[template] Use placement new for template text restore saver (#15883)

esphome/components/template/text/__init__.py

@@ -3,6 +3,7 @@ import esphome.codegen as cg
 from esphome.components import text
 import esphome.config_validation as cv
 from esphome.const import (
+    CONF_ID,
     CONF_INITIAL_VALUE,
     CONF_LAMBDA,
     CONF_MAX_LENGTH,
@@ -12,6 +13,7 @@ from esphome.const import (
     CONF_RESTORE_VALUE,
     CONF_SET_ACTION,
 )
+from esphome.core import ID
 
 from .. import template_ns
 
@@ -84,8 +86,15 @@ async def to_code(config):
         if initial_value_config := config.get(CONF_INITIAL_VALUE):
             cg.add(var.set_initial_value(initial_value_config))
         if config[CONF_RESTORE_VALUE]:
-            args = cg.TemplateArguments(config[CONF_MAX_LENGTH])
+            saver_id = ID(
-            saver = TextSaverTemplate.template(args).new()
+                f"{config[CONF_ID].id}_value_saver",
+                is_declaration=True,
+                type=TextSaverBase,
+            )
+            saver_type = TextSaverTemplate.template(
+                cg.TemplateArguments(config[CONF_MAX_LENGTH])
+            )
+            saver = cg.Pvariable(saver_id, saver_type.new())
             cg.add(var.set_value_saver(saver))
 
     if CONF_SET_ACTION in config:

tests/component_tests/template/config/template_text_restore.yaml

@@ -0,0 +1,14 @@
+esphome:
+  name: test
+
+host:
+
+text:
+  - platform: template
+    name: "Test Text Restore"
+    id: test_text_restore
+    optimistic: true
+    max_length: 10
+    mode: text
+    initial_value: "hello"
+    restore_value: true

tests/component_tests/template/test_template_text.py

@@ -0,0 +1,44 @@
+"""Tests for the template text component."""
+
+from __future__ import annotations
+
+from collections.abc import Callable
+from pathlib import Path
+
+
+def test_template_text_saver_uses_placement_new_with_templated_subclass(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """Regression test for template text restore saver using placement new.
+
+    When ``restore_value: true``, the saver is its own Pvariable with
+    placement new: storage is sized for ``TextSaver<MAX_LENGTH>``, the
+    declared pointer stays at ``TemplateTextSaverBase *`` for polymorphism,
+    and the templated subclass constructor runs. A regression would either
+    reintroduce the heap ``new TextSaver<...>()`` expression or size the
+    storage for the base class and silently skip the subclass ctor.
+    """
+    main_cpp = generate_main(component_config_path("template_text_restore.yaml"))
+
+    # Storage is sized and aligned for the templated subclass.
+    assert "sizeof(template_::TextSaver<10>)" in main_cpp
+    assert "alignas(template_::TextSaver<10>)" in main_cpp
+    # Pointer declared as base type for polymorphism.
+    assert (
+        "static template_::TemplateTextSaverBase *const test_text_restore_value_saver"
+        in main_cpp
+    )
+    # Placement new runs the templated subclass constructor.
+    assert "new(test_text_restore_value_saver) template_::TextSaver<10>()" in main_cpp
+    # Base-class default ctor must NOT be used.
+    assert (
+        "new(test_text_restore_value_saver) template_::TemplateTextSaverBase()"
+        not in main_cpp
+    )
+    # No heap `new TextSaver<...>()` left over — the pre-fix pattern.
+    assert "new template_::TextSaver<" not in main_cpp
+    # Saver is wired into the text component.
+    assert (
+        "test_text_restore->set_value_saver(test_text_restore_value_saver)" in main_cpp
+    )