refactor: xo-gc: + GCObjectVisitor.generation_of()

Concession to narrow MutationLogStore to only use GCObjectVisitor
instead of assuming X1Collector.

include/xo/alloc2/Generation.hpp

@@ -16,7 +16,8 @@ namespace xo {
         /** @class generation
          *  @brief type-safe generation number
          **/
-        struct Generation {
+        class Generation {
+        public:
             using value_type = std::uint32_t;
 
             constexpr Generation() = default;

include/xo/alloc2/gc/AGCObjectVisitor.hpp

@@ -14,6 +14,8 @@
 #pragma once
 
 // includes (via {facet_includes})
+#include <xo/alloc2/Generation.hpp>
+#include <xo/alloc2/role.hpp>
 #include <xo/arena/AllocInfo.hpp>
 #include <xo/facet/obj.hpp>
 #include <xo/facet/facet_implementation.hpp>
@@ -22,6 +24,7 @@
 // see GCObject.hpp, also in xo-alloc2/
 namespace xo { namespace mm { class AGCObject; }}
 namespace xo { namespace mm { class AllocInfo; }}
+namespace xo { namespace mm { class Generation; }}
 
 namespace xo {
 namespace mm {
@@ -58,6 +61,9 @@ public:
     /** allocation metadata for gc-aware data at address @p gco.
 @p gco must be the result of a call to collector's alloc() function **/
     virtual AllocInfo alloc_info(Copaque data, void * addr)  const = 0;
+    /** generation to which pointer @p addr belongs, given role @p r;
+sentinel if @p addr is not owned by collector **/
+    virtual Generation generation_of(Copaque data, role r, const void * addr)  const  noexcept = 0;
 
     // nonconst methods
     /** allocate copy of source object at address @p src.

include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp

@@ -59,6 +59,7 @@ namespace mm {
 
         // const methods
         [[noreturn]] AllocInfo alloc_info(Copaque, void *)  const override { _fatal(); }
+        [[noreturn]] Generation generation_of(Copaque, role, const void *)  const  noexcept override { _fatal(); }
 
         // nonconst methods
         [[noreturn]] void * alloc_copy(Opaque, std::byte *)  const override;

include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp

@@ -13,6 +13,8 @@
 
 #pragma once
 
+#include <xo/alloc2/Generation.hpp>
+#include <xo/alloc2/role.hpp>
 #include <xo/arena/AllocInfo.hpp>
 
 namespace xo {
@@ -46,6 +48,9 @@ namespace mm {
         AllocInfo alloc_info(Copaque data, void * addr)  const override {
             return I::alloc_info(_dcast(data), addr);
         }
+        Generation generation_of(Copaque data, role r, const void * addr)  const  noexcept override {
+            return I::generation_of(_dcast(data), r, addr);
+        }
 
         // non-const methods
         void * alloc_copy(Opaque data, std::byte * src)  const override {

include/xo/alloc2/gc/RGCObjectVisitor.hpp

@@ -91,6 +91,9 @@ public:
     AllocInfo alloc_info(void * addr)  const {
         return O::iface()->alloc_info(O::data(), addr);
     }
+    Generation generation_of(role r, const void * addr)  const  noexcept {
+        return O::iface()->generation_of(O::data(), r, addr);
+    }
 
     // non-const methods (still const in router!)
     void * alloc_copy(std::byte * src)  {