Guard the prefix for C edges, gate it on the pch knob, pin its resolution

esphome/components/host/__init__.py

@@ -1,4 +1,4 @@
-from esphome.build_helpers.pch import pch_extra_scripts
+from esphome.build_helpers.pch import pch_enabled, pch_extra_scripts
 import esphome.codegen as cg
 import esphome.config_validation as cv
 from esphome.const import (
@@ -19,6 +19,9 @@ from .const import KEY_HOST
 # force import gpio to register pin schema
 from .gpio import host_pin_to_code  # noqa: F401
 
+# Guarded wrapper: build_src_flags reaches C/assembly edges too
+HOST_PCH_PREFIX = "esphome/core/pch_prefix.h"
+
 CODEOWNERS = ["@esphome/core", "@clydebarrow"]
 AUTO_LOAD = ["network", "preferences"]
 IS_TARGET_PLATFORM = True
@@ -57,14 +60,13 @@ async def to_code(config: ConfigType) -> None:
     cg.add_platformio_option("lib_ldf_mode", "off")
     cg.add_platformio_option("lib_compat_mode", "strict")
     cg.add_platformio_option("extra_scripts", ["pre:ccache.py", *pch_extra_scripts()])
-    # Curated prefix for the pch (the script folds these plus defines.h):
-    # host has no framework force-includes, and the per-TU cost is the STL
-    # closure behind these core headers. Measured -43% compile CPU.
-    # macOS system clang cannot load a GCC .gch; the load probe falls back.
-    cg.add_platformio_option(
-        "build_src_flags",
-        "-include esphome/core/application.h -include esphome/core/automation.h",
-    )
+    if pch_enabled():
+        # Curated prefix for the pch (the script folds it plus defines.h):
+        # host has no framework force-includes, and the per-TU cost is the
+        # STL closure behind the core headers. Measured -43% compile CPU.
+        # Gated so ESPHOME_PCH_ENABLE=0 restores the strict view; a
+        # toolchain that cannot load its own pch hits the script's probe.
+        cg.add_platformio_option("build_src_flags", f"-include {HOST_PCH_PREFIX}")
 
 
 # Called by writer.py

esphome/core/pch_prefix.h

@@ -0,0 +1,8 @@
+#pragma once
+// Curated precompiled-header prefix for backends that force-include it via
+// build_src_flags (the pch script folds it into the .gch). Guarded because
+// build_src_flags also reaches C and assembly src edges.
+#ifdef __cplusplus
+#include "esphome/core/application.h"
+#include "esphome/core/automation.h"
+#endif

tests/unit_tests/test_host_pch_prefix.py

@@ -0,0 +1,21 @@
+"""The host pch prefix must keep resolving; a rename would silently
+collapse the precompiled set to defines.h with strict CI still green."""
+
+from pathlib import Path
+import re
+
+from esphome.components.host import HOST_PCH_PREFIX
+
+REPO = Path(__file__).parents[2]
+
+
+def test_host_pch_prefix_resolves() -> None:
+    prefix = REPO / HOST_PCH_PREFIX
+    assert prefix.is_file()
+    body = prefix.read_text()
+    includes = re.findall(r'#include "([^"]+)"', body)
+    assert includes, "prefix wrapper folds nothing"
+    for name in includes:
+        assert (REPO / name).is_file(), f"{name} does not resolve"
+    # The C guard is what keeps build_src_flags safe on C/assembly edges
+    assert "#ifdef __cplusplus" in body