Run the formatter in the project root instead of the working directory (#15440)

Closes https://github.com/astral-sh/uv/issues/15430

This PR is a branch from https://github.com/astral-sh/uv/pull/15438,
opening it as a draft until that is merged to main and I'll merge main
with this branch to have a cleaner diff

---------

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

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

@@ -10,6 +10,7 @@ use uv_client::BaseClientBuilder;
 use uv_configuration::{Preview, PreviewFeatures};
 use uv_pep440::Version;
 use uv_warnings::warn_user;
+use uv_workspace::{DiscoveryOptions, VirtualProject, WorkspaceCache};
 
 use crate::child::run_to_completion;
 use crate::commands::ExitStatus;
@@ -36,6 +37,12 @@ pub(crate) async fn format(
             PreviewFeatures::FORMAT
         );
     }
+
+    let workspace_cache = WorkspaceCache::default();
+    let project =
+        VirtualProject::discover(project_dir, &DiscoveryOptions::default(), &workspace_cache)
+            .await?;
+
     // Parse version if provided
     let version = version.as_deref().map(Version::from_str).transpose()?;
 
@@ -55,7 +62,8 @@ pub(crate) async fn format(
         .context("Failed to install ruff {version}")?;
 
     let mut command = Command::new(&ruff_path);
-    command.current_dir(project_dir);
+    // Run ruff in the project root
+    command.current_dir(project.root());
     command.arg("format");
 
     if check {

crates/uv/tests/it/format.rs

@@ -55,6 +55,60 @@ fn format_project() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn format_from_project_root() -> 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 = []
+    "#})?;
+
+    // Create an unformatted Python file
+    let main_py = context.temp_dir.child("main.py");
+    main_py.write_str(indoc! {r#"
+        import sys
+        def   hello():
+            print(  "Hello, World!"  )
+        if __name__=="__main__":
+            hello(   )
+    "#})?;
+
+    let subdir = context.temp_dir.child("subdir");
+    fs_err::create_dir_all(&subdir)?;
+
+    // Using format from a subdirectory should still run in the project root
+    uv_snapshot!(context.filters(), context.format().current_dir(&subdir), @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#"
+    import sys
+
+
+    def hello():
+        print("Hello, World!")
+
+
+    if __name__ == "__main__":
+        hello()
+    "#);
+
+    Ok(())
+}
+
 #[test]
 fn format_relative_project() -> Result<()> {
     let context = TestContext::new_with_versions(&[]);