Close Rao-Wu and global-Rust warning-path coverage gap

CI AI review on PR #324 flagged (P3) that the delta changed the Rao-Wu
local, Rao-Wu global, and global-Rust warning branches without direct
assertions on those paths. Add three targeted tests mirroring the
pattern of the existing four (mocked inner-fit side effects, direct
method invocation, pytest.warns with the context string in the match
regex):

- `_bootstrap_variance_global` Rust happy path
- `_bootstrap_rao_wu_local` (survey design with per-unit PSU)
- `_bootstrap_rao_wu_global` (same survey setup)

All six changed warning sites now have direct regression coverage.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

tests/test_trop.py

@@ -3892,6 +3892,157 @@ def _fake_rust_boot(*args, **kwargs):
         assert np.isfinite(se)
         assert len(dist) == 11
 
+    def test_global_rust_bootstrap_warns_above_5pct_failure(self):
+        """Global Rust happy path: 3/20 Rust successes (85% fail) warns."""
+        import sys
+        from unittest.mock import patch
+
+        df = TestTROPNValidTreated._make_panel()
+
+        trop_est = TROP(
+            method="global",
+            lambda_time_grid=[1.0],
+            lambda_unit_grid=[1.0],
+            lambda_nn_grid=[np.inf],
+            n_bootstrap=20,
+            seed=42,
+        )
+
+        trop_global_module = sys.modules["diff_diff.trop_global"]
+        rng = np.random.default_rng(0)
+        fake_boot = rng.normal(size=3)
+
+        def _fake_rust_boot_global(*args, **kwargs):
+            return fake_boot, float(np.std(fake_boot, ddof=1))
+
+        with (
+            patch.object(trop_global_module, "HAS_RUST_BACKEND", True),
+            patch.object(
+                trop_global_module,
+                "_rust_bootstrap_trop_variance_global",
+                side_effect=_fake_rust_boot_global,
+            ),
+        ):
+            with pytest.warns(
+                UserWarning,
+                match=r"3/20 bootstrap iterations succeeded in TROP global bootstrap \(Rust\)",
+            ):
+                se, dist = trop_est._bootstrap_variance_global(
+                    df, "outcome", "treated", "unit", "time", (1.0, 1.0, 1e10), 3
+                )
+
+        assert np.isfinite(se)
+        assert len(dist) == 3
+
+    @staticmethod
+    def _make_survey_panel_and_design():
+        """Build a panel with per-unit PSU + weight columns and the matching
+        SurveyDesign/ResolvedSurveyDesign needed to reach the Rao-Wu path."""
+        from diff_diff import SurveyDesign
+        from diff_diff.survey import ResolvedSurveyDesign
+
+        df = TestTROPNValidTreated._make_panel().copy()
+        all_units = sorted(df["unit"].unique())
+        unit_to_psu = {u: i for i, u in enumerate(all_units)}
+        df["psu"] = df["unit"].map(unit_to_psu).astype(np.int64)
+        df["weight"] = 1.0
+        n_obs = len(df)
+
+        survey_design = SurveyDesign(weights="weight", psu="psu")
+        resolved_survey = ResolvedSurveyDesign(
+            weights=np.ones(n_obs, dtype=np.float64),
+            weight_type="pweight",
+            strata=None,
+            psu=df["psu"].values.astype(np.int64),
+            fpc=None,
+            n_strata=0,
+            n_psu=len(all_units),
+            lonely_psu="remove",
+        )
+        return df, survey_design, resolved_survey
+
+    def test_local_rao_wu_bootstrap_warns_above_5pct_failure(self):
+        """Local Rao-Wu survey bootstrap: forced failures → proportional warn."""
+        from unittest.mock import patch
+
+        df, survey_design, resolved_survey = self._make_survey_panel_and_design()
+
+        trop_est = TROP(
+            method="local",
+            lambda_time_grid=[1.0],
+            lambda_unit_grid=[1.0],
+            lambda_nn_grid=[np.inf],
+            n_bootstrap=20,
+            seed=42,
+        )
+
+        with patch.object(
+            TROP,
+            "_fit_with_fixed_lambda",
+            side_effect=self._make_failing_fit(20, 4),
+        ):
+            with pytest.warns(
+                UserWarning,
+                match=r"4/20 bootstrap iterations succeeded in TROP local Rao-Wu bootstrap",
+            ):
+                se, dist = trop_est._bootstrap_rao_wu_local(
+                    df,
+                    "outcome",
+                    "treated",
+                    "unit",
+                    "time",
+                    (1.0, 1.0, 1e10),
+                    resolved_survey,
+                    survey_design,
+                )
+
+        assert np.isfinite(se)
+        assert len(dist) == 4
+
+    def test_global_rao_wu_bootstrap_warns_above_5pct_failure(self):
+        """Global Rao-Wu survey bootstrap: forced failures → proportional warn."""
+        from unittest.mock import patch
+
+        df, survey_design, resolved_survey = self._make_survey_panel_and_design()
+
+        trop_est = TROP(
+            method="global",
+            lambda_time_grid=[1.0],
+            lambda_unit_grid=[1.0],
+            lambda_nn_grid=[np.inf],
+            n_bootstrap=20,
+            seed=42,
+        )
+
+        n_calls = {"count": 0}
+
+        def _flaky_solve(*args, **kwargs):
+            n_calls["count"] += 1
+            if n_calls["count"] <= 3:
+                n_periods, n_units = args[0].shape
+                return 0.0, np.zeros(n_units), np.zeros(n_periods), np.zeros((n_periods, n_units))
+            raise ValueError("forced Rao-Wu failure")
+
+        with patch.object(TROP, "_solve_global_model", side_effect=_flaky_solve):
+            with pytest.warns(
+                UserWarning,
+                match=r"3/20 bootstrap iterations succeeded in TROP global Rao-Wu bootstrap",
+            ):
+                se, dist = trop_est._bootstrap_rao_wu_global(
+                    df,
+                    "outcome",
+                    "treated",
+                    "unit",
+                    "time",
+                    (1.0, 1.0, 1e10),
+                    3,
+                    resolved_survey,
+                    survey_design,
+                )
+
+        assert np.isfinite(se) or np.isnan(se)
+        assert len(dist) == 3
+
 
 class TestTROPModuleSplit:
     """Regression tests for the trop.py -> trop_global.py / trop_local.py split."""