From 55cd5e87c0996b68ae9d7d469f674e7fb9f4d4ea Mon Sep 17 00:00:00 2001
From: Nathaniel Wesley Filardo <nfilardo@microsoft.com>
Date: Tue, 21 Sep 2021 12:23:10 +0100
Subject: [PATCH] NFC: Add FreeObject::from_next_ptr

Rather than open-code the conversion from &f->next_object to f, add a static
method to FreeObject and take the opportunity to add a static_assert that our
reinterpret_cast is sound.
---
 src/mem/freelist.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/mem/freelist.h b/src/mem/freelist.h
index 0b882a3..9dd8d9b 100644
--- a/src/mem/freelist.h
+++ b/src/mem/freelist.h
@@ -184,6 +184,17 @@ namespace snmalloc
       return p.template as_static<FreeObject::T<BQueue>>();
     }
 
+    /**
+     * A container-of operation to convert &f->next_object to f
+     */
+    template<SNMALLOC_CONCEPT(capptr::ConceptBound) BQueue>
+    static FreeObject::T<BQueue>*
+    from_next_ptr(CapPtr<FreeObject::T<BQueue>, BQueue>* ptr)
+    {
+      static_assert(offsetof(FreeObject::T<BQueue>, next_object) == 0);
+      return reinterpret_cast<FreeObject::T<BQueue>*>(ptr);
+    }
+
   private:
     /**
      * Involutive encryption with raw pointers
@@ -655,7 +666,7 @@ namespace snmalloc
       // to the actual object.  This isn't true if the builder is
       // empty, but you are not allowed to call this in the empty case.
       auto last = FreeObject::HeadPtr<BView, BQueue>(
-        reinterpret_cast<FreeObject::T<BQueue>*>(end[0]));
+        FreeObject::from_next_ptr(cast_end(0)));
       init();
       return {first, last};
     }