From 0ae24f75368ba90e2528b4e29d6a548644edcfae Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Fri, 9 Oct 2026 16:00:41 -1000
Subject: [PATCH] [core] Catch heap allocating helper calls with explicit
 template arguments (#20391)

---
 esphome/components/debug/debug_zephyr.cpp | 18 +++++++++---------
 script/ci-custom.py                       |  3 ++-
 tests/script/test_ci_custom.py            | 22 ++++++++++++++++++++++
 3 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/esphome/components/debug/debug_zephyr.cpp b/esphome/components/debug/debug_zephyr.cpp
index cec2ab0be8..f3a5f3d770 100644
--- a/esphome/components/debug/debug_zephyr.cpp
+++ b/esphome/components/debug/debug_zephyr.cpp
@@ -1,5 +1,6 @@
 #include "debug_component.h"
 #ifdef USE_ZEPHYR
+#include <cinttypes>
 #include <climits>
 #include "esphome/core/alloc_helpers.h"
 #include "esphome/core/log.h"
@@ -400,18 +401,17 @@ size_t DebugComponent::get_device_info_(std::span<char, DEVICE_INFO_BUFFER_SIZE>
 #endif
   }
 #endif
-  auto uicr = [](volatile uint32_t *data, uint8_t size) {
-    std::string res;
+  // 13 words of 8 hex digits, separated by spaces
+  char uicr_buf[13 * 9];
+  auto uicr = [&uicr_buf](volatile uint32_t *data, uint8_t size) {
+    size_t len = 0;
     for (size_t i = 0; i < size; i++) {
-      if (i > 0) {
-        res += ' ';
-      }
-      res += format_hex_pretty<uint32_t>(data[i], '\0', false);
+      len = buf_append_printf(uicr_buf, sizeof(uicr_buf), len, i > 0 ? " %08" PRIX32 : "%08" PRIX32, data[i]);
     }
-    return res;
+    return uicr_buf;
   };
-  ESP_LOGD(TAG, "  NRFFW %s", uicr(NRF_UICR->NRFFW, 13).c_str());
-  ESP_LOGD(TAG, "  NRFHW %s", uicr(NRF_UICR->NRFHW, 12).c_str());
+  ESP_LOGD(TAG, "  NRFFW %s", uicr(NRF_UICR->NRFFW, 13));
+  ESP_LOGD(TAG, "  NRFHW %s", uicr(NRF_UICR->NRFHW, 12));
 #ifdef ESPHOME_LOG_HAS_VERBOSE
   log_peripherals_info();
 #endif
diff --git a/script/ci-custom.py b/script/ci-custom.py
index 7839038a8c..8361387af0 100755
--- a/script/ci-custom.py
+++ b/script/ci-custom.py
@@ -1065,7 +1065,8 @@ HEAP_ALLOCATING_HELPERS = {
     r"str_sprintf|"
     r"str_snprintf|"
     r"value_accuracy_to_string"
-    r")\s*\(" + CPP_RE_EOL,
+    # Explicit template arguments at any nesting depth, e.g. format_hex<std::array<uint8_t, 4>>(
+    r")\s*(?:<[^;{}()\n]*>\s*)?\(" + CPP_RE_EOL,
     include=cpp_include,
     exclude=[
         # The definitions themselves
diff --git a/tests/script/test_ci_custom.py b/tests/script/test_ci_custom.py
index 998650079e..f303d19bb0 100644
--- a/tests/script/test_ci_custom.py
+++ b/tests/script/test_ci_custom.py
@@ -444,6 +444,28 @@ def test_log_tag_as_string_ignores_logging(line: str) -> None:
     assert not ci_custom.lint_log_tag_as_string("test.cpp", line + "\n")
 
 
+@pytest.mark.parametrize(
+    ("content", "flagged"),
+    [
+        ("auto s = format_hex_pretty(data, len);\n", True),
+        # Explicit template arguments must not hide the call
+        ("auto s = format_hex_pretty<uint32_t>(v, '\\0', false);\n", True),
+        ("auto s = format_hex<std::array<uint8_t, 4>>(arr);\n", True),
+        (
+            "auto s = format_hex<std::conditional_t<true, std::make_unsigned_t<int>, uint64_t>>(v);\n",
+            True,
+        ),
+        ("auto s = format_hex < 3;\n", False),
+        ("format_hex_pretty_to(buf, sizeof(buf), data, len);\n", False),
+    ],
+)
+def test_heap_allocating_helpers_detection(content: str, flagged: bool) -> None:
+    errs = ci_custom.lint_no_heap_allocating_helpers(
+        Path("esphome/components/x/x.cpp"), content
+    )
+    assert bool(errs) is flagged
+
+
 # --- rule: every component __init__.py defines DOMAIN as its own name ---