Bake the probe stamp into the rule text; edge bindings cannot expand $out

esphome/build_gen/arduino8266.py

@@ -1295,21 +1295,22 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
             if pch_strict():
                 # Consumers wait on the probe stamp, so an unloadable .gch
                 # reds the build here instead of warning ~100 times
+                # $out only expands in rule text, so the stamp command is
+                # baked into the rule (generation host == build host)
+                stamp = (
+                    "cmd /c copy /y nul $out >nul" if os.name == "nt" else "touch $out"
+                )
                 lines.append("rule pchprobe")
                 lines.append(
                     "  command = $cxx $cxxflags $flags -Winvalid-pch"
                     " -Werror=invalid-pch"
                     f" -include {PCH_HEADER_NAME} -fsyntax-only -x c++"
-                    f" {_q(Path(os.devnull))} && $stamp"
+                    f" {_q(Path(os.devnull))} && {stamp}"
                 )
                 lines.append("  description = PCHPROBE $out")
                 lines.append(f"build esphome_pch.probe: pchprobe {gch}")
                 if src_other:
                     lines.append(f"  flags = {' '.join(src_other)}")
-                stamp = (
-                    "cmd /c copy /y nul $out >nul" if os.name == "nt" else "touch $out"
-                )
-                lines.append(f"  stamp = {stamp}")
                 pch_dep = f"{gch} esphome_pch.probe"
             src_cxx_override = ("$srccxxflags", pch_dep)
             mark_pch_emitted()

tests/unit_tests/build_gen/test_arduino8266.py

@@ -1871,6 +1871,10 @@ def test_write_project_pch_strict_emits_probe_edge(
     content = _write_ninja(paths, ccache="/usr/bin/ccache")
     assert "build esphome_pch.probe: pchprobe esphome_pch.h.gch" in content
     assert "-Werror=invalid-pch" in content
+    # $out only expands in rule text; an edge-level binding would emit a
+    # bare "touch " and fail every strict build
+    assert "&& touch $out" in content
+    assert "$stamp" not in content
 
     # With extra src flags the probe edge carries them like the .gch edge
     CORE.platformio_options["build_src_flags"] = (