[build] Add clang::nonallocating support

BUILD.bazel

@@ -624,6 +624,7 @@ cc_library(
     strip_include_prefix = strip_include_prefix_config(),
     deps = [
         ":assert_or_abort",
+        ":compiler_compat",
         ":concepts",
         ":fixed_vector",
         ":preconditions",
@@ -640,6 +641,7 @@ cc_library(
     strip_include_prefix = strip_include_prefix_config(),
     deps = [
         ":algorithm",
+        ":compiler_compat",
         ":concepts",
         ":iterator_utils",
         ":memory",
@@ -789,6 +791,9 @@ cc_library(
     hdrs = ["include/fixed_containers/memory.hpp"],
     includes = includes_config(),
     strip_include_prefix = strip_include_prefix_config(),
+    deps = [
+        ":compiler_compat",
+    ],
     copts = ["-std=c++20"],
 )
 
@@ -1071,6 +1076,14 @@ cc_library(
     copts = ["-std=c++20"],
 )
 
+cc_library(
+    name = "compiler_compat",
+    hdrs = ["include/fixed_containers/compiler_compat.hpp"],
+    includes = includes_config(),
+    strip_include_prefix = strip_include_prefix_config(),
+    copts = ["-std=c++20"],
+)
+
 cc_library(
     name = "enums_test_common",
     hdrs = ["test/enums_test_common.hpp"],

README.md

@@ -420,13 +420,15 @@ ctest -C Debug
 
 ### bazel
 #### clang
+- clang++-20 and above [support function side effect verification](https://clang.llvm.org/docs/FunctionEffectAnalysis.html)
+
 1) Build separately (optional)
 ```
 CC=clang++-13 bazel build --config=clang ...
 ```
 2) Run tests
 ```
-CC=clang++-13 bazel test --config=clang :all_tests
+CC=clang++-13 bazel test --config=clang --build_tests_only --nocache_test_results --runs_per_test=1 :all_tests
 ```
 
 #### clang with libc++
@@ -458,7 +460,7 @@ The macro is needed to avoid analysis on some particularly slow places.
 
 ## Tested Compilers
 
-- Clang  13
+- Clang 13 (and 20)
 - GCC 11
 - MSVC++ 14.29 / Visual Studio 2019
 

include/fixed_containers/compiler_compat.hpp

@@ -0,0 +1,50 @@
+#pragma once
+
+/**
+ * @file compiler_compat.hpp
+ * @brief Compiler compatibility utilities and attributes.
+ */
+
+/**
+ * @brief FIXED_CONTAINERS_NONALLOCATING expands to [[clang::nonallocating]] when supported.
+ *
+ * This attribute helps clang's function effects analysis detect potentially allocating
+ * functions. It's only applied when:
+ * 1. Using clang compiler
+ * 2. Clang version supports the nonallocating attribute (clang 20+)
+ *    - https://releases.llvm.org/20.1.0/tools/clang/docs/ReleaseNotes.html#static-analyzer
+ * 3. _GLIBCXX_ASSERTIONS is not enabled (to avoid false positives from debug assertions)
+ *
+ * @note _GLIBCXX_ASSERTIONS enables debug assertions in the standard library which contain calls
+ * to __builtin_printf that clang's function effects analysis considers potentially allocating,
+ * even though the code under test may not allocate in normal operation.
+ */
+#if defined(__clang__) && __clang_major__ >= 20 && !defined(_GLIBCXX_ASSERTIONS)
+#define FIXED_CONTAINERS_NONALLOCATING [[clang::nonallocating]]
+#else
+#define FIXED_CONTAINERS_NONALLOCATING
+#endif
+
+/**
+ * @brief FIXED_CONTAINERS_SUPPRESS_FUNCTION_EFFECTS disables nonallocating and nonblocking checks.
+ *
+ * This macro applies diagnostic pragmas to suppress function effects warnings for code
+ * that is expected to be non-allocating but may trigger false positives in the compiler's
+ * conservative analysis.
+ *
+ * Usage:
+ * ```cpp
+ * FIXED_CONTAINERS_SUPPRESS_FUNCTION_EFFECTS(
+ *     some_function_that_might_trigger_false_positive();
+ * )
+ * ```
+ */
+#if defined(__clang__)
+#define FIXED_CONTAINERS_SUPPRESS_FUNCTION_EFFECTS(...)                  \
+    _Pragma("clang diagnostic push")                                     \
+        _Pragma("clang diagnostic ignored \"-Wunknown-warning-option\"") \
+            _Pragma("clang diagnostic ignored \"-Wfunction-effects\"")   \
+                __VA_ARGS__ _Pragma("clang diagnostic pop")
+#else
+#define FIXED_CONTAINERS_SUPPRESS_FUNCTION_EFFECTS(...) __VA_ARGS__
+#endif