Enable global configuration files (#3049)

## Summary

Enables `uv` to read configuration from (e.g.)
`/Users/crmarsh/.config/uv/uv.toml` on macOS and Linux, and
`C:\Users\Charlie\AppData\Roaming\uv\uv.toml` on Windows.

This differs slightly from Ruff, which uses the `Application Support`
directory on macOS. But I've deviated here. based on the preferences
expressed in https://github.com/astral-sh/ruff/issues/10739.

Cargo.lock

@@ -5026,6 +5026,7 @@ dependencies = [
 name = "uv-workspace"
 version = "0.0.1"
 dependencies = [
+ "dirs-sys",
  "distribution-types",
  "fs-err",
  "install-wheel-rs",

Cargo.toml

@@ -75,6 +75,7 @@ dashmap = { version = "5.5.3" }
 data-encoding = { version = "2.5.0" }
 derivative = { version = "2.2.0" }
 directories = { version = "5.0.1" }
+dirs-sys = { version = "0.4.1" }
 dunce = { version = "1.0.4" }
 either = { version = "1.9.0" }
 encoding_rs_io = { version = "0.1.7" }

crates/uv-workspace/Cargo.toml

@@ -24,6 +24,7 @@ uv-resolver = { workspace = true, features = ["schemars", "serde"] }
 uv-toolchain = { workspace = true, features = ["schemars", "serde"] }
 uv-warnings = { workspace = true }
 
+dirs-sys = { workspace = true }
 fs-err = { workspace = true }
 schemars = { workspace = true, optional = true }
 serde = { workspace = true }

crates/uv-workspace/src/workspace.rs

@@ -15,6 +15,19 @@ pub struct Workspace {
 }
 
 impl Workspace {
+    /// Load the user [`Workspace`].
+    pub fn user() -> Result<Option<Self>, WorkspaceError> {
+        let Some(dir) = config_dir() else {
+            return Ok(None);
+        };
+        let root = dir.join("uv");
+        let file = root.join("uv.toml");
+        Ok(Some(Self {
+            options: find_in_directory(&file)?.unwrap_or_default(),
+            root,
+        }))
+    }
+
     /// Find the [`Workspace`] for the given path.
     ///
     /// The search starts at the given path and goes up the directory tree until a workspace is
@@ -53,6 +66,27 @@ impl Workspace {
     }
 }
 
+/// Returns the path to the user configuration directory.
+///
+/// This is similar to the `config_dir()` returned by the `dirs` crate, but it uses the
+/// `XDG_CONFIG_HOME` environment variable on both Linux _and_ macOS, rather than the
+/// `Application Support` directory on macOS.
+fn config_dir() -> Option<PathBuf> {
+    // On Windows, use, e.g., C:\Users\Alice\AppData\Roaming
+    #[cfg(windows)]
+    {
+        dirs_sys::known_folder_roaming_app_data()
+    }
+
+    // On Linux and macOS, use, e.g., /home/alice/.config.
+    #[cfg(not(windows))]
+    {
+        std::env::var_os("XDG_CONFIG_HOME")
+            .and_then(dirs_sys::is_absolute_path)
+            .or_else(|| dirs_sys::home_dir().map(|path| path.join(".config")))
+    }
+}
+
 /// Read a `uv.toml` or `pyproject.toml` file in the given directory.
 fn find_in_directory(dir: &Path) -> Result<Option<Options>, WorkspaceError> {
     // Read a `uv.toml` file in the current directory.

crates/uv/src/main.rs

@@ -109,11 +109,16 @@ async fn run() -> Result<ExitStatus> {
         }
     };
 
-    // Load the workspace settings.
+    // Load the workspace settings, prioritizing (in order):
+    // 1. The configuration file specified on the command-line.
+    // 2. The configuration file in the current directory.
+    // 3. The user configuration file.
     let workspace = if let Some(config_file) = cli.config_file.as_ref() {
         Some(uv_workspace::Workspace::from_file(config_file)?)
+    } else if let Some(workspace) = uv_workspace::Workspace::find(env::current_dir()?)? {
+        Some(workspace)
     } else {
-        uv_workspace::Workspace::find(env::current_dir()?)?
+        uv_workspace::Workspace::user()?
     };
 
     // Resolve the global settings.