Add ESPHOME_PCH_STRICT and enforce it in the esp8266/esp32/rp2 image compile tests

tests/unit_tests/build_gen/test_arduino8266.py

@@ -1842,3 +1842,17 @@ def test_write_project_pch_no_device_path_poison(tmp_path: Path) -> None:
             (CORE.relative_pioenvs_path(name) / "esphome_pch.h.gch.sum").read_text()
         )
     assert sums[0] == sums[1]
+
+
+def test_write_project_pch_strict_raises_on_skip(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    from esphome.core import EsphomeError
+
+    monkeypatch.setenv("ESPHOME_PCH_STRICT", "1")
+    paths = _make_framework(tmp_path)
+    _set_flags(
+        "-DPIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH", "-include foo.h"
+    )
+    with pytest.raises(EsphomeError, match="precedes the pch"):
+        _write_ninja(paths, ccache="/usr/bin/ccache")

tests/unit_tests/build_gen/test_espidf.py

@@ -1078,3 +1078,22 @@ def test_prepare_pch_transient_compiler_failure_does_not_latch(
         prepare_pch()
     assert not (dev / "build" / "esphome_pch.h.gch.failed").exists()
     assert len(calls) == 2
+
+
+def test_prepare_pch_strict_raises_on_missing_db(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """ESPHOME_PCH_STRICT turns the silent skip into a failure."""
+    from esphome.build_gen.espidf import prepare_pch
+    from esphome.core import EsphomeError
+
+    monkeypatch.setenv("ESPHOME_PCH_STRICT", "1")
+    dev = _make_pch_device(tmp_path, "dev_st")
+    (dev / "build" / "compile_commands.json").unlink()
+    CORE.build_path = dev
+    with (
+        patch.object(CORE, "name", "test"),
+        patch("esphome.build_helpers.pch.subprocess.run", side_effect=AssertionError),
+        pytest.raises(EsphomeError, match="no usable compile command"),
+    ):
+        prepare_pch()

tests/unit_tests/build_helpers/test_pch.py

@@ -216,3 +216,29 @@ def test_ccache_pch_env_warns_on_falsy_extsum(
         env = pch.ccache_pch_env()
     assert "CCACHE_PCH_EXTSUM" not in env
     assert "disables pch caching" in caplog.text
+
+
+@pytest.mark.parametrize(
+    ("value", "expected"),
+    [(None, False), ("0", False), ("1", True), ("true", True)],
+)
+def test_pch_strict(
+    value: str | None, expected: bool, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    if value is None:
+        monkeypatch.delenv("ESPHOME_PCH_STRICT", raising=False)
+    else:
+        monkeypatch.setenv("ESPHOME_PCH_STRICT", value)
+    assert pch.pch_strict() is expected
+
+
+def test_pch_degraded_raises_only_in_strict(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    from esphome.core import EsphomeError
+
+    monkeypatch.delenv("ESPHOME_PCH_STRICT", raising=False)
+    pch.pch_degraded("reason")
+    monkeypatch.setenv("ESPHOME_PCH_STRICT", "1")
+    with pytest.raises(EsphomeError, match="reason"):
+        pch.pch_degraded("reason")

tests/unit_tests/test_platformio_pch_script.py

@@ -458,3 +458,14 @@ def test_pch_script_unreadable_local_header_skips_pch(
     assert not (proj / "esphome_pch.h.gch.sum").exists()
     assert scons_env.prepended == []
     assert "skipping precompiled header" in capsys.readouterr().out
+
+
+def test_pch_script_strict_raises_when_pch_not_used(tmp_path: Path) -> None:
+    """ESPHOME_PCH_STRICT fails the build instead of degrading."""
+    with pytest.raises(Exception, match="ESPHOME_PCH_STRICT|boom"):
+        _run_script(tmp_path, fail=True, env_vars={"ESPHOME_PCH_STRICT": "1"})
+
+
+def test_pch_script_strict_passes_on_success(tmp_path: Path) -> None:
+    scons_env = _run_script(tmp_path, env_vars={"ESPHOME_PCH_STRICT": "1"})
+    assert scons_env.prepended