Extract make_app_name_cpp as testable free function with unit tests

Move the pure logic (string building, escaping, byte length
calculation) out of the nested closure into a module-level function
that can be tested without codegen mocks. Tests cover both copilot
review concerns: empty friendly_name with MAC suffix, and UTF-8
byte length for non-ASCII characters.

esphome/core/config.py

@@ -64,6 +64,32 @@ _APP_NAME_BUF_VAR = "esphome_app_name_buf"
 _APP_NAME_MAC_SEP = "-"
 _APP_FRIENDLY_NAME_BUF_VAR = "esphome_app_friendly_name_buf"
 _APP_FRIENDLY_NAME_MAC_SEP = " "
+# Placeholder suffix for MAC address (last 6 hex chars)
+_MAC_SUFFIX_PLACEHOLDER = "XXXXXX"
+
+
+def make_app_name_cpp(
+    value: str, var_name: str, sep: str, *, add_mac_suffix: bool
+) -> tuple[str, str | None, int]:
+    """Compute C++ expression and optional global declaration for an app name.
+
+    Returns (cpp_expr, global_decl_or_none, byte_length).
+    - cpp_expr: The C++ expression to pass to pre_setup (var name or string literal).
+    - global_decl: A static char[] declaration string, or None if not needed.
+    - byte_length: The UTF-8 byte length of the string value.
+    """
+    if add_mac_suffix:
+        buf_value = "" if not value else f"{value}{sep}{_MAC_SUFFIX_PLACEHOLDER}"
+        escaped = cpp_string_escape(buf_value)
+        return (
+            var_name,
+            f"static char {var_name}[] = {escaped};",
+            len(buf_value.encode("utf-8")),
+        )
+    if not value:
+        return '""', None, 0
+    return cpp_string_escape(value), None, len(value.encode("utf-8"))
+
 
 StartupTrigger = cg.esphome_ns.class_(
     "StartupTrigger", cg.Component, automation.Trigger.template()
@@ -562,37 +588,19 @@ async def to_code(config: ConfigType) -> None:
     friendly_name = config[CONF_FRIENDLY_NAME]
     name_add_mac_suffix = config[CONF_NAME_ADD_MAC_SUFFIX]
 
-    def _make_app_name_expr(
+    def _emit_app_name(
         value: str, var_name: str, sep: str
     ) -> tuple[cg.Expression, int]:
-        """Create a name expression for pre_setup.
-
-        With MAC suffix: emits a static mutable buffer with placeholder suffix.
-        Without: passes the string literal directly as const char*.
-        Returns (expression, byte_length).
-        """
-        if name_add_mac_suffix:
-            value_with_placeholder = "" if not value else f"{value}{sep}XXXXXX"
-            cg.add_global(
-                cg.RawStatement(
-                    f"static char {var_name}[] = {cpp_string_escape(value_with_placeholder)};"
-                )
-            )
-            return (
-                cg.RawExpression(var_name),
-                len(value_with_placeholder.encode("utf-8")),
-            )
-        if not value:
-            return cg.RawExpression('""'), 0
-        return (
-            cg.RawExpression(cpp_string_escape(value)),
-            len(value.encode("utf-8")),
+        """Emit codegen for an app name and return (expression, byte_length)."""
+        cpp_expr, global_decl, byte_len = make_app_name_cpp(
+            value, var_name, sep, add_mac_suffix=name_add_mac_suffix
         )
+        if global_decl is not None:
+            cg.add_global(cg.RawStatement(global_decl))
+        return cg.RawExpression(cpp_expr), byte_len
 
-    name_expr, name_len = _make_app_name_expr(
-        name, _APP_NAME_BUF_VAR, _APP_NAME_MAC_SEP
-    )
-    friendly_expr, friendly_len = _make_app_name_expr(
+    name_expr, name_len = _emit_app_name(name, _APP_NAME_BUF_VAR, _APP_NAME_MAC_SEP)
+    friendly_expr, friendly_len = _emit_app_name(
         friendly_name, _APP_FRIENDLY_NAME_BUF_VAR, _APP_FRIENDLY_NAME_MAC_SEP
     )
     if name_add_mac_suffix:

tests/unit_tests/core/test_config.py

@@ -23,6 +23,7 @@ from esphome.const import (
 from esphome.core import CORE, config
 from esphome.core.config import (
     Area,
+    make_app_name_cpp,
     preload_core_config,
     valid_include,
     valid_project_name,
@@ -969,3 +970,79 @@ def test_config_hash_different_for_different_configs() -> None:
     hash2 = CORE.config_hash
 
     assert hash1 != hash2
+
+
+def test_make_app_name_cpp_no_mac_simple() -> None:
+    """Test simple name without MAC suffix returns string literal."""
+    cpp_expr, global_decl, byte_len = make_app_name_cpp(
+        "my-device", "buf", "-", add_mac_suffix=False
+    )
+    assert cpp_expr == '"my-device"'
+    assert global_decl is None
+    assert byte_len == 9
+
+
+def test_make_app_name_cpp_no_mac_empty() -> None:
+    """Test empty name without MAC suffix."""
+    cpp_expr, global_decl, byte_len = make_app_name_cpp(
+        "", "buf", "-", add_mac_suffix=False
+    )
+    assert cpp_expr == '""'
+    assert global_decl is None
+    assert byte_len == 0
+
+
+def test_make_app_name_cpp_mac_suffix() -> None:
+    """Test name with MAC suffix emits static buffer."""
+    cpp_expr, global_decl, byte_len = make_app_name_cpp(
+        "my-device", "esphome_app_name_buf", "-", add_mac_suffix=True
+    )
+    assert cpp_expr == "esphome_app_name_buf"
+    assert global_decl is not None
+    assert "static char esphome_app_name_buf[]" in global_decl
+    assert "my-device-XXXXXX" in global_decl
+    assert byte_len == len("my-device-XXXXXX")
+
+
+def test_make_app_name_cpp_mac_suffix_empty() -> None:
+    """Test empty name with MAC suffix emits empty static buffer."""
+    cpp_expr, global_decl, byte_len = make_app_name_cpp(
+        "", "esphome_app_name_buf", "-", add_mac_suffix=True
+    )
+    assert cpp_expr == "esphome_app_name_buf"
+    assert global_decl is not None
+    assert "static char esphome_app_name_buf[]" in global_decl
+    assert byte_len == 0
+
+
+def test_make_app_name_cpp_mac_suffix_space_sep() -> None:
+    """Test friendly name uses space separator for MAC suffix."""
+    cpp_expr, global_decl, byte_len = make_app_name_cpp(
+        "My Device", "esphome_app_friendly_name_buf", " ", add_mac_suffix=True
+    )
+    assert cpp_expr == "esphome_app_friendly_name_buf"
+    assert global_decl is not None
+    assert "My Device XXXXXX" in global_decl
+    assert byte_len == len("My Device XXXXXX")
+
+
+def test_make_app_name_cpp_non_ascii_utf8_length() -> None:
+    """Test non-ASCII characters use UTF-8 byte length."""
+    _, global_decl, byte_len = make_app_name_cpp(
+        "café", "buf", "-", add_mac_suffix=False
+    )
+    assert byte_len == len("café".encode())  # 5 bytes, not 4 chars
+    assert global_decl is None
+
+
+def test_make_app_name_cpp_non_ascii_mac_suffix_utf8_length() -> None:
+    """Test non-ASCII with MAC suffix uses UTF-8 byte length."""
+    _, _, byte_len = make_app_name_cpp("café", "buf", "-", add_mac_suffix=True)
+    assert byte_len == len("café-XXXXXX".encode())
+
+
+def test_make_app_name_cpp_special_chars_escaped() -> None:
+    """Test special characters are properly escaped in C++ string."""
+    cpp_expr, _, _ = make_app_name_cpp('my "device"', "buf", "-", add_mac_suffix=False)
+    # cpp_string_escape uses octal escapes for quotes
+    assert '"' not in cpp_expr[1:-1]  # no unescaped quotes inside the outer quotes