -----
+    warning: `uv format` is experimental and may change without warning. Pass `--preview-features format` to disable this warning.
+    ");
+
+    // Check that the file was formatted
+    let formatted_content = fs_err::read_to_string(&main_py)?;
+    assert_snapshot!(formatted_content, @r"
+        x = 1
+    ");
+
+    Ok(())
+}
+
+#[test]
+fn format_unmanaged_project() -> Result<()> {
+    let context = TestContext::new_with_versions(&[]);
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = []
+
+        [tool.uv]
+        managed = false
+    "#})?;
+
+    // Create an unformatted Python file
+    let main_py = context.temp_dir.child("main.py");
+    main_py.write_str(indoc! {r"
+        x    = 1
+    "})?;
+
+    uv_snapshot!(context.filters(), context.format(), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    1 file reformatted
+
+    ----- stderr -----
+    warning: `uv format` is experimental and may change without warning. Pass `--preview-features format` to disable this warning.
+    ");
+
+    // Check that the file was formatted
+    let formatted_content = fs_err::read_to_string(&main_py)?;
+    assert_snapshot!(formatted_content, @r"
+        x = 1
+    ");
+
+    Ok(())
+}
+
 #[test]
 fn format_from_project_root() -> Result<()> {
     let context = TestContext::new_with_versions(&[]);
@@ -135,6 +237,50 @@ fn format_relative_project() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn format_fails_malformed_pyproject() -> Result<()> {
+    let context = TestContext::new_with_versions(&[]);
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str("malformed pyproject.toml")?;
+
+    // Create an unformatted Python file
+    let main_py = context.temp_dir.child("main.py");
+    main_py.write_str(indoc! {r"
+        x    = 1
+    "})?;
+
+    uv_snapshot!(context.filters(), context.format(), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: Failed to parse `pyproject.toml` during settings discovery:
+      TOML parse error at line 1, column 11
+        |
+      1 | malformed pyproject.toml
+        |           ^
+      key with no value, expected `=`
+
+    warning: `uv format` is experimental and may change without warning. Pass `--preview-features format` to disable this warning.
+    error: Failed to parse: `pyproject.toml`
+      Caused by: TOML parse error at line 1, column 11
+      |
+    1 | malformed pyproject.toml
+      |           ^
+    key with no value, expected `=`
+    ");
+
+    // Check that the file is not formatted
+    let formatted_content = fs_err::read_to_string(&main_py)?;
+    assert_snapshot!(formatted_content, @r"
+        x    = 1
+    ");
+
+    Ok(())
+}
+
 #[test]
 fn format_check() -> Result<()> {
     let context = TestContext::new_with_versions(&[]);