Assemble plain .s without the preprocessor, allow unflagging operand-less linker flags

esphome/build_gen/arduino8266.py

@@ -631,6 +631,10 @@ _PLAIN_LINKER_FLAGS = (
     "-nostdlib",
     "-rdynamic",
 )
+# The subset whose next token is an operand; unflagging the bare flag
+# would strand the operand. Operand-less members of the list above filter
+# whole-token from both the compile and link lines, as PlatformIO allows.
+_PLAIN_LINKER_OPERAND_FLAGS = ("-u", "-e")
 _PLAIN_LINKER_PREFIXES = ("-T", "-Xlinker")
 # Driver options, not ld options: -Wl, has no equivalent for these
 _PLAIN_DRIVER_LINK_PREFIXES = ("-fuse-ld=", "--specs=", "-specs=")
@@ -993,7 +997,7 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
     if plain := sorted(
         u
         for u in unflags
-        if u in _PLAIN_LINKER_FLAGS or u.startswith(_PLAIN_LINKER_PREFIXES)
+        if u in _PLAIN_LINKER_OPERAND_FLAGS or u.startswith(_PLAIN_LINKER_PREFIXES)
     ):
         raise EsphomeError(
             f"build_unflags cannot remove plain linker flag(s) "
@@ -1040,7 +1044,7 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
         f"buildtool = {_q(build_tool)}",
         f"ccache = {_q(ccache) if ccache else ''}",
         "",
-        # Rule names match SOURCE_KIND_FOR_SUFFIX values (c, cxx, asm)
+        # Rule names match SOURCE_KIND_FOR_SUFFIX values (c, cxx, asm, aspp)
         "rule c",
         "  command = $ccache $cc -MMD -MF $out.d $cflags $flags -c $in -o $out",
         "  depfile = $out.d",
@@ -1051,11 +1055,16 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
         "  depfile = $out.d",
         "  deps = gcc",
         "  description = CXX $out",
-        "rule asm",
+        "rule aspp",
         "  command = $ccache $cc -MMD -MF $out.d -x assembler-with-cpp $asflags $flags -c $in -o $out",
         "  depfile = $out.d",
         "  deps = gcc",
         "  description = AS $out",
+        # Plain assembler, as SCons's ASCOM: no preprocessor, so no
+        # depfile and no $flags (defines/includes) either
+        "rule asm",
+        "  command = $ccache $cc -x assembler $asflags -c $in -o $out",
+        "  description = AS $out",
         "rule ar",
         f"  command = $python $buildtool ar {_q(toolchain_tool(paths.toolchain, 'ar'))} $out $out.rsp",
         "  rspfile = $out.rsp",

tests/unit_tests/build_gen/test_arduino8266.py

@@ -346,7 +346,7 @@ def test_write_project_link_line_and_exclusions(tmp_path: Path) -> None:
     assert "core_esp8266_waveform_phase.cpp" not in content
     assert "core_esp8266_main.cpp.o" in content
     # Assembly and C sources compile through their own rules
-    assert "cont.S.o: asm" in content
+    assert "cont.S.o: aspp" in content
     assert "abi.c.o: c" in content
     # throw_stubs is force-included for ESPHome sources only, via one shared
     # srcflags variable rather than a copy of the flags line per edge
@@ -688,6 +688,30 @@ def test_build_config_nonosdk_precedence() -> None:
     assert _resolve_build_config(_defines()).nonosdk == "NONOSDK221"
 
 
+def test_write_project_plain_asm_rule_skips_preprocessor(tmp_path: Path) -> None:
+    """A lowercase .s source assembles plain (SCons AS), never through the
+    preprocessor rule that a .S source gets."""
+    paths = _make_framework(tmp_path)
+    core_dir = paths.framework / "cores" / "esp8266"
+    (core_dir / "lowlevel.s").write_text("nop\n")
+    _set_flags()
+    content = _write_ninja(paths)
+    assert "lowlevel.s.o: asm " in content
+    assert "rule asm\n  command = $ccache $cc -x assembler $asflags -c $in -o $out" in (
+        content
+    )
+
+
+def test_write_project_unflags_operandless_linker_flag(tmp_path: Path) -> None:
+    """build_unflags: -nostdlib filters whole-token from both lines, as
+    PlatformIO allows; only operand-taking flags hard-error."""
+    paths = _make_framework(tmp_path)
+    _set_flags()
+    CORE.build_unflags = {"-nostdlib"}
+    content = _write_ninja(paths)
+    assert "-nostdlib" not in content
+
+
 def test_write_project_unflagged_symbol_takes_its_dash_u(tmp_path: Path) -> None:
     """Unflagging a -u symbol drops the -u that carried it; a dangling -u
     would consume the next token and hand ld a symbol as an input file."""