Warn on empty index directory (#13940)

Close #13922

## Summary

Add a warning if the directory given by the `--index` argument is empty.

## Test Plan

Added test case `add_index_empty_directory` in `edit.rs`

crates/uv-distribution-types/src/index_url.rs

@@ -511,30 +511,23 @@ impl<'a> IndexUrls {
     /// iterator.
     pub fn defined_indexes(&'a self) -> impl Iterator<Item = &'a Index> + 'a {
         if self.no_index {
-            Either::Left(std::iter::empty())
-        } else {
-            Either::Right(
-                {
-                    let mut seen = FxHashSet::default();
-                    self.indexes
-                        .iter()
-                        .filter(move |index| {
-                            index.name.as_ref().is_none_or(|name| seen.insert(name))
-                        })
-                        .filter(|index| !index.default)
-                }
-                .chain({
-                    let mut seen = FxHashSet::default();
-                    self.indexes
-                        .iter()
-                        .filter(move |index| {
-                            index.name.as_ref().is_none_or(|name| seen.insert(name))
-                        })
-                        .find(|index| index.default)
-                        .into_iter()
-                }),
-            )
+            return Either::Left(std::iter::empty());
         }
+
+        let mut seen = FxHashSet::default();
+        let (non_default, default) = self
+            .indexes
+            .iter()
+            .filter(move |index| {
+                if let Some(name) = &index.name {
+                    seen.insert(name)
+                } else {
+                    true
+                }
+            })
+            .partition::<Vec<_>, _>(|index| !index.default);
+
+        Either::Right(non_default.into_iter().chain(default))
     }
 
     /// Return the `--no-index` flag.

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

@@ -512,8 +512,9 @@ pub(crate) async fn add(
     )?;
 
     // Validate any indexes that were provided on the command-line to ensure
-    // they point to existing directories when using path URLs.
-    for index in &indexes {
+    // they point to existing non-empty directories when using path URLs.
+    let mut valid_indexes = Vec::with_capacity(indexes.len());
+    for index in indexes {
         if let IndexUrl::Path(url) = &index.url {
             let path = url
                 .to_file_path()
@@ -521,8 +522,14 @@ pub(crate) async fn add(
             if !path.is_dir() {
                 bail!("Directory not found for index: {url}");
             }
+            if fs_err::read_dir(&path)?.next().is_none() {
+                warn_user_once!("Index directory `{url}` is empty, skipping");
+                continue;
+            }
         }
+        valid_indexes.push(index);
     }
+    let indexes = valid_indexes;
 
     // Add any indexes that were provided on the command-line, in priority order.
     if !raw {

crates/uv/tests/it/edit.rs

@@ -9434,6 +9434,38 @@ fn add_index_with_non_existent_relative_path_with_same_name_as_index() -> Result
     Ok(())
 }
 
+#[test]
+fn add_index_empty_directory() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    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 = []
+    "#})?;
+
+    let packages = context.temp_dir.child("test-index");
+    packages.create_dir_all()?;
+
+    uv_snapshot!(context.filters(), context.add().arg("iniconfig").arg("--index").arg("test-index"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: Index directory `file://[TEMP_DIR]/test-index` is empty, skipping
+    Resolved 2 packages in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + iniconfig==2.0.0
+    ");
+
+    Ok(())
+}
+
 /// Add a PyPI requirement.
 #[test]
 fn add_group_comment() -> Result<()> {