[error] force error checking (#4784)

include/openthread/error.h

@@ -55,7 +55,7 @@ extern "C" {
  * This enumeration represents error codes used throughout OpenThread.
  *
  */
-typedef enum otError
+typedef enum OT_MUST_USE_RESULT otError
 {
     /**
      * No error.

include/openthread/platform/toolchain.h

@@ -63,6 +63,24 @@
 extern "C" {
 #endif
 
+/**
+ * @def OT_MUST_USE_RESULT
+ *
+ * Compiler-specific indication that a class or enum must be used when it is
+ * the return value of a function.
+ *
+ * @note This is currently only available with clang (C++17 implements it
+ *       as attribute [[nodiscard]]).
+ * @note To suppress the 'unused-result' warning/error, please use the
+ *       '-Wno-unused-result' compiler option.
+ *
+ */
+#if defined(__clang__)
+#define OT_MUST_USE_RESULT __attribute__((warn_unused_result))
+#else
+#define OT_MUST_USE_RESULT
+#endif
+
 /**
  * @def OT_TOOL_PACKED_BEGIN
  *