[core] Address Copilot review: robust brace depth, accurate docstrings

- Count { and } characters per line instead of matching whole-line
  tokens. Current codegen only emits scope braces as standalone lines
  (from cg.with_local_variable()), but the defensive change is robust
  against future codegen emitting inline control flow like
  `if (cond) {` or `} else {` on one line.
- Add a regression test covering those inline-brace patterns.
- Fix stale docstrings on ComponentMarker and cpp_main_section that
  still claimed "stack frame released on return" and described the
  IIFEs as "noinline". The IIFEs have no noinline attribute and rely
  on scope-based lifetime shortening rather than guaranteed frames.

esphome/core/__init__.py

@@ -565,11 +565,18 @@ def _wrap_in_iifes(lines: list[str], max_statements: int) -> list[str]:
 
     for line in lines:
         chunk.append(line)
-        stripped = line.strip()
-        if stripped == "{":
-            depth += 1
-        elif stripped == "}":
-            depth -= 1
+        # Track brace depth by counting ``{`` and ``}`` characters per line
+        # rather than matching whole-line tokens. Today the only codegen
+        # that emits scope braces as separate statements is
+        # ``cg.with_local_variable()`` (standalone ``{`` / ``}`` lines),
+        # but counting is robust against future codegen that emits inline
+        # control-flow like ``if (cond) {`` or ``} else {`` on a single
+        # line. Multi-line statements (inline lambdas) carry balanced
+        # braces within one list entry so they contribute no net depth.
+        # Braces inside string literals would throw the count off, but
+        # esphome's generated main.cpp does not currently emit strings
+        # that contain unbalanced braces in main_statements.
+        depth += line.count("{") - line.count("}")
         if depth == 0 and len(chunk) >= max_statements:
             flush()
     flush()
@@ -1069,13 +1076,17 @@ class EsphomeCore:
         if not components:
             return "\n".join(prefix) + "\n\n"
 
-        # Each component's block is wrapped in IIFE lambdas so its stack
-        # frame is released on return, bounding peak stack during setup().
-        # Large blocks are sub-split to cap single heavy components (e.g.
-        # sensor platforms with many filter registrations). "begin X" and
-        # "end X" marker comments bracket the IIFE so the generated
-        # main.cpp is easy to scan by component; a comment-only component
-        # gets a single "begin X" marker (no IIFE, no end marker).
+        # Each component's block is wrapped in an IIFE lambda that
+        # introduces a nested scope, shortening the lifetimes of
+        # temporaries so GCC can bound peak setup-time stack usage.
+        # The IIFE has no noinline attribute, so the compiler is free
+        # to inline the block when that produces smaller code without
+        # regressing peak stack. Large blocks are sub-split to cap
+        # single heavy components (e.g. sensor platforms with many
+        # filter registrations). "begin X" and "end X" marker comments
+        # bracket the IIFE so the generated main.cpp is easy to scan by
+        # component; a comment-only component gets a single "begin X"
+        # marker (no IIFE, no end marker).
         pieces = list(prefix)
         for name, body in components:
             wrapped = _wrap_in_iifes(body, max_statements=50)

esphome/cpp_generator.py

@@ -437,7 +437,12 @@ class LineComment(Statement):
 class ComponentMarker(Statement):
     """Sentinel marker recorded in main_statements when a component's
     to_code begins emitting code. ``cpp_main_section`` consumes these
-    to bracket each component's IIFE with begin/end comment markers."""
+    to bracket each component's generated block with begin/end comment
+    markers and to wrap it in an IIFE scope. The IIFE introduces a
+    nested scope so GCC can shorten temporary lifetimes and help reduce
+    peak setup-time stack usage; the lambda has no ``noinline``
+    attribute, so the compiler may still inline the block when that
+    produces smaller code without regressing peak stack."""
 
     __slots__ = ("name",)
 

tests/unit_tests/test_core.py

@@ -934,6 +934,40 @@ def test_wrap_in_iifes_unbalanced_braces_fall_through() -> None:
     assert [line for line in result if line in lines] == lines
 
 
+def test_wrap_in_iifes_never_splits_inline_brace_lines() -> None:
+    # Defensive: if codegen ever emits control flow with braces on the
+    # same line (if/else/for), the depth tracker should keep the whole
+    # scoped block together even with aggressive max_statements.
+    lines = [
+        "before();",
+        "if (cond) {",
+        "then_branch();",
+        "} else {",
+        "for (;;) {",
+        "loop_body();",
+        "}",
+        "}",
+        "after();",
+    ]
+    assert core._wrap_in_iifes(lines, max_statements=1) == [
+        "[]() {",
+        "before();",
+        "}();",
+        "[]() {",
+        "if (cond) {",
+        "then_branch();",
+        "} else {",
+        "for (;;) {",
+        "loop_body();",
+        "}",
+        "}",
+        "}();",
+        "[]() {",
+        "after();",
+        "}();",
+    ]
+
+
 def test_wrap_in_iifes_skips_comment_only_chunks() -> None:
     # Components that emit only a ComponentMarker + config dump (no C++
     # statements) should not be wrapped in an empty IIFE.