Require bodies on the MMU segment sizes, split driver link flags, warn on a stuck note

A bare -DMMU_IRAM_SIZE would preprocess to len = 1 and fail far away in
ld (or as a misleading testing-mode message), so MMU_CUSTOM requires a
body on both segment sizes before the shape check. -fuse-ld=/-specs=/
--specs= are driver options with no -Wl, spelling, so they get their
own rejection instead of advice that would make things worse. An
unremovable stale stderr note now warns with the clean hint, since its
consequence is a permanently unstamped cache.

esphome/build_gen/arduino8266.py

@@ -353,6 +353,12 @@ def _resolve_build_config(defines: dict[str, str]) -> _BuildConfig:
                 "PIO_FRAMEWORK_ARDUINO_MMU_CUSTOM requires MMU_IRAM_SIZE and "
                 "MMU_ICACHE_SIZE build flags"
             )
+        for name in _MMU_SEGMENT_SIZE_NAMES:
+            # A bare -Dname would preprocess to len = 1 and fail far away
+            if "=" not in defines[name]:
+                raise EsphomeError(
+                    f"{name} must be a hex literal (e.g. 0x8000), got (no value)"
+                )
         for name, body in defines.items():
             if not name.startswith("MMU_") or "=" not in body:
                 # Valueless flags (MMU_IRAM_HEAP) are legitimate switches
@@ -488,6 +494,12 @@ def _project_flags(
         elif tok.startswith("-l"):
             libs.append(tok[2:])
         else:
+            if tok.startswith(_PLAIN_DRIVER_LINK_PREFIXES):
+                # Driver options with no -Wl, spelling; ld would reject them
+                raise EsphomeError(
+                    f"Link flag {tok} in build_flags is not supported by the "
+                    "native toolchain"
+                )
             if tok in _PLAIN_LINKER_FLAGS or tok.startswith(_PLAIN_LINKER_PREFIXES):
                 raise EsphomeError(
                     f"Linker flag {tok} in build_flags is not routed to the "
@@ -511,7 +523,9 @@ _PLAIN_LINKER_FLAGS = (
     "-nostdlib",
     "-rdynamic",
 )
-_PLAIN_LINKER_PREFIXES = ("-T", "-Xlinker", "-fuse-ld=", "--specs=")
+_PLAIN_LINKER_PREFIXES = ("-T", "-Xlinker")
+# Driver options, not ld options: -Wl, has no equivalent for these
+_PLAIN_DRIVER_LINK_PREFIXES = ("-fuse-ld=", "--specs=", "-specs=")
 
 
 def _stat_sig(path: Path) -> str:
@@ -641,7 +655,13 @@ def generate_ld_scripts(
             except OSError as err:
                 # A kept stale note would re-emit an obsolete diagnostic on
                 # every cache hit; skip the stamp so -E re-derives the truth
-                _LOGGER.debug("Could not remove %s: %s", stderr_note, err)
+                _LOGGER.warning(
+                    "Could not remove %s (%s); the linker script will "
+                    "regenerate every build until it is removable; %s",
+                    stderr_note,
+                    err,
+                    _CLEAN_HINT,
+                )
                 note_persisted = False
         if "SECTIONS" not in result.stdout:
             # A degenerate zero-exit run must not be stamped as a good cache

tests/unit_tests/build_gen/test_arduino8266.py

@@ -702,6 +702,24 @@ def test_generate_ld_scripts_unremovable_stale_note_vetoes_the_stamp(
     run3.assert_called_once()
 
 
+@pytest.mark.parametrize("name", ["MMU_IRAM_SIZE", "MMU_ICACHE_SIZE"])
+def test_mmu_custom_valueless_segment_size_raises(name: str) -> None:
+    """A bare -Dname would preprocess to len = 1 and fail far away in ld."""
+    other = "MMU_ICACHE_SIZE" if name == "MMU_IRAM_SIZE" else "MMU_IRAM_SIZE"
+    with pytest.raises(EsphomeError, match=f"{name} must be a hex literal"):
+        _resolve("-DPIO_FRAMEWORK_ARDUINO_MMU_CUSTOM", f"-D{name}", f"-D{other}=0x8000")
+
+
+@pytest.mark.parametrize(
+    "flag", ["-fuse-ld=lld", "--specs=nano.specs", "-specs=nano.specs"]
+)
+def test_driver_link_flags_rejected_without_wl_advice(flag: str) -> None:
+    """No -Wl, spelling exists for these; the message must not suggest one."""
+    CORE.build_flags = {flag, "-DFOO"}
+    with pytest.raises(EsphomeError, match="not supported by the native toolchain"):
+        arduino8266._project_flags(set(), arduino8266._lexed_build_flags())
+
+
 def test_build_config_mmu_knob_with_raw_mmu_flag_raises() -> None:
     """A variant knob plus a raw MMU_* define would split the compile line
     from the linker script; refuse like the no-knob case."""