Fix vendor Rust: limit to manifests from backend

Fromager was vendoring crates from all `Cargo.toml` files in a project.
This approach is causing issues for projects that have cargo files in
tests and example directories.

The `vendor_rust()` function now only vendors crates from `Cargo.toml`
in the project's root directory and additional cargo files listed in
`tools.maturin` or `tools.setuptools-rust` entries.

Fixes: python-wheel-build#529
Signed-off-by: Christian Heimes <[email protected]>

src/fromager/vendor_rust.py

@@ -86,28 +86,27 @@ def _cargo_config(project_dir: pathlib.Path) -> None:
         tomlkit.dump(cfg, f)
 
 
-def _should_vendor_rust(req: Requirement, project_dir: pathlib.Path) -> bool:
+def _detect_rust_build_backend(
+    req: Requirement, pyproject_toml: dict[str, typing.Any]
+) -> str | None:
     """Detect if project has build requirement on Rust
 
     Detects setuptools-rust and maturin.
     """
-    pyproject_toml = dependencies.get_pyproject_contents(project_dir)
-    if not pyproject_toml:
-        logger.debug(f"{req.name}: has no pyproject.toml")
-        return False
-
     build_backend = dependencies.get_build_backend(pyproject_toml)
+    if build_backend["build-backend"] == "maturin":
+        return "maturin"
 
     for reqstring in build_backend["requires"]:
         req = Requirement(reqstring)
         if req.name in RUST_BUILD_REQUIRES:
             logger.debug(
-                f"{req.name}: build-system requires {req.name}, vendoring crates"
+                f"{req.name}: build-system requires '{req.name}', vendoring crates"
             )
-            return True
+            return req.name
 
     logger.debug(f"{req.name}: no Rust build plugin detected")
-    return False
+    return None
 
 
 def vendor_rust(
@@ -119,11 +118,42 @@ def vendor_rust(
     ``setuptools-rust`` or ``maturin``, and has a ``Cargo.toml``, otherwise
     ``False``.
     """
-    if not _should_vendor_rust(req, project_dir):
+    pyproject_toml = dependencies.get_pyproject_contents(project_dir)
+    if not pyproject_toml:
+        logger.debug(f"{req.name}: has no pyproject.toml")
         return False
 
-    # check for Cargo.toml
-    manifests = list(project_dir.glob("**/Cargo.toml"))
+    backend = _detect_rust_build_backend(req, pyproject_toml)
+    if backend is None:
+        return False
+
+    manifests: list[pathlib.Path] = []
+    # check for Cargo.toml in project root
+    root_cargo_toml = project_dir / "Cargo.toml"
+    if root_cargo_toml.is_file():
+        manifests.append(root_cargo_toml)
+
+    # maturin and setuptools-rust can have Cargo.toml in other directories.
+    # tool.setuptools-rust and tool.maturin are optional.
+    if backend == "maturin":
+        try:
+            tool_maturin: dict[str, typing.Any] = pyproject_toml["tool"]["maturin"]
+        except KeyError as e:
+            logger.debug(f"{req.name}: No additional maturin settings: {e}")
+        else:
+            if "manifest-path" in tool_maturin:
+                manifests.append(project_dir / tool_maturin["manifest-path"])
+    elif backend == "setuptools-rust":
+        ext_modules: list[dict[str, typing.Any]]
+        try:
+            ext_modules = pyproject_toml["tool"]["setuptools-rust"]["ext-modules"]
+        except KeyError as e:
+            logger.debug(f"{req.name}: No additional setuptools-rust settings: {e}")
+        else:
+            for ext_module in ext_modules:
+                if "path" in ext_module:
+                    manifests.append(project_dir / ext_module["path"])
+
     if not manifests:
         logger.debug(f"{req.name}: has no Cargo.toml files")
         return False