Show a dedicated error for missing subdirectories (#9761)

## Summary

On `main`, if you ask for a source but name a missing subdirectory, you
just get:

```
{source} does not appear to be a Python project, as neither `pyproject.toml` nor `setup.py` are present in the directory
```

But, in reality, the directory doesn't exist at all.

crates/uv-distribution/src/error.rs

@@ -92,6 +92,8 @@ pub enum Error {
     MissingEggInfo,
     #[error("The source distribution is missing a `requires.txt` file")]
     MissingRequiresTxt,
+    #[error("The source distribution `{}` has no subdirectory `{}`", _0, _1.display())]
+    MissingSubdirectory(Url, PathBuf),
     #[error("Failed to extract static metadata from `PKG-INFO`")]
     PkgInfo(#[source] uv_pypi_types::MetadataError),
     #[error("Failed to extract metadata from `requires.txt`")]

crates/uv-distribution/src/source/mod.rs

@@ -393,6 +393,16 @@ impl<'a, T: BuildContext> SourceDistributionBuilder<'a, T> {
         let cache_shard = cache_shard.shard(revision.id());
         let source_dist_entry = cache_shard.entry(SOURCE);
 
+        // Validate that the subdirectory exists.
+        if let Some(subdirectory) = subdirectory {
+            if !source_dist_entry.path().join(subdirectory).is_dir() {
+                return Err(Error::MissingSubdirectory(
+                    url.clone(),
+                    subdirectory.to_path_buf(),
+                ));
+            }
+        }
+
         // If there are build settings, we need to scope to a cache shard.
         let config_settings = self.build_context.config_settings();
         let cache_shard = if config_settings.is_empty() {
@@ -496,6 +506,16 @@ impl<'a, T: BuildContext> SourceDistributionBuilder<'a, T> {
         let cache_shard = cache_shard.shard(revision.id());
         let source_dist_entry = cache_shard.entry(SOURCE);
 
+        // Validate that the subdirectory exists.
+        if let Some(subdirectory) = subdirectory {
+            if !source_dist_entry.path().join(subdirectory).is_dir() {
+                return Err(Error::MissingSubdirectory(
+                    url.clone(),
+                    subdirectory.to_path_buf(),
+                ));
+            }
+        }
+
         // If the metadata is static, return it.
         if let Some(metadata) =
             Self::read_static_metadata(source, source_dist_entry.path(), subdirectory).await?
@@ -1303,6 +1323,16 @@ impl<'a, T: BuildContext> SourceDistributionBuilder<'a, T> {
             )
             .await?;
 
+        // Validate that the subdirectory exists.
+        if let Some(subdirectory) = resource.subdirectory {
+            if !fetch.path().join(subdirectory).is_dir() {
+                return Err(Error::MissingSubdirectory(
+                    resource.url.to_url(),
+                    subdirectory.to_path_buf(),
+                ));
+            }
+        }
+
         let git_sha = fetch.git().precise().expect("Exact commit after checkout");
         let cache_shard = self.build_context.cache().shard(
             CacheBucket::SourceDistributions,
@@ -1390,6 +1420,16 @@ impl<'a, T: BuildContext> SourceDistributionBuilder<'a, T> {
             )
             .await?;
 
+        // Validate that the subdirectory exists.
+        if let Some(subdirectory) = resource.subdirectory {
+            if !fetch.path().join(subdirectory).is_dir() {
+                return Err(Error::MissingSubdirectory(
+                    resource.url.to_url(),
+                    subdirectory.to_path_buf(),
+                ));
+            }
+        }
+
         let git_sha = fetch.git().precise().expect("Exact commit after checkout");
         let cache_shard = self.build_context.cache().shard(
             CacheBucket::SourceDistributions,
@@ -1438,12 +1478,6 @@ impl<'a, T: BuildContext> SourceDistributionBuilder<'a, T> {
                 .await?
                 .filter(|metadata| metadata.matches(source.name(), source.version()))
             {
-                let path = if let Some(subdirectory) = resource.subdirectory {
-                    Cow::Owned(fetch.path().join(subdirectory))
-                } else {
-                    Cow::Borrowed(fetch.path())
-                };
-
                 let git_member = GitWorkspaceMember {
                     fetch_root: fetch.path(),
                     git_source: resource,

crates/uv/tests/it/pip_install.rs

@@ -7589,3 +7589,45 @@ fn build_tag() {
     ----- stderr -----
     "###);
 }
+
+#[test]
+fn missing_subdirectory_git() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.touch()?;
+
+    uv_snapshot!(context.pip_install()
+        .arg("source-distribution @ git+https://github.com/astral-sh/workspace-in-root-test#subdirectory=missing"), @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+
+    ----- stderr -----
+      × Failed to download and build `source-distribution @ git+https://github.com/astral-sh/workspace-in-root-test#subdirectory=missing`
+      ╰─▶ The source distribution `git+https://github.com/astral-sh/workspace-in-root-test#subdirectory=missing` has no subdirectory `missing`
+    "###
+    );
+
+    Ok(())
+}
+
+#[test]
+fn missing_subdirectory_url() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.touch()?;
+
+    uv_snapshot!(context.pip_install()
+        .arg("source-distribution @ https://files.pythonhosted.org/packages/1f/e5/5b016c945d745f8b108e759d428341488a6aee8f51f07c6c4e33498bb91f/source_distribution-0.0.3.tar.gz#subdirectory=missing"), @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+
+    ----- stderr -----
+      × Failed to download and build `source-distribution @ https://files.pythonhosted.org/packages/1f/e5/5b016c945d745f8b108e759d428341488a6aee8f51f07c6c4e33498bb91f/source_distribution-0.0.3.tar.gz#subdirectory=missing`
+      ╰─▶ The source distribution `https://files.pythonhosted.org/packages/1f/e5/5b016c945d745f8b108e759d428341488a6aee8f51f07c6c4e33498bb91f/source_distribution-0.0.3.tar.gz#subdirectory=missing` has no subdirectory `missing`
+    "###
+    );
+
+    Ok(())
+}