go/zodb/internal/weak: Assert that the object was not moved in the finalizer

This is good to do since we are relying on non-moving property of
current GC. It never triggered but it removed one uncertainty while
debugging GC crash issue. Good to have it in place.

/reviewed-by @levin.zimmermann
/reviewed-on !11

go/zodb/internal/weak/weak.go

@@ -31,9 +31,12 @@
 package weak
 
 import (
+	"fmt"
 	"runtime"
 	"sync"
 	"unsafe"
+
+	_ "go4.org/unsafe/assume-no-moving-gc"
 )
 
 
@@ -76,6 +79,12 @@ func NewRef[T any](obj *T) *Ref[T] {
 
 	var release func(*T)
 	release = func(obj *T) {
+		// assert that the object was not moved
+		iptr := (uintptr)(unsafe.Pointer(obj))
+		if w.iptr != iptr {
+			panic(fmt.Sprintf("weak: release: object moved:  w.iptr=%x  obj=%x", w.iptr, iptr))
+		}
+
 		// GC decided that the object is no longer reachable and
 		// scheduled us to run as finalizer. During the time till we
 		// actually run, Ref.Get might have been come to run and