From: Mark Thom <markjordanthom@gmail.com>
Date: Wed, 3 Feb 2021 18:11:22 +0000 (-0700)
Subject: do not traverse data structures when binding to local variables in unify_with_occurs_... 
X-Git-Tag: v0.9.0~153
X-Git-Url: https://git.sagredo.dev/?a=commitdiff_plain;h=f5d808e68f0763cbe86f5b7c82e29a4cc6ee7823;p=scryer-prolog.git

do not traverse data structures when binding to local variables in unify_with_occurs_check (#781)
---

diff --git a/src/machine/machine_state_impl.rs b/src/machine/machine_state_impl.rs
index 5de07140..f7cda917 100644
--- a/src/machine/machine_state_impl.rs
+++ b/src/machine/machine_state_impl.rs
@@ -201,6 +201,14 @@ impl MachineState {
     }
 
     fn bind_with_occurs_check(&mut self, r: Ref, addr: Addr) {
+        if let Ref::StackCell(..) = r {
+            // local variable optimization -- r cannot occur in the
+            // data structure bound to addr, so don't bother
+            // traversing it.
+            self.bind(r, addr);
+            return;
+        }
+
         let mut fail = false;
 
         for addr in self.acyclic_pre_order_iter(addr) {