`AssociatedItem` metadata gets entangled with item contents across crates

Within a crate, changes to the body of an impl item (with def-id X) are not considered to affect `AssociatedItem(X)` (because computing `AssociatedItem(X)` only reads from the impl). However, across crates, the data is stored with the item X along with things that are dependent on the body, and hence the results get entangled. 

At least I think this is the cause. =)