[common] add Clang lifetime safety macros and annotate Heap::String (#13381)

This commit introduces initial support for Clang's Lifetime Safety
Analysis across the OpenThread codebase:

1. Defines abstraction macros in `toolchain.h`:
   - `OT_LIFETIME_BOUND` (`[[clang::lifetimebound]]`)
   - `OT_NOESCAPE` (`[[clang::noescape]]`)
   - `OT_GSL_OWNER` (`[[gsl::Owner]]`)
   - `OT_GSL_POINTER` (`[[gsl::Pointer]]`)
   These macros expand to Clang attributes under C++ LLVM builds
   and gracefully degrade to no-ops or GNU attributes elsewhere.

2. Enables Clang lifetime warning flags in `CMakeLists.txt`:
   - `-Wdangling` and `-Wdangling-gsl`
   - `-Wdangling-assignment` and `-Wdangling-field`
   - `-Wreturn-stack-address`

3. Annotates `Heap::String` (`heap_string.hpp`):
   - Marks class `String` with `OT_GSL_OWNER`.
   - Marks `AsCString()` and `Move()` with `OT_LIFETIME_BOUND` to
     ensure returned C string pointers do not outlive the owner.

CMakeLists.txt

@@ -99,7 +99,7 @@ if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_C_COMPILER_ID MATCHES "(Apple)?[Cc]lang")
     set(OT_CFLAGS
         $<$<COMPILE_LANGUAGE:C>:${OT_CFLAGS} -Wall -Wformat-nonliteral -Wextra -Wshadow>
         $<$<COMPILE_LANGUAGE:CXX>:${OT_CFLAGS} -Wall -Wformat-nonliteral -Wextra -Wshadow -Wno-c++14-compat -fno-exceptions>
-        $<$<CXX_COMPILER_ID:Clang>:-Wc99-extensions>
+        $<$<CXX_COMPILER_ID:Clang>:-Wc99-extensions -Wno-unknown-warning-option -Wdangling -Wdangling-gsl -Wdangling-assignment -Wdangling-field -Wreturn-stack-address>
     )
 endif()
 

include/openthread/instance.h

@@ -52,7 +52,7 @@ extern "C" {
  *
  * @note This number versions both OpenThread platform and user APIs.
  */
-#define OPENTHREAD_API_VERSION (613)
+#define OPENTHREAD_API_VERSION (614)
 
 /**
  * @addtogroup api-instance

include/openthread/platform/toolchain.h

@@ -310,6 +310,57 @@ extern "C" {
 
 #endif
 
+/**
+ * @def OT_LIFETIME_BOUND
+ *
+ * Compiler-specific indication that a function or method return value's lifetime is bound to a parameter or `this`.
+ */
+
+/**
+ * @def OT_NOESCAPE
+ *
+ * Compiler-specific indication that a pointer or reference parameter does not escape the function scope.
+ */
+
+/**
+ * @def OT_GSL_OWNER
+ *
+ * Compiler-specific indication that a class or struct is a resource owner for lifetime safety analysis.
+ */
+
+/**
+ * @def OT_GSL_POINTER
+ *
+ * Compiler-specific indication that a class or struct is a non-owning view or pointer for lifetime safety analysis.
+ */
+#if defined(__cplusplus) && defined(__clang__) && defined(__has_cpp_attribute)
+#if __has_cpp_attribute(clang::lifetimebound)
+#define OT_LIFETIME_BOUND [[clang::lifetimebound]]
+#endif
+#if __has_cpp_attribute(clang::noescape)
+#define OT_NOESCAPE [[clang::noescape]]
+#endif
+#if __has_cpp_attribute(gsl::Owner)
+#define OT_GSL_OWNER [[gsl::Owner]]
+#endif
+#if __has_cpp_attribute(gsl::Pointer)
+#define OT_GSL_POINTER [[gsl::Pointer]]
+#endif
+#endif
+
+#ifndef OT_LIFETIME_BOUND
+#define OT_LIFETIME_BOUND
+#endif
+#ifndef OT_NOESCAPE
+#define OT_NOESCAPE
+#endif
+#ifndef OT_GSL_OWNER
+#define OT_GSL_OWNER
+#endif
+#ifndef OT_GSL_POINTER
+#define OT_GSL_POINTER
+#endif
+
 /**
  * @}
  */

src/core/common/heap_string.hpp

@@ -50,7 +50,7 @@ namespace Heap {
  * be reused and/or freed and reallocated when the string is set. The `Heap::String` destructor will always free the
  * allocated buffer.
  */
-class String : public Unequatable<String>
+class OT_GSL_OWNER String : public Unequatable<String>
 {
 public:
     /**
@@ -92,7 +92,7 @@ public:
      *
      * @returns A pointer to C string buffer or `nullptr` if the `String` is null (never set or freed).
      */
-    const char *AsCString(void) const { return mStringBuffer; }
+    const char *AsCString(void) const OT_LIFETIME_BOUND { return mStringBuffer; }
 
     /**
      * Sets the string from a given C string.
@@ -132,7 +132,7 @@ public:
      *
      * @returns An rvalue reference to this `String`.
      */
-    String &&Move(void) { return static_cast<String &&>(*this); }
+    String &&Move(void) OT_LIFETIME_BOUND { return static_cast<String &&>(*this); }
 
     /**
      * Frees any buffer allocated by the `String`.