Validate PEP 517 backend paths (#19834)

## Summary

Prior to this change, we added `backend-path` entries to `sys.path`
without checking that they existed in the current source tree. When an
in-tree backend was omitted from an sdist, the subsequent wheel build
failed with `ModuleNotFoundError` and suggested that the backend was an
undeclared build dependency.

This validates each `backend-path` entry before resolving or invoking
the backend. Missing directories now produce a direct error explaining
that the configured path does not exist, matching the behavior of PyPA's
`build` frontend.

The regression test builds an sdist with a Flit wrapper in
`backend_dir`, intentionally omits that directory from the sdist, and
verifies the diagnostic during the wheel-from-sdist build.

Closes https://github.com/astral-sh/uv/issues/19771.

crates/uv-build-frontend/src/error.rs

@@ -69,6 +69,8 @@ pub enum Error {
         "`pyproject.toml` does not match the required schema. When the `[project]` table is present, `project.name` must be present and non-empty."
     )]
     InvalidPyprojectTomlSchema(#[from] toml_edit::de::Error),
+    #[error("`backend-path` entry `{0}` does not exist or is not a directory")]
+    InvalidBackendPath(String),
     #[error("Failed to resolve requirements from {0}")]
     RequirementsResolve(&'static str, #[source] AnyErrorBuild),
     #[error("Failed to install requirements from {0}")]
@@ -105,6 +107,7 @@ impl IsBuildBackendError for Error {
             | Self::InvalidSourceDist(_)
             | Self::InvalidPyprojectTomlSyntax(_)
             | Self::InvalidPyprojectTomlSchema(_)
+            | Self::InvalidBackendPath(_)
             | Self::RequirementsResolve(_, _)
             | Self::RequirementsInstall(_, _)
             | Self::Virtualenv(_)

crates/uv-build-frontend/src/lib.rs

@@ -610,6 +610,19 @@ impl SourceBuild {
             .build_system
             .as_ref()
             .and_then(|build_system| build_system.build_backend.as_deref());
+
+        if let Some(backend_path) = pyproject_toml
+            .build_system
+            .as_ref()
+            .and_then(|build_system| build_system.backend_path.as_ref())
+        {
+            for path in backend_path.iter() {
+                if !source_tree.join(path).is_dir() {
+                    return Err(Box::new(Error::InvalidBackendPath(path.to_string())));
+                }
+            }
+        }
+
         // Only show the warning for first party and URL dependencies, not for registry dependencies
         // (which have sources disabled).
         if !no_sources.all()

crates/uv/tests/build/build.rs

@@ -148,6 +148,45 @@ fn build_basic() -> Result<()> {
     Ok(())
 }
 
+/// A source distribution must include an in-tree build backend referenced by `backend-path`.
+/// Regression test for <https://github.com/astral-sh/uv/issues/19771>.
+#[test]
+fn build_sdist_missing_backend_path() -> Result<()> {
+    let context = uv_test::test_context!("3.12");
+    let project = context.temp_dir.child("project");
+
+    project.child("pyproject.toml").write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        description = "A test project"
+
+        [build-system]
+        requires = ["flit_core >=3.4,<4"]
+        build-backend = "my_backend:builder"
+        backend-path = ["backend_dir"]
+    "#})?;
+    project.child("src/project/__init__.py").touch()?;
+    project
+        .child("backend_dir/my_backend.py")
+        .write_str("from flit_core import buildapi\n\nbuilder = buildapi\n")?;
+
+    uv_snapshot!(context.filters(), context.build().arg(project.path()), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    Building source distribution...
+    Building wheel from source distribution...
+      × Failed to build `[TEMP_DIR]/project`
+      ╰─▶ `backend-path` entry `backend_dir` does not exist or is not a directory
+    ");
+
+    Ok(())
+}
+
 #[test]
 fn build_sdist() -> Result<()> {
     let context = uv_test::test_context!("3.12");