To store the alias relationships among arguments and return values.
Each function may have multiple return instructions, leading to different RetAlias.
If local (typically 1 = self) with fields in def_id’s body
corresponds to a struct field, return its identity.
Trace a multi-level field path (self.node.ptr, self.mid.leaf.ptr) down
to the innermost raw-pointer field, so the encapsulation check targets
Inner::ptr / Leaf::ptr rather than the intermediate Outer::node /
Outer::mid field. Intermediate reference and ADT layers are dereferenced
through; if the path ends without reaching a raw pointer, None is returned.