Refactor `extract_pep517_backend`

Warn when using `uv_build` settings without `uv_build`

To help with cases such as https://github.com/astral-sh/uv/issues/15655.

A question is when to show this warning. I've used sources as a proxy as URL dependencies are likely those controlled by the user, and they include workspace and `git clone`d path dependencies.

Fixes #15740

Update crates/uv/tests/it/build_backend.rs

Co-authored-by: Zanie Blue <contact@zanie.dev>

Review

De-deduplicate message

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

@@ -109,6 +109,8 @@ struct Tool {
 #[serde(rename_all = "kebab-case")]
 struct ToolUv {
     workspace: Option<de::IgnoredAny>,
+    /// To warn users about ignored build backend settings.
+    build_backend: Option<de::IgnoredAny>,
 }
 
 impl BackendPath {
@@ -308,6 +310,7 @@ impl SourceBuild {
             &source_tree,
             install_path,
             fallback_package_name,
+            fallback_package_version,
             locations,
             source_strategy,
             workspace_cache,
@@ -560,17 +563,62 @@ impl SourceBuild {
         source_tree: &Path,
         install_path: &Path,
         package_name: Option<&PackageName>,
+        package_version: Option<&Version>,
         locations: &IndexLocations,
         source_strategy: SourceStrategy,
         workspace_cache: &WorkspaceCache,
         credentials_cache: &CredentialsCache,
     ) -> Result<(Pep517Backend, Option<Project>), Box<Error>> {
-        match fs::read_to_string(source_tree.join("pyproject.toml")) {
+        let pyproject_toml = match fs::read_to_string(source_tree.join("pyproject.toml")) {
             Ok(toml) => {
                 let pyproject_toml = toml_edit::Document::from_str(&toml)
                     .map_err(Error::InvalidPyprojectTomlSyntax)?;
-                let pyproject_toml = PyProjectToml::deserialize(pyproject_toml.into_deserializer())
-                    .map_err(Error::InvalidPyprojectTomlSchema)?;
+                PyProjectToml::deserialize(pyproject_toml.into_deserializer())
+                    .map_err(Error::InvalidPyprojectTomlSchema)?
+            }
+            Err(err) if err.kind() == io::ErrorKind::NotFound => {
+                // We require either a `pyproject.toml` or a `setup.py` file at the top level.
+                if !source_tree.join("setup.py").is_file() {
+                    return Err(Box::new(Error::InvalidSourceDist(
+                        source_tree.to_path_buf(),
+                    )));
+                }
+
+                // If no `pyproject.toml` is present, by default, proceed with a PEP 517 build using
+                // the default backend, to match `build`. `pip` uses `setup.py` directly in this
+                // case,  but plans to make PEP 517 builds the default in the future.
+                // See: https://github.com/pypa/pip/issues/9175.
+                return Ok((DEFAULT_BACKEND.clone(), None));
+            }
+            Err(err) => return Err(Box::new(err.into())),
+        };
+
+        if source_strategy == SourceStrategy::Enabled
+            && pyproject_toml
+                .tool
+                .as_ref()
+                .and_then(|tool| tool.uv.as_ref())
+                .map(|uv| uv.build_backend.is_some())
+                .unwrap_or(false)
+            && pyproject_toml
+                .build_system
+                .as_ref()
+                .and_then(|build_backend| build_backend.build_backend.as_deref())
+                != Some("uv_build")
+            && let Some(package_name) =
+                package_name.or(pyproject_toml.project.as_ref().map(|project| &project.name))
+            && let Some(package_version) = package_version.or(pyproject_toml
+                .project
+                .as_ref()
+                .and_then(|project| project.version.as_ref()))
+        {
+            // Show name/version where available to avoid showing a (duplicate) warning with
+            // a temporary path.
+            warn_user_once!(
+                "`pyproject.toml` of {package_name}=={package_version} defines settings for \
+                `uv_build` in `tool.uv.build-backend`, but does not use `uv_build`",
+            );
+        }
 
         let backend = if let Some(build_system) = pyproject_toml.build_system {
             // If necessary, lower the requirements.
@@ -592,8 +640,7 @@ impl SourceBuild {
                             locations,
                             source_strategy,
                             workspace_cache,
-                                    credentials_cache,
-                                )
+                        credentials_cache,)
                         .await
                         .map_err(Error::Lowering)?;
                         build_requires.requires_dist
@@ -666,23 +713,6 @@ impl SourceBuild {
         };
         Ok((backend, pyproject_toml.project))
     }
-            Err(err) if err.kind() == io::ErrorKind::NotFound => {
-                // We require either a `pyproject.toml` or a `setup.py` file at the top level.
-                if !source_tree.join("setup.py").is_file() {
-                    return Err(Box::new(Error::InvalidSourceDist(
-                        source_tree.to_path_buf(),
-                    )));
-                }
-
-                // If no `pyproject.toml` is present, by default, proceed with a PEP 517 build using
-                // the default backend, to match `build`. `pip` uses `setup.py` directly in this
-                // case,  but plans to make PEP 517 builds the default in the future.
-                // See: https://github.com/pypa/pip/issues/9175.
-                Ok((DEFAULT_BACKEND.clone(), None))
-            }
-            Err(err) => Err(Box::new(err.into())),
-        }
-    }
 
     /// Try calling `prepare_metadata_for_build_wheel` to get the metadata without executing the
     /// actual build.

crates/uv/tests/it/build_backend.rs

@@ -1222,3 +1222,51 @@ fn invalid_pyproject_toml() -> Result<()> {
 
     Ok(())
 }
+
+/// Warn for cases where `tool.uv.build-backend` is used without the corresponding build backend
+/// entry.
+#[test]
+fn tool_uv_build_backend_without_build_backend() -> 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"
+
+        [tool.uv]
+        package = true
+
+        [tool.uv.build-backend.data]
+        data = "assets"
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.build().arg("--no-build-logs"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Building source distribution...
+    warning: `pyproject.toml` of project==0.1.0 defines settings for `uv_build` in `tool.uv.build-backend`, but does not use `uv_build`
+    Building wheel from source distribution...
+    Successfully built dist/project-0.1.0.tar.gz
+    Successfully built dist/project-0.1.0-py3-none-any.whl
+    ");
+
+    uv_snapshot!(context.filters(), context.pip_install().arg("."), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 1 package in [TIME]
+    warning: `pyproject.toml` of project==0.1.0 defines settings for `uv_build` in `tool.uv.build-backend`, but does not use `uv_build`
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + project==0.1.0 (from file://[TEMP_DIR]/)
+    ");
+
+    Ok(())
+}