struct: replace sys.getrefcount with a cleaner method

sx/types/struct.py

@@ -19,6 +19,16 @@ from ..core.expr import VarExpr, VarSource
 
 T = TypeVar('T')
 
+class StructProxy:
+    def __init__(self):
+        self._sx_fields_ = {}
+
+    def __getattr__(self, name: str) -> VarExpr:
+        e = VarExpr(name)
+        self._sx_fields_[name].append(e)
+        return e
+
+
 class StructType(G[T], Type[T]):
     __slots__ = ('fields', 'cls', 'partial', 'union', 'generics', 'bound')
 
@@ -222,21 +232,23 @@ class Struct:
         localns.update({g.name: g for g in generics})
 
         # Evaluate annotations into fields
+        proxy = StructProxy()
+        localns['self'] = proxy
         fields = {}
-        refs: Mapping[str, VarExpr] = {}
         for name, (globalns, value) in annots.items():
             val = eval(value, globalns, localns)
             if isinstance(val, Annotated):
                 val = next(v for v in val.__metadata__ if isinstance(v, Type))
             fields[name] = val
-            localns[name] = refs[name] = VarExpr(name)
+            proxy._sx_fields_[name] = []
         
         del localns
-        for name, r in refs.items():
-            count = sys.getrefcount(r) - 4 # cursed
+        for name, exprs in proxy._sx_fields_.items():
+            count = len(exprs)
             if count:
                 fields[name] = VarSource(fields[name], count)
-                r._sx_resolve_(fields[name])
+                for e in exprs:
+                    e._sx_resolve_(fields[name])
 
         cls._sx_type_ = StructType(fields, cls, generics=generics, bound=bound, **kwargs)