Clojure currently doesn't clear unused locals. This is problematic as some form of destructuring can generate unused/unusable locals that the compiler cannot clear and thus can cause head retention:
Here's a macroexpansion that exposes this issue:
Cause: The first two bindings of a and b will hold onto the head of c since they are never used and not accessible from the loop body they cannot be cleared.
Approach: Track whether local bindings are used. After evaluating the binding expression, if the local binding is not used and can be cleared, then pop the result rather than storing it.
Screened by: Alex Miller