Don't warn about uv_build settings for in-tree build backend

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

@@ -611,11 +611,12 @@ impl SourceBuild {
             .as_ref()
             .and_then(|build_system| build_system.build_backend.as_deref());
 
-        if let Some(backend_path) = pyproject_toml
+        let backend_path = pyproject_toml
             .build_system
             .as_ref()
-            .and_then(|build_system| build_system.backend_path.as_ref())
-        {
+            .and_then(|build_system| build_system.backend_path.as_ref());
+
+        if let Some(backend_path) = backend_path {
             let source_tree = fs_err::canonicalize(source_tree).map_err(Error::Io)?;
             for path in backend_path.iter() {
                 if Path::new(path).is_absolute() {
@@ -639,8 +640,10 @@ impl SourceBuild {
         }
 
         // Only show the warning for first party and URL dependencies, not for registry dependencies
-        // (which have sources disabled).
+        // (which have sources disabled). In-tree build backends may wrap `uv_build`, so we don't
+        // warn for them either.
         if !no_sources.all()
+            && backend_path.is_none()
             && pyproject_toml
                 .tool
                 .as_ref()

crates/uv/tests/build/build_backend.rs

@@ -1764,6 +1764,51 @@ fn tool_uv_build_backend_wrong_build_backend() -> Result<()> {
     Ok(())
 }
 
+/// Don't warn about uv build backend setting when an in-tree build backend, it might be wrapping
+/// `uv_build`.
+///
+/// See <https://github.com/astral-sh/uv/issues/20128>.
+#[test]
+#[cfg(feature = "test-pypi")]
+fn tool_uv_build_backend_in_tree_backend() -> Result<()> {
+    // We need to use a real `uv_build` package.
+    let context = uv_test::test_context!("3.12").with_exclude_newer("2025-05-27T00:00:00Z");
+
+    let project = context.temp_dir.child("project");
+    let pyproject_toml = project.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+
+        [tool.uv.build-backend]
+        source-include = ["backend/**"]
+
+        [build-system]
+        requires = ["uv_build>=0.7,<10000"]
+        build-backend = "backend"
+        backend-path = ["."]
+    "#})?;
+    project.child("src/project/__init__.py").touch()?;
+    project
+        .child("backend/__init__.py")
+        .write_str("from uv_build import *\n")?;
+
+    uv_snapshot!(context.filters(), context.build().arg("--no-build-logs").arg(project.path()), @"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Building source distribution...
+    Building wheel from source distribution...
+    Successfully built project/dist/project-0.1.0.tar.gz
+    Successfully built project/dist/project-0.1.0-py3-none-any.whl
+    ");
+
+    Ok(())
+}
+
 /// Show a warning when the project uses deprecated `License ::` classifiers.
 #[test]
 fn warn_on_license_classifier() -> Result<()> {