Don't require setuptools_rust at runtime

changelog.d/13952.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in v1.68.0 where Synapse would require `setuptools_rust` at runtime, even though the package is only required at build time.

synapse/util/check_dependencies.py

@@ -66,6 +66,17 @@ def _is_dev_dependency(req: Requirement) -> bool:
     )
 
 
+def _should_ignore_runtime_requirement(req: Requirement) -> bool:
+    # This is a build-time dependency. Irritatingly, `poetry build` ignores the
+    # requirements listed in the [build-system] section of pyproject.toml, so in order
+    # to support `poetry install --no-dev` we have to mark it as a runtime dependency.
+    # Workaround this by ignoring it here. (It might be slightly cleaner to put
+    # `setuptools_rust` in a `build` extra or similar, but . But for now I'
+    if req.name == "setuptools_rust":
+        return True
+    return False
+
+
 class Dependency(NamedTuple):
     requirement: Requirement
     must_be_installed: bool
@@ -77,7 +88,7 @@ def _generic_dependencies() -> Iterable[Dependency]:
     assert requirements is not None
     for raw_requirement in requirements:
         req = Requirement(raw_requirement)
-        if _is_dev_dependency(req):
+        if _is_dev_dependency(req) or _should_ignore_runtime_requirement(req):
             continue
 
         # https://packaging.pypa.io/en/latest/markers.html#usage notes that

tests/util/test_check_dependencies.py

@@ -40,7 +40,10 @@ class TestDependencyChecker(TestCase):
     def mock_installed_package(
         self, distribution: Optional[DummyDistribution]
     ) -> Generator[None, None, None]:
-        """Pretend that looking up any distribution yields the given `distribution`."""
+        """Pretend that looking up any package yields the given `distribution`.
+
+        If `distribution = None`, we pretend that the package is not installed.
+        """
 
         def mock_distribution(name: str):
             if distribution is None:
@@ -81,7 +84,7 @@ def test_version_reported_as_none(self) -> None:
                 self.assertRaises(DependencyException, check_requirements)
 
     def test_checks_ignore_dev_dependencies(self) -> None:
-        """Bot generic and per-extra checks should ignore dev dependencies."""
+        """Both generic and per-extra checks should ignore dev dependencies."""
         with patch(
             "synapse.util.check_dependencies.metadata.requires",
             return_value=["dummypkg >= 1; extra == 'mypy'"],
@@ -142,3 +145,16 @@ def test_release_candidates_satisfy_dependency(self) -> None:
             with self.mock_installed_package(new_release_candidate):
                 # should not raise
                 check_requirements()
+
+    def test_setuptools_rust_ignored(self) -> None:
+        """Test a workaround for a `poetry build` problem. Reproduces #13926."""
+        with patch(
+            "synapse.util.check_dependencies.metadata.requires",
+            return_value=["setuptools_rust >= 1.3"],
+        ):
+            with self.mock_installed_package(None):
+                # should not raise, even if setuptools_rust is not installed
+                check_requirements()
+            with self.mock_installed_package(old):
+                # We also ignore old versions of setuptools_rust
+                check_requirements()