Remove mInfo field from Cancelable by default.

It's big, and totally unused.

The opt-in to enable it is for backwards compat.  If we care about preserving
compat while also ensuring that some internal Cancelable uses never have this
field, that is also possible, but more complicated.

src/lib/core/BUILD.gn

@@ -64,6 +64,7 @@ buildconfig_header("chip_buildconfig") {
     "CHIP_CONFIG_TRANSPORT_PW_TRACE_ENABLED=${chip_enable_transport_pw_trace}",
     "CHIP_CONFIG_MINMDNS_DYNAMIC_OPERATIONAL_RESPONDER_LIST=${chip_config_minmdns_dynamic_operational_responder_list}",
     "CHIP_CONFIG_MINMDNS_MAX_PARALLEL_RESOLVES=${chip_config_minmdns_max_parallel_resolves}",
+    "CHIP_CONFIG_CANCELABLE_HAS_INFO_STRING_FIELD=${chip_config_cancelable_has_info_string_field}",
   ]
 }
 

src/lib/core/CHIPCallback.h

@@ -26,6 +26,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <lib/core/CHIPConfig.h>
+
 namespace chip {
 
 namespace Callback {
@@ -49,7 +51,13 @@ class Cancelable
      */
     Cancelable * mNext;
     Cancelable * mPrev;
+
+    // CHIP_CONFIG_CANCELABLE_HAS_INFO_STRING_FIELD allows consumers that were
+    // using this field to opt into having it (and the resulting memory bloat)
+    // while allowing everyone else to save the memory.
+#if CHIP_CONFIG_CANCELABLE_HAS_INFO_STRING_FIELD
     alignas(uint64_t) char mInfo[24];
+#endif // CHIP_CONFIG_CANCELABLE_HAS_INFO_STRING_FIELD
 
     /**
      * @brief when non-null, indicates the Callback is registered with

src/lib/core/core.gni

@@ -84,6 +84,10 @@ declare_args() {
 
   # When using minmdns, set the number of parallel resolves
   chip_config_minmdns_max_parallel_resolves = 2
+
+  # If set to true, adds a string "info" field to Cancelable.
+  # Only here for backwards compat.  Generally should not be set to true.
+  chip_config_cancelable_has_info_string_field = false
 }
 
 if (chip_target_style == "") {