Check if relative URL is valid directory before treating as index (#13917)

As per #13874, passing a relative URL like `test` to `--index` for `uv
add` causes unexpected behavior if the directory does not exist. The
non-existent index is effectively ignored and uv falls back to PyPI. If
a package is found there, the spurious index is then written to
`pyproject.toml`. This doesn't happen for `--default-index` since
resolution will fail without fallback to PyPI.

This PR adds a validation step for indexes provided on the command line.
If a directory does not exist, uv will fail with an error.

Closes #13874

crates/uv-pep508/src/verbatim_url.rs

@@ -106,10 +106,8 @@ impl VerbatimUrl {
         let (path, fragment) = split_fragment(&path);
 
         // Convert to a URL.
-        let mut url = DisplaySafeUrl::from(
-            Url::from_file_path(path.clone())
-                .unwrap_or_else(|()| panic!("path is absolute: {}", path.display())),
-        );
+        let mut url = DisplaySafeUrl::from_file_path(path.clone())
+            .unwrap_or_else(|()| panic!("path is absolute: {}", path.display()));
 
         // Set the fragment, if it exists.
         if let Some(fragment) = fragment {

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

@@ -23,8 +23,8 @@ use uv_configuration::{
 use uv_dispatch::BuildDispatch;
 use uv_distribution::DistributionDatabase;
 use uv_distribution_types::{
-    Index, IndexName, IndexUrls, NameRequirementSpecification, Requirement, RequirementSource,
-    UnresolvedRequirement, VersionId,
+    Index, IndexName, IndexUrl, IndexUrls, NameRequirementSpecification, Requirement,
+    RequirementSource, UnresolvedRequirement, VersionId,
 };
 use uv_fs::{LockedFile, Simplified};
 use uv_git::GIT_STORE;
@@ -473,6 +473,19 @@ pub(crate) async fn add(
         &mut toml,
     )?;
 
+    // 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 {
+        if let IndexUrl::Path(url) = &index.url {
+            let path = url
+                .to_file_path()
+                .map_err(|()| anyhow::anyhow!("Invalid file path in index URL: {url}"))?;
+            if !path.is_dir() {
+                bail!("Directory not found for index: {url}");
+            }
+        }
+    }
+
     // Add any indexes that were provided on the command-line, in priority order.
     if !raw {
         let urls = IndexUrls::from_indexes(indexes);

crates/uv/tests/it/edit.rs

@@ -9303,6 +9303,101 @@ fn add_index_without_trailing_slash() -> Result<()> {
     Ok(())
 }
 
+/// Add an index with an existing relative path.
+#[test]
+fn add_index_with_existing_relative_path_index() -> 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 = []
+    "#})?;
+
+    // Create test-index/ subdirectory and copy our "offline" tqdm wheel there
+    let packages = context.temp_dir.child("test-index");
+    packages.create_dir_all()?;
+
+    let wheel_src = context
+        .workspace_root
+        .join("scripts/links/ok-1.0.0-py3-none-any.whl");
+    let wheel_dst = packages.child("ok-1.0.0-py3-none-any.whl");
+    fs_err::copy(&wheel_src, &wheel_dst)?;
+
+    uv_snapshot!(context.filters(), context.add().arg("iniconfig").arg("--index").arg("test-index"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + iniconfig==2.0.0
+    ");
+
+    Ok(())
+}
+
+/// Add an index with a non-existent relative path.
+#[test]
+fn add_index_with_non_existent_relative_path() -> 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 = []
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("iniconfig").arg("--index").arg("test-index"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Directory not found for index: file://[TEMP_DIR]/test-index
+    ");
+
+    Ok(())
+}
+
+/// Add an index with a non-existent relative path with the same name as a defined index.
+#[test]
+fn add_index_with_non_existent_relative_path_with_same_name_as_index() -> 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 = []
+
+        [[tool.uv.index]]
+        name = "test-index"
+        url = "https://pypi-proxy.fly.dev/simple"
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("iniconfig").arg("--index").arg("test-index"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Directory not found for index: file://[TEMP_DIR]/test-index
+    ");
+
+    Ok(())
+}
+
 /// Add a PyPI requirement.
 #[test]
 fn add_group_comment() -> Result<()> {