Split record_var_binding into methods for expected and actual

This allows unify to maintain the same subtype relationship between
expected and actual throughout unify, which we are going to need
for mutable? and for function types.

Author: brson

src/comp/middle/ty.rs

@@ -1840,13 +1840,35 @@ mod unify {
           }
         }
     }
-    fn record_var_binding(cx: @ctxt, key: int, typ: t) -> result {
+
+    fn record_var_binding_for_expected(
+        cx: @ctxt, key: int, typ: t) -> result {
+        record_var_binding(
+            cx, key, typ,
+            fn (cx: @ctxt, old_type: t, new_type: t) -> result {
+                unify_step(cx, old_type, new_type)
+            })
+    }
+
+    fn record_var_binding_for_actual(
+        cx: @ctxt, key: int, typ: t) -> result {
+        record_var_binding(
+            cx, key, typ,
+            fn (cx: @ctxt, old_type: t, new_type: t) -> result {
+                unify_step(cx, new_type, old_type)
+            })
+    }
+
+    fn record_var_binding(
+        cx: @ctxt, key: int, typ: t,
+        unify_types: fn(@ctxt, t, t) -> result) -> result {
+
         ufind::grow(cx.vb.sets, (key as uint) + 1u);
         let root = ufind::find(cx.vb.sets, key as uint);
         let result_type = typ;
         alt smallintmap::find::<t>(cx.vb.types, root) {
           some(old_type) {
-            alt unify_step(cx, old_type, typ) {
+            alt unify_types(cx, old_type, typ) {
               ures_ok(unified_type) { result_type = unified_type; }
               rs { ret rs; }
             }
@@ -2099,7 +2121,7 @@ mod unify {
               }
               _ {
                 // Just bind the type variable to the expected type.
-                alt record_var_binding(cx, actual_id, expected) {
+                alt record_var_binding_for_actual(cx, actual_id, expected) {
                   ures_ok(_) {/* fall through */ }
                   rs { ret rs; }
                 }
@@ -2113,7 +2135,7 @@ mod unify {
           ty::ty_var(expected_id) {
             // Add a binding. (`actual` can't actually be a var here.)
 
-            alt record_var_binding(cx, expected_id, actual) {
+            alt record_var_binding_for_expected(cx, expected_id, actual) {
               ures_ok(_) {/* fall through */ }
               rs { ret rs; }
             }